Impedance matching (auto) transformer and common-mode choke for the JPC-7 dipole and other electrically-short (loaded) dipoles and verticals

Figure 1:
The JPC-7 loaded dipole out in the wild!
Click on the image for a larger version.

Loading coils and "electrically-short" antennas

It is well-known that you can make a "short" wire (e.g. one that is significantly shorter than 1/4 wavelength at the operating frequency) resonant by putting in series with it a coil.  There is no "magic" in this as the inductance of the coil, appropriately chosen, can completely cancel out the capacitance of the electrically-short wire, result being that at "resonance" we are left only with a pure resistance.

In an ideal situation, what we would be left with would be just the radiation resistance of this antenna and for such an antenna, this would mean that the feedpoint resistance would be less than 50 Ohms - probably much less!  In reality, the feedpoint resistance would really a combination of "ground" (counterpoise) losses, conductor losses of the antenna, and losses of the coil itself.

What this means is that if you have an electrically short antenna such as a loaded dipole or vertical with only a series loading coil tuned to resonance at the frequency of interest and no other matching scheme, its feedpoint impedance should be well under 50 Ohms on some bands if it is operating efficiently.

This is often not the case with portable antennas!

Figure 2:
The original stainless steel coil (top) for the
JPC-7 (and JPC-12) with the coil rewound with
silver-plated "jewelry" wire (bottom).
Click on the image for a larger version.

The JPC-7

Some time ago I wrote extensively about the JPC-7 (See the article, "Observations, analysis and field use of the JPC-7 portable "dipole" antenna" - LINK) where I discussed the bits and pieces comprising it:  I have used it in the field a number of times, finding it to work as advertised.

In short, this is a loaded dipole - at least on the lower amateur bands (especially 40 and 30 meters) that is intended for portable use:  On these bands (including 20 and 17 meters) it is physically shorter than 1/2 wavelength and it requires the adjustment of its series inductors to resonate.  On the higher bands (15 and above) its overall length approaches and exceeds a half wavelength meaning that it's a full-sized dipole and is (generally) tuned by adjusting the length of the telescoping sections.

Lossy coils!

There is a down-side:  As sold, it has loading coils that are wound with stainless steel:  As noted in the original article, these coils are very lossy, with MOST of the RF power being dissipated as heat on the lower bands (40 and 30 meters in particular - roughly an "S" unit of signal loss) where a fair bit of inductance is required.

Figure 3:
An example of heating of a stainless steel
loading coil on a short vertical - here, made by
Wolf River.  On 40 meters the temperature of
the coil rose by more than 30F (17C) with
60 watts of RF applied for 60 seconds.
Click on the image for a larger version.

The reason for this is that an electrically-short antenna (one that is physically short compared to the wavelength.)  The total length of the telescoping sections alone put together is about 198" (5 meters) - which is about 12.5% of a wavelength at 40 meters implies that the feedpoint resistance would, were there no loss at all, be around 8-10 Ohms, resulting in a VSWR of more than 4:1.

Calculations and measurements indicate that the approximate Ohmic loss of the original stainless-steel loading coil - if we optimistically presume it to have a Q of 47 - would be about 19 Ohms per coil (remember that there are two coils!) and the sum of the two coils would push feedpoint resistance near-ish 50 Ohms.  The result is that roughly 1 "S-unit" (about 6dB) is lost in the coils alone:  Contacts would still be made, but running a "compromised" antenna (e.g. physically small) that already would be less-efficient than its full-sized counterpart and adding another S-unit of loss doesn't sound like an optimal solution!

Using silver-plated copper "Jewelry Wire" (found on Amazon) to rewind the original loading coils dramatically improved the "Q" (approximately 200) and lowering the Ohmic loss to around 4 Ohms.  The result of this is that rather than something in the 40-50 Ohms for the feedpoint resistance, it dropped to "about 15" Ohms on 40 meters - a VSWR of around 3:1 - and even lower impedance than that (higher VSWR) when I reconfigured the antenna for 60 meters (e.g. added extra screw-together sections, moved the coils next to the feedpoint and added extra "drooping" wires to the ends of the dipole).  At the higher bands (20 meters and up) the feedpoint impedance is close enough to 50 Ohms that one can probably forego the auto transformer at all.

For more information about the "Silver-plated versus Stainless Steel" topic, see the blog entry "Rewinding the Stainless Steel coils with Silver-Plated copper wire on the JPC-7 and JPC-12 antennas" - link.

When a worse VSWR is a good thing!

The first thought when being faced with a higher VSWR on an antenna might be that it was made to be worse - but here is a instance where this is not the case.  As noted earlier, an electrically short antenna like a dipole or vertical can be made to be "longer" (from an RF standpoint) with the addition of a "loading" coil to a "short" (in terms of wavelength) antenna - but the job of the coil is to cancel out the capacitance of the, leaving only the resistive portion of the antenna's feedpoint impedance.

For a full-sized dipole or vertical, this resistance is "close enough" to 50 Ohms (perhaps 35-70 Ohms, depending on the antenna and its environment) to provide a decent load to a modern radio - even one without tuner.  But a very small antenna - where a lot more "coil" is required - will have a lower feedpoint resistance unless your coil is very lossy, as was the case with the stainless steel coils on the JPC-7.  With the lower-loss silver plated coil we (mostly) eliminate it as a lossy component - but end up with a different problem.

Figure 4:
Inside the LDG Z-11 antenna tuner.  The center
toroid shows evidence of have been heated,
apparently due to matching very low "R".
Click on the image for a larger version.

With a feedpoint resistance of 13-15 Ohms on 40 meters with the JPC-7 and silver plated coil and its resulting 3-ish:1 VSWR one can "fix" this with an antenna tuner to make the radio happy - and I have done this many times, placing the tuner (an LDG Z-11 Pro) right at the antenna (only a few feet/a meter of coax) but almost all common antenna tuners have quite high losses at these low impedances.

Testing with the cover of the tuner removed, I have noted that one ore more of its toroids in particular will run very warm with just 100 watts of power - Figure 4 shows the inside of this tuner showing one of its toroids discolored because of this.  Fortunately, iron-powder toroids are very forgiving of heating with very high Curie temperatures and other than cosmetic (e.g. discoloring the paint) moderate heating won't have any lasting effects as long as it remains intact (e.g. not cracked) and there aren't problems with (possibly-degraded) insulation between turns of the windings.

The other issue is that the balun originally supplied with the JPC-7 - intended for 50 Ohm operation - also got very warm, and after a bit more than a minute of continuous 100 watts at 40 meters the VSWR would start to rise due to its ferrite reaching the Curie temperature, causing the permeability to drop like a rock:  Essentially, the ferrite would "go away" when it got hot - likely not a problem on SSB or CW, but it might be on "key down" digital modes at full power.  This heating seemed to be more severe at the low impedances (below 20 Ohms) than at 50 Ohms.

Eliminating the tuner

By definition, we can remove the reactive component of the short antenna with the loading coil:  Its inductance will cancel out the capacitance of the antenna at resonance (which is the very definition of resonance) leaving only a pure resistance.  While an antenna tuner is able to cancel out capacitive and inductive reactance - or just pure resistance - we have a situation where, with a properly-tuned loading coil - we have only resistance and for that we don't need a tuner and we can use just a transformer, to change the impedance from whatever it is to 50 Ohms.

An easy way to do this is with an autotransformer.  This is a device with just one winding and in this case - where we are trying to tune to a feedpoint resistance lower than 50 Ohms - we can feed our power across the ends of the entire coil and tap it at various points along the winding to get our desired (lower) impedance.  For my application, having several taps between about 10 and 40 Ohms (plus the natural 50 Ohm feed impedance) would assure the ability to attain a VSWR of better than 1.5:1 for any purely resistive impedance between 7 and 75 Ohms.

