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, other 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. it had 79 Ohms X_L in a 50 Ohm system rather than around 150 Ohms X_L) 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 field use of the JPC-7 portable "dipole" antenna

Figure 1:
The JPC-7 and its original set of components in the case.  On
the left is a zippered section with the balun, strap, feedpoint
and mounting hardware for the elements.  On the right
can be seen the two telescoping sections, the two loading
coils and the four screw-together mast sections.
Click on the image for a larger version.

The JPC-7 (apparently by BD7JPC) is a portable dipole antenna - somewhat similar to the "Buddipole" - in that it is tripod-mounted, with telescoping elements that can be oriented horizontally.  Both use loading coils to increase the electrical length of the antenna, allowing them to operate down to 40 meters in their standard configuration.

I was able to get mine, shipped, via Ali Express for about US$170, but it is also sold domestically (in the U.S.) from a number of vendors - sometimes under the brand name of "Chelegance" using the model number of "JPC-7".

Notes:

• Silver-plated coils:  Since this article was posted I have added an article describing the effects of the stainless-steel coils in the JPC-7 and JPC-12 and how to rewind with silver-plated copper, found HERE.  This article also discusses the effects of stainless-steel versus copper loading coils in general.
  
• JPC-12 Vertical:  I have also discussed the JPC-12 vertical antenna made by the same folks - you may read about that antenna HERE.
  

A portable antenna is not the same as a "home" antenna

As you might expect, this antenna is intended for portable use - and easy-to-assemble, quickly-deployable antennas are not likely to offer high performance compared to their "ful-sized, high up in the tree" counterparts that you might have at your home QTH.  Rather, this antenna's height is limited by the tripod on which it is mounted - which, for the lower bands where its height above ground is definitely below 1/4 wavelength - is likely to put it squarely in the "NVIS" (Near Vertical Incident Skywave) category - that is, an antenna with a rather high radiation angle that better-favors nearer stations than being a DX antenna.

Additionally, its total element length as-shipped (with the two screw-in sections and the telescoping whip fully-extended, sans coil on each side) is 125" (3.175 meters) - approximately a quarter-wavelength at 22MHz - near, but above the 15 meter band meaning - that for all HF amateur bands 15 meters and below it requires the addition of the coils' inductance to resonate the two elements.  Being a loaded antenna - and with a small-ish aperture and with coils losses - means that its efficiency IS going to be less than that of its full-sized antenna (e.g. half-wave dipole) counterpart.

Of course, the entire reason for using a "portable" antenna is to enjoy the convenience of an antenna that is quick to deploy and fairly easy to transport - and anyone doing this knows (or should know) that one must often sacrifice performance when doing this!

Having said this, after using the JPC-7 in the field several times I've found that it holds up pretty well against a similar "full size" antenna (e.g. dipole) on the higher bands (20 and up) while on 40 meters, subjective analysis indicates that it's down by "about an S-unit" (e.g. the standard 6 dB IRU S-unit).  For SSB (voice) operation, this is usually tolerable under reasonable conditions and for digital or CW, it may hardly be noticeable.

Figure 2:
The components included with the JPC-7 - except the
strap and the manual.
Click on the image for a larger version.

What is included with the JPC-7:

• Four aluminum mast sections.  These are hollow tubes with (pressed in?) 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.  These are 3/4" (1.9cm) diameter.  There are two of these sections per element to achieve the  125" (3.175 meter) length of each.
  

• Telescoping sections.  These are stainless steel telescoping rods that are 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, it is strongly recommended that you lubricate the sections as soon as you receive them.  As with about every telescoping whip you will ever see, these sections are "stainless on stainless" and as with many friction surfaces between the same type of metal, they will eventually gall and become increasingly difficult to operate as they scratch each other.  I use PTFE (Teflon) based "Super Lube" for this purpose as it does not dry out and become gummy as normal distillate oils like "3-in-1" or "household" do.  Do not use "lubricants" like "WD-40" as these aren't actually lubricants in the traditional sense in that they tend to evaporate and leave a varnish behind.  If the sections do get stiff over time due to surface abrasion, a buffing with very fine steel wool and/or very fine (1000 grit or higher) sandpaper followed by wiping down and lubricating may help loosen them.