The tyranny of the "electrically small antenna" and efficiency

It's worth noting several things about electrically-small low-band HF antennas - which includes portable antennas like the JPC-7, JPC-12 as well as mobile antennas - and how they interact with common antenna tuners (which an autotransformer is not):

• Any efficient, electrically-small vertical antenna will have a very low impedance once it is resonated:  For example, a "perfect", loss-less 1.5 meter (4.9 foot) long vertical antenna system on 40 meters would have a radiation resistance of about half an Ohm.
• Without losses due to the coils and stainless-steel telescoping rods, etc., the feedpoint resistance of the JPC-7 would, at 40 meters, be in the vicinity of 3-5 Ohms, depending on how many screw-together sections are used (e.g. the longer, the higher).
  
• Any automatic (or manual) antenna tuner that you are likely to ever use for portable operation will have rather poor efficiency when trying to match at lower than 20 Ohms or so - which translates to heat as demonstrated in Figure 4.

These facts - among others - conspire against having a small, efficient mobile antenna for the lower HF bands (e.g. 80-40 meters).  In the real world, losses (coil, antenna wire, ground) will conspire to make the feedpoint impedance much higher than the "less than an Ohm" that the would theoretically be - and any difference between the feedpoint resistance at resonance and the predicted radiation resistance is where most of the power in such an antenna system is lost:  In a typical antenna of this sort, the vast majority of transmitted power is lost in heat rather than radiated.

With significant efforts, it may be practical to get the losses of such an antenna system (which includes not just the antenna, but the series matching coil and ground losses an other factors) down to about 10 Ohms - still far above the 0.5-5 Ohms of our "perfect" antennas in the examples above - but as we know, physics conspires against us as trying to force-feed such an antenna with a tuner will probably put it into the impedance range where it is very inefficient.

It's worth noting that many simple and inexpensive mobile antennas achieve at least part of their "matching" to 50 Ohms simply by being lossy as depicted in Figure 3:  Most of the power is simply burned up in the coil.  This method is convenient in that it simplifies the problem with matching and is often accompanied by much wider tuning bandwidth (reducing the need to frequently re-tune when one changes frequency) than with our hypothetical "high efficiency" antenna, but the trade-off is poor efficiency.

Auto transformer for impedance matching 

Another way to handle this is to simply transform (pun intended!) the impedance downwards from 50 Ohms - and one way that this could be done is with a transformer of some type - and the simplest of these is one with a single winding with one or more taps, called an auto transformer.  Such transformers are commonly used to match a random wire (9:1 matching to about 450 Ohms) and for end-fed half-wave antennas (49:1 matching to about 2450 Ohms) - but we can also efficiently transform the impedance downwards.  By designing appropriately, this transformer can be made to be very efficient.

Commercial RF Auto Transformers:  The Atlas MT-1

It would seem that the use of an auto transformer for matching a low-impedance antenna - such as a low-band mobile antenna on a vehicle - used to be more common decades ago, but has fallen out of favor, possibly due to the easy and cheap availability of automatic antenna tuners:  Devices that do this function include the Atlas MT-1 (see Figure 5) and the Swan MMBX (described later), both of which have a number of low-impedance taps. 

Figure 5:
The Atlas MT-1 autotransformer,  The variety of
taps available provide the possibility of achieving a 1.5:1
match to any resistive loads between 9 and 75 Ohms.
Click on the image for a larger version.

My initial thought was to use a ferrite toroid as the core for the auto transformer.  As a general rule of thumb, a transformer should ideally have an inductive reactance of about ten times that of the operating impedance at the lowest frequency (e.g. 500 Ohms for a 50 Ohm system) but, in a pinch, just three times the operating impedance (e.g. 150 Ohms for a 50 Ohm system) was "OK".  With this in mind I wound 7 turns on an FT140-43 toroid with multiple taps.  The inductance of this arrangement was about 45uH which correlates with about 1900 Ohms at 7 MHz - well above the target inductive reactance but it would have been difficult to achieve the multiple taps needed to attain the impedance steps with fewer turns.

This transformer - wound on ferrite - did not work well at all!  When testing it on the antenna, I could not achieve a sensible match and I quickly realized that the problem was due to leakage inductance of the transformer itself.  An ideal transformer would simply transform the voltage according to the tap's turns ratio, but any practical transformer will place some amount of inductance in series with the supposedly ideal tap, and it was likely this spurious series inductance (which needed only to be a few uH to make it "un-matchable") was totally messing up the attempt to tune the antenna, departing far from the ideal transformer at RF.

Measuring the self-inductance of the Atlas MT-1 confirmed this:  Its end-to-end inductance was about 2uH and the inductances between the taps and ground - the results of these measurements made using my HP-4275A LCR Meter (at 4 and 10 MHz - interpolated at 7 MHz) are as follows:

Tap marking (Ohms)	@ 4 MHz Inductance uH (XL Ohms)	@ 7 MHz (Interpolated) Inductance uH (XL Ohms)	@ 10 MHz Inductance uH (XL Ohms)
52	1.87uH (46.6)	1.8uH (79)	1.74uH (116)
23	0.95uH (24.4)	0.95uH (41.8)	0.95uH (61)
18	0.77uH (18.1)	0.75uH (33)	0.72uH (47)
13	0.61uH (14.3)	0.57uH (25)	0.53uH (35.8)

Figure 6:
The impedances (X_L ) of the taps on the Atlas MT-1 auto transformer versus frequency.

While "about 2uH" of inductance at 40 meters (7 MHz) doesn't fit the "3x reactance" rule-of-thumb (e.g. 79 Ohms X_L in a 50 Ohm system) it will still work OK, acting as a parallel inductance across the antenna - but the important part is that there will be a fraction of the leakage inductance compared to the version with the ferrite core mentioned above:  A small amount of this inductance would lower the resonance frequency slightly, but not disastrously so.

Figure 7:
A close-up view of the MMBX matcher.
Top:  The front panel of the MMBX.
Bottom:  Inside the MMBX showing the
tapped transformer and (underneath)
the ceramic switch.
Click on the image for a larger version.

The Swan/Cubic MMBX RF Auto Transformer

A similar device is the Swan/Cubic MMBX which operates on the same principle, but uses a 7-position rotary switch to select the output impedance rather than using banana plugs like the Atlas MT-1.  Depending on the vintage, the switch is labeled with letters as A-G or as impedance levels of 50, 38, 28, 19, 12, 7 and 3 Ohms, respectively.

As seen in Figure 7, the MMBX consists of a tapped inductor wound on a pair of stacked toroidal cores (each being 1.125" O.D., 0.75" I.D. and 0.375" tall).  I've seen several of these MMBXs over the years and their construction seemed to vary slightly in the case of this unit, it's technically not an auto transformer:  

Careful inspection of Figure 7 will reveal that there are two windings - the red one connecting to the 50 Ohm side (left-hand side on the lower portion) consisting of eight turns while the other winding (the tinned wire) also consists of eight turns in parallel with taps connected to a good-quality ceramic rotary switch on every other turn.  I have seen examples of this same model where there is just a single, tapped winding which would be easier to construct and likely to be slightly more efficient (e.g. somewhat tighter coupling than one might get with two, separate windings) - but functionally identical.  In other words, this could have been built as a conventional auto transformer with one winding, so for the purpose of discussion, we'll do so.

Because of the rather low inductance value (see Figure 8) it is likely that the toroidal cores are not typical Amidon cores familiar to most amateurs due to the fact that not only are the not a standard size from their catalog.  A bit of calculation indicates that they are approximately equal to a hypothetical FT125-61 ferrite core - likely a material from a different manufacture with slightly lower permeability than the modern "61" ferrite material.

I was curious as to the inductance values used by the MMBX, and a quick test showed to be as follows:

Switch setting (approx Ohms)	Nominal Inductance	XL@4MHz	XL@7MHz	XL@10MHz
"A" (50)	5.8uH	146 Ohms	255 Ohms	364 Ohms
"B" (38)	4.5uH	113 Ohms	198 Ohms	283 Ohms
"C" (28)	3.4uH	84 Ohms	148 Ohms	211 Ohms
"D" (19)	2.4uH	60 Ohms	105 Ohms	151 Ohms
"E" (12)	1.6uH	40 Ohms	70 Ohms	100 Ohms
"F" (7)	0.95uH	24 Ohms	42 Ohms	60 Ohms
"G" (3)	0.45uH	11 Ohms	20 Ohms	28 Ohms

 Figure 8:
The inductance values and calculated impedances (X_L ) of the taps on the Cubic/Swan MMBX at various frequencies.

Figure 9:
The auto-transformer, wound on a T157-2 iron-powder
toroid with taps terminated with 2.5mm banana plugs.
Click on the image for a larger version.

Comparing the tables for Figure 6 (for the Atlas MT-1) and Figure 8 (for the MMBX) we note several things:  The inductance of the MMBX is quite a bit higher than that of the MT-1 - and at the lowest frequency of 4 MHz (close enough to 80 meters) it more or less meets the "3x rule-of-thumb" design suggestion where the inductive reactance (X_L) of the entire tapped coils (e.g. auto transformer) is three times that of the operating impedance.

 

The other thing that is worth noting is that whereas the MT-1 had four impedance taps (including 50 ohms), the MMBX has seven taps.  In reality, the various impedances of the MT-1 are, as noted earlier, able to match anything between 7 and 75 Ohms (resistive) to an SWR of 1.5:1 or better, the larger number of taps on the MMBX means that one could  likely find a better match than 1.5:1 between its rated 3-50 ohms and possibly 1.5:1 down to 2 Ohms presuming that the antenna itself was tuned to be resonant (e.g. reactance tuned out).

 

Replicating the auto transformer

Rather than reinventing the wheel, I decided to (more or less) replicate the electrical properties of the MT-1 (and similar devices) and for this I chose a T157-2 Iron-powder toroid.  With a target inductance of "about" 2uH I wound 13 turns of 16AWG silver-plated PTFE (Teflon) insulated wire which should, in theory yield about 2.4uH - but when compressed together on the core it yielded about 3.6uH which correlates with about 158 Ohm at 7 MHz -  almost exactly 3x the 50 Ohm system impedance.

As can be seen in Figure 9, taps were placed at 6, 7, 8, 9 and 11 turns (from ground) by scraping the insulation off the side if the wire and tack-soldering wires to it providing impedance taps of approximately 11, 14, 19, 24, 36 Ohms - plus another wire across the 50 Ohm feed for the higher bands:  These impedances resulted from where the turns landed and it was convenient to attach taps rather than from any attempts to obtain specific or precise impedances:  After construction, I labeled the leads with the approximate impedances - for obvious reasons!

I used five taps to allow a selection of an impedance to be able to obtain about 1.25:1 VSWR or better, but if I were happy with just 1.5:1, I could have chosen fewer taps in the manner of the Atlas MT-1 discussed, above.

As the impedance of a tap is related to square relation of the number of turns (e.g. twice the number of turns results in 4x the impedance) there's a pretty simple formula to follow to calculate the impedance of a tap:

Z_tap = (Z_sys) / ((Turns_total/Turns_tap)²)

Where:

Z_tap = Impedance of the autotransformer tap

Z_sys = System impedance (typically 50 Ohms)

Turns_total = Total number of turns on the autotransformer (13 turns in our example)

Turns_tap = Number of turns from the bottom (ground) end of the autotransformer to the tap

In other words:

 Z_tap = (50) / ((Turns_total/Turns_tap)²)

Taking our 13 turn autotransformer as an example, we can calculate the impedance at any turn.  Taking the 8th turn as an example:

Z_tap = (50) / ((13/8)²)  therefore,

Z_tap = 18.9 Ohms 

Or, if you know the desired target impedance and want to calculate the turn on which to make that tap, here's the above formula rewritten to solve for it:

Turns_tap = Turns_total / √(Z_sys /Z_tap)

I also included a "50 Ohm" tap (which is connected at the "top" of the transformer, across all of the windings) so that I could still use the common-mode choke (described below) even when operating on the higher bands (20 meters and above) where the natural impedance was close enough to 50 Ohms that I probably wouldn't have needed the autotransformer for impedance transformation, anyway.

At the end of the flying leads are 2.5mm "banana" plugs - which plug in to the feedpoint of the JPC-7.  These allow the selection of taps on the auto transformer which permits the VSWR to be minimized for those bands for which the feedpoint impedance is significantly lower than 50 Ohms:  A bit of care is required to prevent the "floating" banana plugs from touching each other (or anything else metal) but this isn't actually much of a problem.

Initial testing using a kludge of clip leads, I verified with my NanoVNA that the auto transformer worked as it should (e.g. I was able to attain less than 1.5:1 VSWR on 60, 40 and 30 meters) and almost as important, the tuning with the auto transformer was only slightly different from that using the original balun indicating that the leakage inductance of the auto transformer was not much different than that of the originally-supplied balun.

Adding a common-mode choke

Feeding a dipole (which is a balanced antenna) with coaxial cable has the inherent hazard of RF appearing on the coaxial cable feedline due to the symmetry of the antenna.  Excessive RF on the feedline can result in a "hot" rig - that is, RF energy appearing on the chassis of the radio as well which can result in distortion (RF getting into the microphone) and/or malfunction of peripherals (outboard keyer malfunctioning, USB interfaces crashing, interference to the sound card) and out "in the field" where one may not have an elaborate ground system already, this may be more likely than at home.

Figure 10:
The auto transformer (left) plus a common-mode coaxial
choke (right).  The choke is wound on an FT140-43 ferrite
toroid.  Both toroids are in the foreground for comparison.
Click on the image for a larger version.

The "input" to the auto transformer is simply the opposite ends of its 13 turn winding which would normally be soldered to an RF connector.  Rather than doing that, I soldered it to a 36" (91cm) piece of RG-316 PTFE coaxial cable - the shield going to the "bottom" (ground) side of the auto transformer, insulating the connections with adhesive-lined heat-shrink tubing.  The rest of this RG-316 was wound on an FT140-43 toroid yielding 13 turns using the "cross-over" technique where about half of the turns are wound on the opposite side of the toroid:  This method is said to (slightly) increase the series choking impedance at higher frequencies (e.g. 15 meters and up).

Not having a UHF connector designed for RG-316 on hand, I used a crimp-type PL-259 intended for RG-58.  I stripped more than usual of the jacket from the end of the coax, folding the shield over the outer sheath.  Using some PTFE tubing and part of the jacket stripped from the coax itself I was able to increase the effective diameter of the inner dielectric.  Assembling the cable - remembering to include the ferrule and pieces of adhesive-lined heat shrink - I was able to fold the outer shield over the ferrule after a bit of tugging on it to increase its inner diameter.  At that point, I was able to crimp the ferrule into place, securing the coaxial cable firmly.

Figure 11:
The auto transformer with the common
mode choke on the JPC-7's feed.
Click on the image for a larger version.

Since RG-316 is fairly small (it's the same size as RG-174) - and because the weight of the connecting coaxial cable and the common-mode choke itself would be hanging from the cable - I protected the connector with several pieces of adhesive-lined shrink tubing - using a smaller piece just behind the connector to increase its outside diameter and then a larger piece over the ferrule, onto the previous piece of tubing.

 

Not content with this, I wound several turns of "miniature" paracord (1.15mm diameter) onto the ferrule and tied it securely, feeding both free ends underneath yet another piece of heat-shrink tubing that was then installed over where I'd tied the paracord - taking careful care not to damage the cord when applying heat to shrink it.

These two strands of mini-paracord were then counter-wound over the RG-316 as can be seen in Figures 8 and 9 and were tied to the ferrite core of the common-mode choke such that when hanging, the weight of the connector was on the cord and not the coaxial cable:  I did a similar thing between the core of the auto transformer and the balun to prevent the cable itself from being pulled.

Putting it on the antenna

Figures 10 and 11 shows the combination auto transformer and common-mode choke at the feedpoint of the JPC-7 loaded vertical.  As noted earlier, testing showed only a slight difference in tuning between the lowest VSWR achieved with the original 1:1 balun and the transformer-choke combination indicating that its effect was minimal:  As figure 12 shows, transmitting 100 watts on 40 meters also resulted in only very slight heating of the auto transformer - certainly a much lower amount of signal loss than that which resulted in the heating and discoloring of the toroid in the antenna tuner pictured in Figure 4.