• Adjustable coils.  These are constructed 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.

The form itself is 4-1/2" (11.4cm) long not including the stud and 1-11/16" (4.3cm) diameter - wound with about 34 turns of #18 (1mm) stainless steel wire (one of my coils had an extra turn) with an inside diameter of approximately 1.66" (4.21cm) over a length of about 2.725" (6.92cm).  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.  I suggest that a drop of lubricant (I recommend the PTFE-based "Super Lube" as it doesn't dry and get gummy) be applied to the slider to make it easier to adjust and to minimize the probability of galling.

 

The coils have 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.  These coils are wound with 1mm diameter (approx. 18 AWG) 316 stainless steel wire:  The maximum inductance is a bit over 20uH and the DC resistance of the full coil is about 4 ohms - more on this later.

• 

  Figure 3:
  A close-up of the feedpoint mount showing the
  brass inserts and index pins.  The holes in the knurled
  knobs are sized to receive the miniature banana plugs
  from the balun.
  Click on the image for a larger version.
  

  Feedpoint mount.   This is a heavy plastic piece molded about pieces of brass into which the elements/coils are threaded.  There are three 10mm x 1.5mm female threads into the brass inserts plus another female thread of larger size (1/2" NPT) into which the aluminum 5/8" gaffer stud mount is screwed.  On the surfaces with the brass inserts and the 10mm x 1.5mm female threads are a series of index holes into which the element mounts (described below) are seated to allow the elements to be adjusted at various angles.  Electrical connection is made via holes in the brass to receive 2.5mm miniature banana plugs (visible in Figure 3) which contact the adjacent 10mm x 1.5mm female thread bodies.
  

• Element mounts.  These are two heavy-duty nickel-plated brass adapters that are held to the feedpoint mount via 10mm x 1.5mm screws with large handles - both included.  Into the mounting surfaces are holes to receive index pins allow the elements to be rotated to various angles - from a horizontal dipole to a "Vee" configuration - and even to an "L" with one element vertical and the other horizontal.  It can also be configured with just a single element as a plain vertical if one so-chooses - the counterpoise/ground needing to be supplied by the user.  Figure 8, below, offers a better view of how this is used.

• 5/8" stud (gaffer) mount.  As mentioned earlier, this kit includes a male 5/8" stud mount commonly found on photographic lighting tripods.  The other side of this has 1/2" NPT pipe threads that screw into the feedpoint mount.  This piece is shown in Figure 4.  Note:  Pipe thread is NOT the same as "normal" thread as would be found on a large bolt as pipe threads have some taper and specific size - see the discussion in the comment section at the bottom of the page.
  

Figure 4:
5/8 stud mount adapter to be used with
lighting tripods.  The "other" side is a 1/2 inch
NPT pipe thread that screws into the feedpoint mount.
Click on the image for a larger version.

• 1:1 balun.  This appears to be a "voltage" balun, with DC continuity between the "balanced" and "unbalanced" sections and across the windings themselves.  This is in contrast to a "current" type balun that would typically consist of feedline, twisted pair or two conductors wound as a common-mode choke on a ferrite core. More near the end of the article.
  

• Hook-and-loop ("Velcro") strap for the balun.  This is used to attach the balun to the mast to prevent the weight of the coax and balun from pulling on the feedpoint mount.  This strap appears to be generic and doesn't really fit the balun too well unless it is cinched up, so I zip-tied it in place to keep both of them together. 
  

• 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 components for the antenna mount, balun, and the instructions.  There is ample room in this case to add additional components such as coaxial cable - and enhancements to the antenna, as discussed below.  Other than the printing on the outside, this case is identical to the one supplied with the JPC-12 vertical.
  