Figure 12:
Thermal infrared view of the autotransformer
(top) and common-mode choke (bottom)
after 60 seconds key-down with 100 watts
on 40 meters.  The temperature of the
autotransformer increased only by about 2F
(1C) while the common-mode choke got about
10F (6C) warmer.
Click for a slightly larger version.

Testing the common-mode choke

 

The efficacy of the common-mode coaxial choke was also verified:  Without it, grasping the shield of the coaxial cable with one's hand would result in slight detuning of the antenna, but with it, there was no detectable effect - and there was no detectable amount of "hot rig" due to the presence of common-mode currents flowing beyond the choke and onto the radio's chassis - even without the use of a counterpoise/ground wire.

 

The presence or lack of effect of the change of antenna tuning when body capacitance is introduced is a simple - but effective - means of determining the presence of RF current on the feedline at the point where it is grasped.  Figure 12 shows that this core heated only minimally - also indicative of low loss.

Does it work?

I have put this configuration pictured in Figure 11 on the air several times since assembling it on 60 through 15 meters.  As expected, the best match on 60 meters (<1.5:1) required the 11 Ohm tap while 40 meters seemed fine with either the 11 or 14 Ohm tap.  20 meters, on the other hand, found the best match using the 36 Ohm tap while 15 meters worked well with either this or the 50 Ohm tap.  Again, the heating of the autotransformer at 100 watts was also minimal on any band - even on the 60 and 40 meters where the losses would probably have been the highest.

Conclusion

The use of an autotransformer rather than an L/C antenna tuner is a time-honored means of matching an "electrically-short" antenna, so what has been presented is nothing new - but it may be "new" to some of the readers.  For a portable antenna such as this, its size and relative simplicity can't be beat as it's far smaller than any antenna tuner that could handle 100 watts at the low impedances that may be presented - and it's certainly lower loss as well!

The only "complication" is that which is already intrinsic to this type of antenna:  As this is a dipole, there are two elements - each with its own coil and telescoping rod making it a bit "fiddly" to tune, something best done with a VNA or antenna analyzer.  With this antenna I keep a card that is marked with the physical locations of the tap positions of the two coils for the various bands:  These are held up to the coil and the sliders adjusted, quickly getting "close" to a match with the analyzer used to do any final touch-ups on the tuning.

* * * * * * *

Related pages:

• For more information about the "Silver-plated versus Stainless Steel" topic, see the blog entry "Rewinding the Stainless Steel coils with Silver-Plated copper wire on the JPC-7 and JPC-12 antennas" - link.
• Analysis, observations about the JPC-7 loaded dipole - link.
• Analysis, observations about the JPC-7 portable vertical - link.
• Reducing RF susceptibility for the HamGadgets "Ultra Pico Keyer" - and mimizing RF issues on portable HF stations in general - link.

* * * * * * *

This page stolen from ka7oei.com

[END]

 

Observations, analysis and modifications of the JPC-12 vertical antenna

The JPC-12 antenna (possibly designed by BD7JPC) is relatively inexpensive a portable vertical antenna - made in China, of course - that may be found for sale at quite a few places  under a few different brand names (including "Chelegance"). The price varies very widely - sometimes well over $200 - but I got mine via AliExpress for about $120, shipped, about a year and a half ago.

Comments: 

I analyzed the JPC-7 loaded dipole antenna - which is made by the same company and uses many of the same components - and reported on it in previous article, and you may find that discussion HERE.

Silver-plated coils:  Since this article was posted I have added an article describing the effects of the stainless-steel coils and how to rewind with silver-plated copper, found HERE.

Figure 1:
All of the standard components of the JPC-12 kit.
Click on the image for a larger version.

As for a vertical antenna, there are only so many variations on a theme.  The JPC-12 is intended to be used as resonant vertical, and with its included coil, it is capable of operation down to 40 meters - but it can be operated sans loading coil at higher bands, adjusting the length of the telescoping section to resonance.

"The perfect is the enemy of the good"

The above statement should be kept in mind when doing any temporary, portable installation.  The idea is to have an antenna that will work "well enough" to do the job.  It's also likely that in the situation where you are portable, you will not (and cannot!) spend an inordinate amount of time tweaking things to eke out the last decibel.

This is not to say that one should not be mindful of good practices as too much corner-cutting can excessively impact performance and potentially replacing enjoyment with frustration.  One should achieve a balance between that which works well and something that will allow more operation than fussing about.

Remember:  The more time you spend trying to get that last bit of performance out of your system is less time that you are spending operating - and I'm presuming that operating is your goal.

What is included with the JPC-12

As shipped and as depicted in Figure 1, the antenna comes with these components:

• Aluminum ground stake.  This is a pointed stake 9-5/8" (24.5cm) long end-to-end about 1/2" (1.3cm) diameter.  This stake has M10-1.5 (coarse) threads on the end - the same as all other male and female threads used on the other mast/coil components of this antenna.
  

This stake is intended to be pushed into the ground and be capable of holding it vertically - which works fine in fairly compact soil, but it may be inadequate for looser soil or sand requiring, instead, a longer stake or a bit of guying.  Faced with the situation where putting the stake in the ground was not an option (the soil was way too rocky) I have also clamped it to existing supports, such as a metal "T" stake using locking pliers.

 

While the antenna is ostensibly "grounded" with the stake, solely stabbing metal into lossy dirt is never going to result in an effective vertical antenna so its being "grounded" by the stake is incidental and not important to its overall performance:  It's going to be the system of radial and/or counterpoise wires that you set up that will form other "half" of any 1/4 wave vertical (which is really a form of dipole with the other half "mirrored" by the ground plane) antenna to make it work effectively.

• Four aluminum mast sections.  These are hollow tubes with (pressed in?) screw fittings on the ends - one male and the other female, both with M10-1.5 coarse threads that may be assembled piece-by-piece into a mast/extension.  End-to-end these measure 13-3/16" (33.5cm) each, including the protruding screw - 12-3/4" (32.4cm) from flat to flat and are 3/4" (1.9cm) diameter.

  

  Figure 2:
  The feedpoint for the JPC-12.  The upper half (right)
  is insulating while the bottom portion is machined aluminum.
  Click on the image for a larger version.
  

• Feedpoint assembly.  This has a (correctly-machined!) SO-239 (female UHF connector) - the shield of which connects to the bottom half while the top - which is isolated by a section of fiberglass tubing - is connected to the center pin.  On both ends are female M10-1.5 threads to receive the screw from the ground stake (on the bottom) and the "hot" portion of the antenna on top.  This piece appears to be well built and is 6-9/16" (16.7cm) long.

• Adjustable coil.  This is a piece of what appears to be thermoplastic or possibly nylon with molded grooves for the wire.  This unit is connected to the others via a male threaded stud on the bottom and female threads on the top, both being M10-1.5 like everything else.

Figure 3: 
The adjustable resonator coil, wound with 1mm stainless-
steel wire.  (The markings are mine.)
Click on the image for a larger version.

The form itself is 4-1/2" (11.4cm) long not including the stud and 1-11/16" (4.3cm) diameter - wound with 34 turns of 18 AWG (1mm) stainless steel wire.  The coil has an inside diameter of approximately 1.66" (4.21cm) over a length of about 2.725" (6.92cm) and it has a slider with a notched spring that makes contact with the coil and this moves along a stainless steel rod about 0.12" (3mm) diameter that is insulated at the top, meaning that as the slider is moved down, the inductance of the coil is increased.

 

The coil has painted markings indicating "approximate" locations of the tap for both 20 and 40 meters when the telescoping section is adjusted as described in the manual using the four (originally) supplied mast sections.  The maximum inductance is a bit over 20uH and the DC resistance of the entire coil (with its original stainless steel wire) is about 4 ohms - more on this later - see comments on rewinding with silver-plated copper wire.

• Telescoping section.  This is a stainless steel telescoping rod that is 13-1/8" (33.4cm) long including the threaded stud (12-7/8" or 32.7cm without) when collapsed and 99-11/16" (8' 3-11/16" or 253.2cm) when fully extended - not including the stud.
  

As with all stainless-steel telescoping whips, you MUST maintain them - keeping them clean and lubricated:  More on this later in this article.