• Instruction manual.  The instructions included with this antenna are only somewhat better than typical "Chinese English" - apparently produced with the help of an online translator rather than someone with intimate knowledge of the English language resulting 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-12 vertical as well, printed on the obverse side of the manual.

Construction and build quality

About a year ago I purchased a JPC-12 vertical antenna and it shares many of the same components as this antenna - the only real differences are that this antenna comes with two telescoping whips and loading coils, the center mount for the elements, a 1:1 balun, and the 5/8" stud adapter for the center mount.

Many of these components are the same as supplied with the JPC-12 vertical:  The loading coils, the telescoping whips, and the screw-together antenna sections.  In other words, if you have both antennas, you can mix-match parts to augment the other.  You can, in fact, buy kits of parts for either antenna to supply the missing pieces to convert from one to the other.

Mechanically, this antenna seems to be quite well built:  During use, I have no sense of anything being "about to come apart" or "just barely good enough".  I suspect that the designers of this antenna did so iteratively, and the end product is a result of some refinement over time.  The only really fragile parts are the telescoping whips, but these things are, by definition, fragile - no matter who makes them!

How it is mounted

This antenna does NOT come with any tripod or other support, but it offers three ways of being mounted:

• 1/2" NPT threads.  The center support, as the primary mounting, has female 1/2" NPT threads.  If you have a piece of pipe with that type of thread on it, you can mount the antenna directly to it.
  

• 5/8" male stud mount.  This antenna comes with a machined aluminum mount (seen in Figure 4) that screws into 1/2" NPT threads in the center support that is a 5/8" stud mount - sometimes referred to as a "Gaffer" or "Grip" mount - of the sort found everywhere on tripods used for holding photographic lights.
  

• 10mm x 1.5mm thread.  If you want to configure this antenna as a dipole, you also have the option of using a 10mm x 1.5mm thread that is on the side opposite the female threads into which the 5/8" stud mount screws - but this should not be your first choice as it will put a fair bit of stress into a small mechanical connection.  While this thread isn't particularly common in the U.S.A., it would seem that this is a common size for portable antennas everywhere else in the world and hardware of this size is available at larger U.S. hardware stores.  As this mounting point may be used as part of the antenna

  

  Figure 5:
  A homebrew double-female 5/8 stud adapter.  These adapters
  have 3/8" threads and were attached using a thread
  coupler.  This piece was necessary as both the antenna and my
  tripod have male 5/8" stud mounts on them!
  Click on the image for a larger version.
  

  (when configured in an "L" shape or if configured as a vertical-only) so it's the same threads as the screw-in element sections.
  

For me the 5/8" male stud mount is the most useful as it happens that I have on hand an old gaffer tripod (light stand) of this sort - but there's a catch:  It, too, has a 5/8" male stud mount!  It would seem that these tripods come both ways - with either a male or female 5/8" mount, but for less than US$15 I was able to construct a "double-female" adapter that solved the problem.  From Amazon, I ordered two 5/8 female stud to 3/8"-16 adapters and coupled them together with a 3/8" thread coupler as seen in Figure 5.  The only "trick" with this was that I had to sort through my collection of flat washers to find the combination of thicknesses that resulted in both knobs facing the same direction when the adapters were tightened to the thread coupler - and the reason for this is so that it would store flat in the case with both knobs oriented as shown above.

Element configuration

As with any antenna that you are likely to come across, the only portions of the antenna that actually radiate energy in the far field are those with current flowing through them:  The higher the current, the more energy is radiated.  By extension, the very ends of the wire - or, in this case, the ends of the telescoping section - have essentially zero current and do not radiate.  As the total length of conductors prior to the loading coil (screw-together sections, feedpoint mount, connecting wires) is about 56" (1.42 meters) this represents only about 3.6% of a wavelength at 40 meters.