Figure 4:
This is the supplied "radial" kit - a 10-strand chunk of ribbon
cable - the ring to be sandwiched on the bottom of the feed.
Click on the image for a larger version.

• Counterpoise/Radial cable.  This is in the form of a chunk of 10 conductor ribbon cable terminated with large (0.4", 1cm I.D.) ring lug on one end sized to fit over the M10-1.5 threads.  This cable is about 203" (16' 11" or 516cm) long, including the ring lug that is intended to be sandwiched between the bottom of the coil and the ground stake. As noted later in this article, this radial isn't as useful/convenient/versatile as one might initially think.
  

• Padded carrying case.  This zippered case is about 14" x 9" (35.5x23cm) with elastic loops to retain the above antenna components and a zippered "net" pocket to contain the counterpoise/radial cable kit and the instructions.  There is ample room in this case to add additional components such as small-diameter coaxial cable - and enhancements to the antenna, as discussed below. 
  

• Instruction manual.  The instructions included with this antenna are marginally better than typical "Chinese English" - apparently produced with the help of an online translator rather than someone with intimate knowledge of the English language.  The result in a combination of head-scratching, laughter and frustration when trying to make sense of them.  Additionally, the instructions that came with my antenna included those for the JPC-7 loaded dipole as well, printed on the obverse side of the manual.

Fully assembled with the originally-supplied components, the length of the antenna is about 13' 5" (411cm) not including the ground spike meaning that it is self resonant - without added inductance - at a bit below the 17 meter band.  This means that at 17 meters and above, the tuning can be done solely with adjustment of the telescoping section and the coil can likely be omitted entirely.  Below 17 meters additional inductance is required which is obtained by moving the slider of the antenna downwards, requiring all but the last 3-4 turns of the coil to obtain resonance on 40 meters.

Comments:

Build quality

I'm quite pleased about the overall build quality:  The design seems to be well thought-out, perhaps inspired by other (similar) products on the market.  The individual mast sections seem to be plenty strong and I've seen no indication of the end sections coming loose.  I have screwed eight of these sections end-to-end and held them horizontal and noticed very little drooping and no "permanent" bends.

The feedpoint - being a combination of aluminum and plastic - seems to be well-built, the bottom section being machined with a flat to accept the SO-239 connector.  The upper section appears to be fiberglass, threaded at the top to accept an aluminum plug into which female threads are tapped to accept threads of the mast sections.

Likewise, the coil itself seems to be well built, the 32 turns of wire set into a spiral groove molded into the body with the coil tap selection having firm, positive action.  As noted previously, the wire comprising the coil is, itself, about 1mm diameter (approximately 18 AWG) and is apparently austenitic (e.g.  non-magnetic) stainless steel.

While this wire is very rugged, the fact that it is stainless means that its resistance is quite high compared to copper - in this case the end-to-end DC resistance is about 4 ohms - but the RF resistance, taking the "skin effect" into account, is likely to be very much higher.

Using Owen Duffy's online skin effect calculator (link to archived page) and assuming 1mm diameter, 316 Stainless, the 4 ohms of DC resistance translate as follows to RF resistance including skin effect:

• 3.5 MHz = 5.2 ohms
• 7 MHz = 7.2 ohms
• 14 MHz = 9.6 ohms
• 28 MHz = 13.6 ohms

While these values would be for the entire coil remember that less than full inductance is typically used  - but the message is clear:  The fewer turns of coil you need to use, the lower the loss!   The total length of 1mm wire is estimated to be about 180 inches (457cm).  By comparison, copper wire of this same diameter and length would have a DC resistance of about 0.1 ohm - or a skin effective resistance of 2 ohms at 28 MHz.  Alternatives using silver-plated copper wire will be discussed later.

Using the supplied radials - or not!

Noted in most reviews is the nature of the included radial/counterpoise wire - particularly since there is little or no mention of how it is to be used in the included manual.  Clearly, the single ring lug is intended to be captured between the bottom of the feedpoint section with the SO-239 connector and the ground stake.

For use as a resonant radial, the length of the this cable (203" or 516cm) is approximately correct for 1/4 wavelength at 20 meters, but this is not really suitable for 40 meters.  For best efficacy, the radials should be elevated above the ground by about a foot (25cm) or so so that the 1/4 wave impedance transformation (e.g. the distal end of the radial being open being transformed to a "short" at the antenna end to make it work effectively) but laying it on the ground directly - particularly if it is dry, sandy soil - will usually work ok.

Being 10 conductor ribbon cable, the opportunity exists to split the wire lengthwise to obtain individual wires to spread radially around the base of the antenna but it is best to NOT split the 10 conductors individually, but rather have them consist of two or three wires each to make them easier to handle.  This wire - with its PVC insulation and rather small gauge conductor (likely 26 AWG) means that it is difficult for it to lay flat unless it is warmed by the sun on hot ground (or with rocks laid on the wire) - plus a large number of connected-together conductors from a split-apart ribbon cable are the makings of a portable rats-nest of wires that cannot easily wound/unwound later.

Most reviewers/users of this antenna - including myself - don't really like the "ribbon cable radial" system and personally, I have never used it - but I keep it in the kit, just in case.  

Location of the loading coil

While it might be tempting to place the loading coil immediately above the feedpoint, this is not the suggested location, but rather at the top of the four supplied screw-together mast sections immediately below the telescoping section.  This makes sense on several counts:

• This elevates the coil above the ground, making it easier to adjust as it is at more convenient height (about 4' 3" or 130cm above ground).
  

• Because it is the portions of the antenna that conduct the most RF current are those that will radiate the most, those same sections below the coil - and above the connection to the counterpoise - will emit the bulk of RF energy
  

• The markings on the coil for 40 and 20 meters assume that you have placed the loading coil in the location described above using the original components in the kit.
  

From a practical standpoint, placing the loading coil immediately above the feedpoint will also work - albeit with some loss of efficiency - and this may be desirable if the base of the antenna (and radials) itself is elevated - perhaps by being clamping it to a fence post or table.  In this case one might place the coil closer to the feed point to keep it at a reasonable (accessible) height rather than needing to access the coil's tuning slider by standing on a ladder or chair. 

Augmenting/improving the JPC-12 with optional accessories

A bit of perusal among the goods of the various sellers of the JPC-12 (and the related JPC-7 dipole) will reveal that spare parts:  It's a pretty good idea that - if you find that you are using this antenna a lot - to get a few "extra" parts (I strongly suggest an extra telescoping section or two) when things inevitably get worn out or broken.  There are also several "accessories" that may be used with the antenna(s) that might be useful - some of which are discussed below - and other components that you can easily assemble and add to the kit.

Improved ground radial system

For a 1/4 wave vertical - and this antenna is exactly that, albeit electrically lengthened with a coil and tophad on the lower bands - half of the antenna is its mirror reflection from the ground.  As it is unlikely that most people will ever set up their antenna atop a metal surface or in salt water, a set of wires is typically deployed to locally simulate the needed reflective "ground" surface.

The common advice in years past has been to bury many, many ground radials just below the surface of the (lossy!) ground - advice that is practical in terms of avoiding trip-hazards and to provide a degree of lightning protection - but equal or better performance may be had by deploying an array of radials that are odd-order quarter-wave multiples (1/4, 3/4, 5/4, etc.) that are elevated slightly above the ground.  Emperical testing (see the linked article below) that as few as three or four elevated, resonant radials can be even more effective for the bands on which they are tuned than a mat of many buried radials - and this number of radials is perfectly manageable in a portable installation.

The accessories described below make it easier to quickly deploy a resonant ground radial system - elevated or not.

The ground radial plate 

Figure 5:
This is the "radial plate" - an add-on accessory.  Spade-lug
terminated radial wires connect easily under the wing nuts.
Click on the image for a larger version.

As shipped, the radial kit (ribbon cable) included with JPC-12 antenna is perfectly usable - but in the opinion of many (including myself) the supplied radials aren't particularly practical or convenient.  From the same seller as the antenna I purchased what is cryptically called a "JPC-12 PAC-12 Network Disk" - seen in Figure 5.

What this really is is an aluminum disk about 4-3/4" (12cm) in diameter with a series of eight wingnuts and screws around the perimeter with a center hole sized appropriate for the M10 stud on the top of the ground rod or one of the antenna elements.  This device makes the connection of individual ground radials equipped with spade lugs much more convenient.

In looking at Figure 5, you may have realized that it's sitting atop its protective pouch to keep the screws from tearing up the inside of the carrying case:  The rear pocket removed from an old pair of blue jeans!

Update:  Thijs, PE1RLN wrote to describe another way to easily connect radials.  Look at the bottom of this article for that information.

Using individual wires for the radials

Figure 6:
Four radials on kite string winders - each long enough for 60
meters - with markers on the wires for the different bands.
Click on the image for a larger version.

Rather than using the original ribbon cable, I have four lengths of 22 AWG hookup wire on kite string cable winders (a pack of ten cost US$10 from Amazon - including the string!)  These four wires are terminated with spade lugs to slide under the screws on this pate and are each 44' (13.4 meters) long corresponding with the quarter-wavelength at 60 meters.  This length of 22 AWG insulated wire just barely fits on these winders:  24 AWG wire would fit better and work about as well as 22 AWG.

Marking the radials' lengths

At various points along the length of each of these wires are pieces of marked heat-shrink tubing to indicate the points corresponding to quarter-wavelengths of the various amateur bands from 60 through 10 meters and only as much wire as needed is unspooled from the cable winder to achieve the desired length for the intended band of operation:  These yellow tags can just be seen in Figure 6 among the wire on the winders.

For these marker tags I used heat-shrink tubing cartridges for my Brother label maker - but I could just have easily have written on  light-colored tubing with an indelible marker prior to shrinking them.  To keep these tags from sliding around I put a dab of "Shoe Goo" (rubber repair adhesive) on the wire and slid the tubing over it before applying heat, locking it into place with much greater tenacity than the compression of the tubing shrinkage alone:  Having used these radials in the field a quite a few times, I have yet to have one come loose.

Using elevated radials

From an operational standpoint, just three or four elevated, resonant radials will perform equally to or better to a large number of radials - resonant or not - buried in the ground.  The reason for this - alluded to earlier - is the fact that any open-ended conductor that is an odd multiple of a quarter-wavelength long (e.g. 1/4, 3/4, 5/4) will exhibit a low impedance on the opposite (antenna) end - which is exactly what we want.

Simply laying such a length on the "average" ground will tend to diminish this effect somewhat, but elevating it even a short distance above the ground will preserve it.  For more information and an analysis of vertical antennas with elevated radial systems see the article "A Closer Look at Vertical Antennas with Elevated Ground Systems"  by Rudy, N6LF - LINK.  It's worth noting the admonition of the author of this page to avoid the use of radials that are around 1/2 wavelength long and multiples thereof - likely for the reason that the nature of a free-space half-wavelength conductor is not to provide a low-impedance on their proximal end when the distal end is unterminated!

The obvious hazard of elevated radials is that of tripping - of you, the operator, others in the area, or animals, so it isn't necessarily practical in every situation.  If it is possible to control access to the area with the antenna - or raise the radial above the height of the average person for much of its length - then this is a good choice.

Figure 7:
Fiberglass driveway markers modified to mark/hold radials.
Click on the image for a larger version.

In my operation - typically out in isolated areas - I don't have much worry about tripping anyone other than myself so I obtained some 4' (1.2 meter) long fiberglass driveway marker stakes.  These bright-orange stakes are about 4" long each (122cm) each - much 1--  long to fit in the antenna case, so eight of them were cut to shorter lengths to allow them to fit in the case, yielding two pieces each - the bottom portion with the sharpened point cut to 11-3/4" (30cm) and the top portion cut to 13-3/8" (34cm).  To the bottom portion, I glued (again using "Shoe Goo" because, unlike epoxy or cyanoacrylate "super" glue, it remains flexible) a 2" (5cm) long of 8.5mm I.D. stainless steel "Capillary" tubing (found on Amazon) so that the two pieces could be assembled to a single (mostly) non-conductive post about 26" (66cm) long.

Eight of these two-piece posts allow the support of four elevated radials at two points along their length, the radial wire being wrapped once or twice around to form a friction fit to keep them from sliding down.  At the distal end, the remaining lump of wire still on the kite string winder is simply wrapped and hung over the post once a slight amount of tension is pulled on it.

Figure 7 also shows something else:  I made a drawstring bag (again, from an old pair of blue jeans) that keeps all of these post pieces together and it can accommodate some of the extra mast sections, all while fitting in the original padded antenna case.

Comment: 

The reader should be conscious of the fact that for the purposes of this discussion, we are talking about a temporary, portable antenna rather than a permanent installation.  In the case of the latter, a different approach (the deployment of many, many radials, perhaps buried) is reasonable - but for a temporary antenna - where less effort to erect and break down is desirable - the use of three or four elevated, resonant (e.g. 1/4 wavelength) radials will outperform the same number of radials laid atop the ground.  In either case, however, the antenna will be usable - and that's the entire point!

On-the-ground radials

The use of elevated radials is arguably most important on the lower frequencies of operation of this antenna - namely 40 and 30 meters - where efficiency of this "electrically small" antenna will suffer due to a number of factors, but maximizing the efficiency of the ground plane is one way to mitigate this.  In those cases where it is not practical to elevate the radials, the wires may simply be laid atop the ground.  

As these posts are intended for marking driveways they are bright orange, making them stand out, but near the top they have a piece of white reflective tape so that they will show up at night.  As I had this type of tape on hand I added a piece of this reflective to the bottom section as well - just below the stainless steel capillary tube - to make them even more visible - particularly if the bottom and top portions are used separately to mark where an on-the-ground radial might be run to warn against a possible trip hazard.

The use of a "Magic Carpet" (e.g. "Faraday Fabric")

There is no reason why one could not use the aforementioned conductive fabric as part of their ground plane - but you would probably have to construct an additional component to connect to the fabric and use it effectively.  For this, a piece of clean aluminum or copper plate laid atop the fabric - possibly weighed down with a rock - should provide a low-impedance connection to it.

While I do own some of this "Faraday Fabric" (obtained from Amazon) I have yet to try it with this antenna - and when I do, I plan to perform an "A/B" comparison.  I have since tested the Faraday Fabric - see below.  

As of the time of this writing I have yet to see a serious, scientific and well thought-out comparison between a simple radial field and the use of just the fabric:  Most of these comparisons simply demonstrate that it is possible to get a good antenna match while using the fabric - but as we all know, simply getting a match does not mean that the antenna will work:  After all, a dummy load has a great match!

I expect that - at least on the sort of desert ground that I'm likely to encounter - the radials will "win" the contest - although I still plan to do a comparison:  I suspect that using both the fabric and radials will offer decent results - even when tuned to a higher band for which the lengths of the radials are not expected to work (e.g. 20 or 10 meters with 40 meter radials.)

Addendum - brief testing with "Faraday Fabric":

I have done some cursory tests with two pieces of "Faraday Fabric" on 20 meters, laid out around the base of the antenna which were overlapped by a few inches/cm and rocks laid atop to make sure that they were electrically connected - all of this on dry, sandy soil.  The connection to the "ground" of the vertical itself was done using an aluminum plate connected to a wire - and the continuity between the antenna's "ground" and the pieces of fabric was verified using an ohmmeter.

The result was not particularly impressive.  Using four resonant elevated radials, the resonance of the antenna was distinct and sharp - but with just the "Faraday Fabric" there were a number of apparent resonance points - and worse, these shifted in frequency when I grasped the coaxial cable with my hand indicating that the feedline itself was doing some radiating and was participating in the "grounding" (e.g. being a counterpoise) and that it - and not the fabric - was a significant portion of the counterpoise:  This did not occur when using the elevated, resonant radials.  Again, this was done on dry, sandy soil (in a desert environment) but it may work better in a wetter environment.