It is for this reason that the preferred configuration is to have the screw-together sections connected directly to the feedpoint mount, then the loading coil and then the telescoping section, placing the loading coils nearly 30" (75cm) from the feed.  As 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 - you might think that they are doing the lions share of radiating - but that's not really the case.

Particularly at lower bands, it is understandable why coil losses are of such importance - and also why even a relatively small amount of lengthening of the antenna can improve performance on the lower frequencies:  Adding two more screw-together sections (one per side) increase the length "before the loading coil" from 56" to 84" (2.13 meters) - or about 5.3% of a wavelength and not only increase the aperture of the antenna, but it will also allow a reduction of the amount of inductance (and coil loss) required to resonate the antenna.

Further improvement can be made by adding a bit of extra length to the telescoping whips by clipping hanging wires to the end of it:  This will further reduce the amount of inductance needed to resonate, but it will also increase the effective portion of the whips that are carrying RF current.  (This is discussed further in the section on 60 meter coverage, below.)

Frequency coverage

(This section presumes that the original, stainless-steel wound coils are being used.)

This antenna is advertised to cover 40 through 6 meters - and this is certainly true:  When the four supplied mast sections are installed (two per side) the lowest frequency at which it can be resonated with the telescoping rods at full extension and the inductors set at maximum is around 6.7-6.8 MHz - well below the entirety of the 40 meter band.

On 40 meters, the 2:1 VSWR bandwidth was typically around 120 kHz:  A 2:1 VSWR is about the maximum mismatch at which most modern radios will operate at full power before SWR "foldback" occurs, reducing transmit power.  Of course, if your radio has a built-in tuner - even one with a limited range - you will certainly be able to make the radio "happy" across the entire 40 meter band without fussing with the antenna, even if it isn't tuned exactly to your operating frequency.

On the other extreme, with the minimum coil inductance and the two telescoping rods at maximum extension the resonant frequency was about 21.7 MHz:  This means that for all amateur bands 15 meters and lower, you will need the inductors - but for 12 meters and up you can omit them entirely (which is recommended!), bringing the antenna to resonance solely by adjusting the length of the telescoping sections.

Tuning the antenna

This may be where some people have issues.  I am very comfortable using a NanoVNA:  I have several of these as they are both cheap and extremely useful - the only down-side really being that their screens are not easily viewed in direct sunlight - but simply standing with my back to the sun was enough to make it usable as all one is trying to see is the trace on the screen rather than any fine detail.

The biggest advantage of the NanoVNA over a traditional antenna analyzer is that you get the "big picture" of what is going on:  You can instantly see where the antenna is resonant  - and how good the match may be.  More importantly, you can see at a glance if the antenna is tuned high (too little inductance) or too low (too much inductance) and make adjustments accordingly whereas using a conventional antenna analyzer will require you to sweep up and down:  Still do-able, but less convenient.

Tuning is somewhat complicated by two factors:

• There are two coils to adjust - and they must both be pretty close to each other in terms of adjustment to get the best match.  Simply looking at the coils one can "eyeball" the settings of the slider/contact to get them very close to each other - something that becomes easier with practice.
  

• The "resolution" of the inductors' adjustments is limited by the fact that one can make adjustments by one turn at a time with the slider.  At 20 meters and higher, being able to only adjust inductance one turn at a time is likely to result in the best match being just above or below the desired frequency.  At lower frequencies (lots of turns) - say 40 and 30 meters - you can likely get 2:1 or better by adjusting the coil taps alone, but at higher frequencies you will likely need to tune for the best match just below the frequency of interest and then shorten the telescoping rods slightly to bring it right onto frequency.

 Once I'd used the antenna a few times I found that I could change bands in 2-3 minutes as I would:

• Lower the antenna to shoulder height so that the coils and telescoping rods may be reached.  If you had previously shortened the telescoping elements for fine-tuning a band you should reset them to full length.
  