I have yet to do more thorough testing of this antenna with the "Faraday Fabric" to compare its performance to one using resonant radials, but the fact that the resonance of the antenna was skewed and that the feedline seemed to be participating in the "ground" seems to indicate that the fabric, alone, is not equivalent to a proper, low-impedance radial system.

Additional antenna height - both real and "virtual"

Figure 8:
The accessory top had kit consisting of a machined piece that
attaches to the top of the vertical with four telescoping rods.
Click on the image for a larger version.

Any antenna that has to be electrically lengthened with inductance is likely to suffer from efficiency loss as that inductor is unlikely to be comparatively lossy.  As the antenna is mechanically "about" 1/4 wavelength on bands above 20 meters, it make sense, then, that the lower bands that it is intended to cover - particularly 30 and 40 meters - need some additional inductance to bring it to resonance.  It further follows that anything that may be done to make the antenna "taller" will reduce the amount of needed inductance and minimize these losses.

Tophat capacitance

Another accessory available for this antenna is a small tophat attachment for the telescoping vertical section.  Often described as a "PAC-12 Capacity Cap" this consists of what looks like a knurled aluminum knob with five holes drilled around its circumference and yet another hole on the bottom sized to receive the "static ball" (really a short cylinder) atop the telescoping section.

Using a set screw to secure it to the top of the antenna, this kit contains four small telescoping whips (3-1/8" 8 cm long collapsed, 12-1/4" 31c fully extended - not including threads) that screw into the 5/8" (2cm) diameter center disk.  Assembled, the end-to-end length of two of the telescoping elements is 25" (98.4cm) which forms a four-spoke "disk" that adds to the effective height of the main telescoping section of the antenna by increasing the capacitance.  The idea (and hope) is that this allows the reduction of the amount of inductance needed to bring the system to resonance - and it also allows potential coverage of 60 meters as noted later.

The size and weight of this attachment is, in my opinion, about right:  Any larger or heavier, it would likely be too much for the fully-extended main whip to handle and expose it to excessive wind loading.  To be sure, one must always be very careful when handling the whip when fully-extended, anyway and adding the tophat increases the risk of damage.  Even so, I've had the antenna up in 20+ MPH (32kPH) and despite a bit of swaying, experienced no damage.

Figure 9:
The top hat kit assembled - but the rods are not extended.
Click on the image for a larger version.

Testing has shown that the addition of the tophat - when the antenna has previously been tuned for 40 meters - lowers the resonant frequency by approximately 1 MHz indicating an increase of virtual height by about 12 percent at that frequency.  Even with the tophat the antenna falls short of being able to resonate at any 60 meter frequency with the normal complement of parts included with the antenna.

For the higher bands, the top-hat may be enough to eliminate the need for the coil on 20 meters - or at least greatly reduce the amount of coil and thus the potential loss.

Of course, the use of this top hat means that the existing coil marking scheme (e.g. the paint marks that show approximate slider position for the bands) is meaningless as tuning is changed - but if one is already prepared in the field for this (e.g. using an antenna analyzer, added markings to the coil for the new configuration, a paper template marked with pre-determined coil positions) then this is of little consequence.

This tophat kit is constructed fairly well, using a small grub screw with a supplied Allen key to attach it to the top of the telescoping section, but I noted that the key and the screw weren't well matched and couldn't be tightened too much with the key slipping.  Rummaging about in my collection of hardware I found several metric machine screws and a hexagonal brass stand-off with matching threads and I replaced the grub screw with the stand-off, allowing it to be attached firmly to the top of the telescoping section using just my finger:  Had I not found the stand-off, I would have rummage around in my parts bin - or gone to the hardware store - to find a metric screw to fit it.

Additional mast sections

It should not be surprising to know that you can buy individual mast sections.  These are often described as being "dedicated lengthened vibrator for JPC-7 (PAC-12) multiband portable antenna".  I purchased two more of these sections when I first purchased the antenna, increasing its fully erect height from 13' 5" (411cm) to 15' 7-5/16" (476cm) and coupled with the tophat and the full inductance of the coil allows the antenna itself to resonate at approximately 4.7 MHz, allowing complete coverage of the 60 meter band.  Having used it for SSB communications on this band a number of times, I can verify that it works pretty well.

Figure 10:
Four more mast sections to be used in a variety of ways - as
a support below the feedpoint, or as the "live" mast itself.
Click on the image for a larger version.

At this extended height and with the tophat, this antenna was used in fairly high winds with gusts of 35 MPH (approx 67 kph) with no issues:  Having clamped the ground stake of this antenna to a metal fence post helped keep the antenna vertical and minimize sway certainly helped!

After using the antenna several times, I purchased yet two more mast sections (for a total of eight) - not only to have as spares, but also to elevate the bottom of the antenna still further.  Living in the desert west of the U.S. (Utah) it's often the case that there is only sand into which the ground stake can be pushed and it simply isn't long enough to adequately support the antenna:  Lengthening the ground rod with the addition of another mast section allows the ground stake to be pushed in farther and support the antenna without burying the radial plate or the feed point below ground level or stealing one of the mast sections from the antenna and reducing its height.  This is mostly a problem on the lower bands (40 and 30 meters) where one needs as much height as one can get to maximize antenna efficiency.

This extension also facilitates the use of an elevated ground radial system, placing the feedpoint - and the ground radial disk - at a reasonable height.

Maintenance

The telescoping whip(s)

The telescoping whip is certainly the most fragile component included and it - like any other telescoping antenna - is easily broken if one is not careful.  The "safest" way to collapse one of these things is to pull it down - section by section - starting from the bottom:  One should NEVER push it down from the top as that is just asking for problems.

As with any telescoping whip that I own, one of the first things that I do when I get it is to make sure that it is clean of dirt and oxidation (particularly if it has been "pre-owned") as this can cause the metal-on-metal - especially when the two metals are the same - to gall and seize up, making it more difficult to extend or collapse.  If I do find a section that is hard to move, I carefully examine it, often discovering slight scratches, buffing them out with very fine sand paper (1000 grit or finer) and/or steel wool (size 0000 or finer).

The final step - after cleaning with paint thinner or alcohol to remove any dust - particularly if it was just buffed with steel wool or sandpaper - is to put a light coating of oil on all sections when they are fully extended:  I prefer to use a PTFE ("Teflon") based lubricant like "Super Lube" (made by Synco) as it does not dry and become "gummy".  Extending and then retracting the whip a few times does a decent job of spreading out the lubrication - even getting inside the individual sections.

Although much smaller, I did a similar thing to the four telescoping whips that comprise the tophat.

I would consider this "cleaning and lubricating" to be a necessary maintenance item when using this antenna, needing to be done occasionally, with constant vigilance toward possible issues every time it is used.

Inductor slider

The adjustable inductor's components are all stainless steel - including the coil wire itself.  Besides being known for the fact that this material "stains less" than others, it is also known for galling - that is, developing tiny burrs on the surface and jamming up when it is used against the same type of metal:  In the case of stainless screws, nuts and bolts - if these gall, even if you ARE able to remove them without breaking them, they have to be replaced.

While the contact area on the slider is small enough that it is unlikely to gall and get "stuck", I noticed immediately a bit of "roughness" in its movement that indicated excessive metal-on-metal friction:  I could not tell if this was on the contact area of where the slider rubbed across the coil's windings or on the sliding rod itself - but it was probably a combination of both.

This "roughness" in movement was relieved with the application of lubricant - the same "Super Lube" used on the telescoping sections - also making the adjustment easier to do.

Improving the coil

As noted previously, the coil is wound with 18 AWG (1mm dia) stainless steel wire.  It is suspected that one of the main reasons why this type of wire is used despite its terrible losses - as compared with copper - is that this resistive loss increases the feedpoint resistance - but at least in relatively cool temperatures (below 90F or 32C) I wouldn't worry about running key-down for several minutes at 100 watts - or higher power with a low duty-cycle mode like CW or FT-4.

As it happens, one can juggle the proportions of a vertical antenna a bit to vary the feedpoint resistance - but if you consider that these same coils are also used in the JPC-7 dipole, the reasoning behind the use of stainless steel wire becomes more clear.  An electrically-short dipole - such as when the JPC-7 is configured for 40 meters - would ideally have a feedpoint resistance of just a few ohms - but this would not match at all well to a 50 ohm system:  Even a very low-loss antenna tuner would have difficulty coping and placing the tuner away from the antenna through a length of coaxial cable would make the situation even worse!