• Set the NanoVNA to cover from the frequency to which it is already tuned and where I want to go:  If I was setting it up for the first time I would set the 'VNA to cover above and below the desired frequency by 5 MHz or so so I could see the resonant point even when it was far off-frequency.  After using it a few times you will remember about where the coil taps need to be set for a particular band.
  
• On the NanoVNA I would then set a marker to the desired operating frequency.
• I would then "walk" both coils up/down to the desired frequency while watching the 'VNA.  As the tuning of the elements interact, you may have to iterate a bit to get the VSWR down.  Again, you may have to tune for best match at a frequency just below the target frequency and then shorten the telescoping sections.
• I would raise the mast to full height again.  I noticed  a slight increase in resonant frequency (particularly on the lower bands - 40 and 30 meters) by raising the antenna on the order of 50 kHz on 40 meters.  Usually, this doesn't matter, but with a bit of practice/experience you'll be able to compensate for this while tuning.
  
• A match of 2:1 or better was easily obtained - but don't expect to get a 1:1 match all of the time as the only adjustments are those of resonating the elements and nothing to take into account the actual feedpoint resistance at resonance.  Practically speaking, there is no performance difference between a 2:1 and 1:1 match unless your radio's power drops back significantly:  An antenna tuner could be used, but this will surely insert more loss than having a modest mismatch!
  

Figure 6:
As with almost any inductor adjustable using sliders, care
should be taken to assure that only one turn is being touched
by the contact, as shown.
Click on the image for a larger version.

All of that sounds complicated - and it may be, the first time doing it - but I found it to be very quick and easy, particularly after even just a little bit of practice!

The "card" trick

Another "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(s) for each band.

Be sure to mark on the card how the antenna is configured (e.g. where the coil is located, number of pipe sections, if you used wires at the ends of the antenna) 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.

Carefully adjusting coil taps

 If you look very carefully at the sliding coil taps you'll notice that if very carefully adjusted that they will contact just one turn of wire - but it is almost easier for the contact spring to bridge two turns of wire, shorting them together.  When this happens the inductance will go down slightly and you may see the resonance go up in frequency unexpectedly.  Additionally, the shorting of two turns can also reduce the "Q" (and efficiency) of the coil slightly.

If you are aware of this situation - which can occur with nearly all tapped inductors adjusted with a slider - you can start to "feel" when the slider bridges two turns of the coil and avoid its happening as you make the adjustments.

* * *

Suggested modifications/additions:

All electrically-short antennas that require series inductance for tuning to resonance - like this one - will lose efficiency due to losses in the coil, but this can be offset - at least somewhat - by increasing the length of the elements themselves.  One of the easiest ways to do this is to purchase a couple of extra screw-on mast sections:  The addition of one on each side will increase the total length of the antenna by about 25" (64cm) and allow a slight decrease in the required inductance - resulting in slightly lower loss and increase the aperture of the antenna slightly.  These additional screw-on sections are typically available from the sellers of the antenna for between US $10 and $15 each but are often called something like "Dedicated lengthened vibrator for JPC-7 (JPC-12)" or similar due to quirks of the translation.

60 meter operation

Figure 7:
The elements may be lengthened by clipping a lead to each
end of the telescoping sections, reducing the amount of
needed inductance - and also allowing resonance on lower
bands - in this case, 60 meters.
Click on the image for a larger version.

While adding two additional sections (on on each side, between the coil and the whip) - and rearranging the antenna with the coils located next to the feedpoint (rather than the usual configuration in which the coils are located away from the feedpoint) - will bring the resonant frequency down to about 5.7 MHz with full inductance and extension of the telescoping sections.  In this configuration, the added length beyond the coil adds significant capacitance, lowering the resonant frequency as compared to the normal coil location

The antenna can be made to cover 60 meters by clipping on short (18" or 46cm) jumper leads to the very end of the antenna elements and let them hang down.  Despite this being a less desirable configuration in terms of RF current distribution, in testing on the air, the signals were about 1 or 2 "S" units below a full-sized dipole, but still quite good for a fairly compact antenna that was  close to the ground in terms of wavelength.