Having said that, testing was done that revealed that on the JPC-17 - while operating on the lower bands (30, 40 meters) a significant amount of power was being dissipated in the coil - enough to raise its temperature by 135F (75C) at 70-100 watts on 40 meters (lower loss on higher bands) - but rewinding the coil with silver-plated copper wire pretty much eliminated that element of loss.

There are links at the top and bottom of this article that link to an article that details the rewinding of this coil with silver-plated copper wire, along with measurements/comparisons between the original stainless steel and rewound coil for both the JPC-7 dipole and this JPC-12 vertical which will eliminate any nagging worries about power handling capability.

Using the JPC-12 vertical in the field

I have used this vertical in the field a number of times, mostly with the "augmented" kit with the extra mast sections, top hat and ground radial plate and elevated radials - typically on 40 and 60 meters SSB, but also for POTA using CW.  While the signal reports comparing my signal with that of others using full-size antennas unsurprisingly indicates that this doesn't to as well as the others on the lower bands, conditions have generally been good enough that there was little difficulty in copying my signal.

Figure 11:
The JPC-12 vertical out in the wild in a slight breeze.
The tophat is installed as is a section below the radial
plate - along with extra sections to increase height.
Click on the image for a larger version.

 

As the ground here in Utah is usually rather poor making it difficult to simulate the "other half" of a vertical antenna, it is even more important that the radial system be effective.  While I usually configure it to have four radials elevated about 18" (0.5 meters) above the ground, I have also simply laid the radial directly on the sandy soil - or found some convenient sagebrush, scrub oak or some other low plant or small tree to support them off them ground - and have always had pretty good results.

While I like this antenna, I find it to be far less convenient than the JPC-7 loaded dipole in that the vertical takes quite a bit more time to set up, needing a bit of assembly of the various pieces and laying out of the radials.  With resonant radials, changing bands - and trying to maintain optimal performance - also makes it a bit awkward by the fact that the radials need to be shortened/lengthened as appropriate/

Practically speaking, having the radials laid out for 40 meters and running on 60, 30 or 15 meters isn't much of a problem, but picking a band that is an even multiple of the radials' base resonant frequency - being an odd half-wave multiple (e.g. 20 or 10 meters with a 40 meter radial) - will not work well as that is the worst possible radial length (other than zero length) to choose:  The half-wave length will not provide the low impedance at the antenna and it's also likely that the coaxial cable will become most of the radial, possibly causing a "hot rig" in terms of RF and the related ill effects (RF into the audio, computer/radio crashing, extra noise) from doing so.

If, however, I have a bit of extra time and I want a better signal that I would otherwise get from the loaded dipole or a mobile-mounted HF antenna, I would definitely set up the JPC-12.

The "card" trick

One "trick" that I have done is to use a piece of thick paper (card stock) and mark along its edge the top and bottom of the coil and then - as you find tuning for each band - mark the position of the slider:  Using this as a "template" will quickly get you to within one turn of the coil for each band.

Be sure to mark on the card how the antenna is configured (e.g. where the coil is located along the antenna, number of mast sections, if you used the tophat, how many radials, etc.) so that the configuration can be replicated in the future.  If this card is laminated and kept with the antenna in its carry bag, it will withstand repeated use.

On-air signal strength comparisons

I was present at the 2025 QuartzFest gathering in western Arizona and we had an assortment of different antennas at our disposal.  As we had Internet access, we decided to do a signal comparison on 40 meters using the Northern Utah WebSDR.  To be sure, this was a test of just one signal path on one day at a specific time of day, but it is nevertheless enlightening.

Using the S-meter graphing feature on the WebSDR - and repeating the tests four times - we did our best to glean signal differences within the normal QSB of midday 40 meters.  Differences of less than 2dB are difficult to spot, but differences greater than this are increasingly obvious - but still with a couple of dB of uncertainty.

The measurements of the various antennas on hand - and their relative differences - are as follows: 

• Near full-size vertical.  This antenna consists of a 27 foot aluminum mast approximately 1.5" (38mm) O.D average diameter with a four wire aluminum top hat to bring it to resonance at 7100 kHz.  There was one 1/4 wave elevated radial and another 1/4 wave wire laying on the sandy ground.
• This was our "reference" antenna to which the others were compared. 
  

• Approx. 20 foot whip on Mercedes Sprinter van.  This antenna used an Icom remote tuner on the roof, at the base of the antenna.  It used only the van itself as its ground/counterpoise, being mounted near the left rear corner.
• This antenna measured about 1-2dB lower than the "reference" antenna.  Doing an "A/B" comparison, it was definitely lower than that of the reference, but with the vagaries of QSB, it was hard to tell exactly how much.

• JPC-12 vertical with five screwed-together sections and tophat.  For this test, two 1/4 wave elevated radials were used along with loading coil having been rewound with silver-plated copper wire.  This antenna was clamped to a "tee" post placing the bottom of the antenna and the attachment point of the radials at height of about four feet (1.2 meters).
  
• This antenna measured almost exactly the same as the 20 foot whip on the Sprinter van.  In other words, this was 1-2dB below the "reference" antenna.

• JPC-7 loaded dipole.  This antenna was raised to a height of about 12 feet (3.7 meters) and used three screw-in sections between the feed and silver-plated copper coil with the telescoping whips fully extended.  As the feedpoint resistance of this antenna is about 15 ohms on 40 meters at resonance so a 100 watt LDG antenna tuner was used with a short (3 foot/1 meter) run of coax between it and the antenna.
  
• This antenna measured about 4 dB lower than the reference antenna.  It's likely that 1-2 dB of this difference was due to the loss of the tuner being used to match at this low impedance.

• Yaesu ATAS-100 vertical.  This antenna is mounted on the right-front fender of my four-door Jeep.
• This antenna measured 6-8 dB below the reference antenna.  As this is a very short antenna (particularly on 40 meters) with a somewhat lossy coil.  I've previously done "A/B" comparisons using this antenna on 40 meters and the general consensus is that it's "one or two S-units" below a "full-size" antenna so this result was not surprising.  This antenna is much more efficient on 20 meters and up.

In the future we may repeat this type of testing, trying different bands and a wider variety of remote receivers to get a better idea of how these antennas compare in performance.

* * * * *

Another way to connect radials

Thijs, PE1RLN, emailed me after this article was posted with a clever way to add radials.  Quoting his email:

Figure 12:
The "feed" section of the JPC-12, shown here
with holes drilled to accommodate some
"banana" plugs to connect radials.
Click on the image for a larger version.

"Recently I read your post about the JPC-12 antenna, from may 29 last year. I purchased this antenna around that time and I’m very happy with it. I was looking for a way to connect the radials in an easy way and stumbled upon the Aliexpress disc with nuts ant bolts but that didn’t look very easy to use.  

"So I started looking for another way and the antenna base itself was the solution! It’s thick enough to hold a few 4mm holes for radials to plug in. Drilled just above the height the ground pin would screw in these holes give enough space for the entire 4mm plug to go in. When the antenna is standing and someone trips over a radial wire, they will pull it out instead of throwing the entire antenna down.  

"Maybe this is a mod to include in your post to help others to do the same as there are no other pages covering modifications about this antenna.

"Keep up the good work and 73!"

As can be seen in Figure 12, Thijs drilled holes for some standard 4mm (5/32) "Banana" plugs in the feed just above the bottom of the connection to the ground stake.  Pre-made jumpers with these plugs are readily available to which you can attach extensions for the full length - and you can buy just the banana connectors themselves.

Thanks, Thijs!

* * * * *

Related articles: 

•  I analyzed the JPC-7 loaded dipole antenna - which is made by the same company and uses many of the same components - and reported on it in previous article, and you may find that discussion HERE.
• Silver-plated coils:  Since this article was posted I have added an article describing the effects of the stainless-steel coils and how to rewind with silver-plated copper, found HERE.

This page stolen from ka7oei.com

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