If you wish to use the "stock" antenna on 60 meters rather than buying two extra screw-together sections, you'll need about 48" (1.25 meters)  of wire on each end:  For this I simply used two pair of 24" (approx. 100cm) clip leads connected end-to-end, each pair hanging from the tips of the telescoping section.

Longer is better

Of course these "extension" leads can be used for all bands for which the coils are needed to lower the inductance and reduce losses:  As it will always be the parts of the antenna that carry the most RF current that radiates the vast majority of the signal - and since those portions will always be the sections right near the coils for this type of antenna - adding these dropping wires at the ends won't appreciably affect the antenna pattern or its polarization.

As there is plenty of room to do so in the zipper case, I have since added two extra sections and two sets of "clip leads" permanently into the kit.

Get extra telescoping sections!

Finally, I would order at least two extra telescoping sections as these are the most fragile parts of the antenna kit.  These can also be ordered from the same folks that sell the antennas for US $12-$16 each and are typically referred as something like "304 stainless steel 2.5M whip antenna for PAC-12 JPC7 portable shortwave antenna". 

The reason for ordering two of them is that if the antenna falls over, both whips are likely to be damaged (ask me how I know!)  The cost of getting two extra whips is likely to be less than the cost of fuel for even a modest road trip to wherever you are going, so their price should be kept in perspective.  As the zippered case for the antenna has plenty of extra elastic loops inside, there is ready storage for these two extra whips with no modification.

A word of caution:  However you store them, do not allow the telescoping whips to lay loosely in the case:  If they bash into something else they can be easily dented which may make it impossible for them to be extended/retracted.  For this reason they should be secured in the elastic strap, or individually in a tubes or padded cases.

Note:  There are also available much heavier and longer telescoping whips with the same M10x1.5 thread that would easily allow 60 meter coverage:  I have not tried these to see how well they would work, mechanically, or if it would even be a good idea to do so (e.g. extra stress on the tubes, coils, mounting point - or how stable such a thing might be on a tripod).

Figure 8:
The mounting of the balun, just below the feedpoint mount.
The index holes allow flexibility in the orientation, the
connection being made by 2.5mm banana plugs.
Here, the antenna is shown with the elements configured
one hole higher than "flat", forming a lazy "Vee"
shape as seen in Figures 9 and 10.
Click on the image for a larger version.

Additional comments:

"To vee, or not to vee"

The feedpoint mount has a number of indexed holes that allow the elements to be mounted in a variety of configurations, from flat, in a number of "Vee" configurations, or even an "L" or vertical configuration.  

Personally, I use the flattest "Vee" configuration as seen in Figures 8, 9 and 10.  This configuration keeps the drooping ends of the telescoping whips higher than the feedpoint and helps clear any local obstacles (trees!)  - and just looks cool!

As can be seen in Figure 8, the connection between the balun and the feedpoint is made by plugging 2.5mm miniature banana plugs into the brass receptacles on the feed.  Shown in the photo are connections to the two sides, typically used for a dipole arrangement, but the third, unused connection on the top could be used to hold an element horizontal while one of the side connections hold it vertical - more on the use of this antenna as a vertical in the next section.

It should be no surprise that these 2.5mm miniature banana plugs are quite small and fragile and if one isn't careful - say, by allowing the weight of the balun to be supported by the wires rather than using the hook-and-loop strap - they can be broken.  For this reason I ordered a pack of ten 2.5mm banana plugs from Amazon and made a pair of short (4", 10cm) leads - one end with a small alligator clip and the other with a 2.5mm banana plug - to allow me to make a temporary connection should one get broken off in the field - something that can torpedo an activation if you didn't have spare parts! 

Operating as a vertical antenna

Because of the flexibility of the mounting point, it is possible to use this same kit as a vertical antenna with the second element as a resonant (rod) ground "plane" if - due to space or personal preference - emitting a signal with a vertically-polarized component is desired.  While this will certainly "work", if you do plan to operate with vertical polarization its recommended that you add several (2 or more) wire "radials" or counterpoises.

Because of the included balun (more on this in a moment) the coaxial feedline itself will not act as an effective part of the counterpoise network so rather than connecting additional radials to the shield, the ends of the wire should be clamped under the washer/bolt that holds the horizontally-configured element in place.  Of course, one need not use the balun and connect the coaxial cable directly, but if you choose this option you will be on your own to supply the means to make such a connection.

For best results with the fewest number of radials, choosing lengths that are odd-number quarter wavelengths long (1/4, 3/4, 5/4) and keeping them elevated a foot (25cm) or more off the ground is suggested as this will help minimize "ground" losses.  Having said this, almost no matter what you do, you will probably be able to radiate a useful amount of signal:  Operating CW or digital modes offers an improvement in "talk" capability owing to their efficiency - but if you are planning to operate SSB, it's worth taking a bit of extra time and effort to maximize performance.  Remember that just two elevated radials oriented in a straight line will produce a (mostly) omnidirectional pattern with nearly the performance as four radials while a single radial.  Meanwhile, if you do use a single resonant radial (e.g. "counterpoise") the pattern will be somewhat lopsided, the peak in the direction of the wire.

Would I operate this antenna in "vertical" mode?  While I don't have plans to do so, I have purchased an extra ground stake of the sort used on the JPC-12 vertical, and the short banana plug/clip lead jumpers that I made could be used to make a temporary connection directly to a coaxial connector.

Nature of the balun

For more details about the supplied balun, see the "Addendum" section at the end of this article.

The supplied balun has a 1:1 impedance ratio and has DC connection between the input and output - but since there is a DC connection between all of the conductors, it is more than a simple current balun (e.g. transmission line wound on ferrite).  As the balun seems to work well, I have no reason to break it open to figure out what's inside, but I did a bit of "buzzing" of the connections with a meter to measure inductance and here are the results:

• Between coax shield and center conductor:  16.9uH
• Between red and black (on antenna side):   16.9uH
• Between center coax and black:  38.5uH
• Between center and red:  3.4uH
• Between Shield and black:  3.4uH
• Between Shield and red:  3.4uH
• The DC resistance between any combination of the leads is well under 1 ohm.

What does this tell us?  The inductance readings of about 16.9uH indicate that this may be a voltage balun providing about 500 ohms of inductive reactance at 5 MHz - more than enough for reasonable efficiency.  The interesting reading is the inductance between the center coaxial connection and the black wire which is only twice the inductance of the input or output windings:  If there was a direct connection between one of the coax and one of the output wires this would imply twice the number of turns and four times the inductance - but since it is only twice, this indicates that the total number of turns in the "center coax to black" route is about sqrt(2) (or 1.414x) as many turns as the primary/secondary - or there is another inductor in there.

Figure 9:
The JPC-7 backgrounded by red rock during a POTA
operating in K-0010.
Click on the image for a larger version.

While I'm sure that the balun is very simple, its exact configuration/wiring escapes me at this time.

Addendum:  I was able to figure out the configuration of the balun - see the note at the very end of this article.

Coil losses

As mentioned earlier, the coil is wound with 18 AWG (1mm diameter) type 316 stainless steel wire.  Fortunately, this wire appears is austenitic - which is to say that it is not of the variety that is magnetic and thus has a permeability of unity:  Were it magnetic, this would negatively impact performance significantly.

Knowing the diameter of the coil form and the fact that there are 34 turns, we know that the total length of the wire used is approximately 180 inches (457cm) and measurement shows that the stainless steel wire coil has a total DC resistance of about 4 ohms.  Using Owen Duffy's online skin effect calculator (link to archived page) and assuming 1mm diameter, 316 Stainless we can calculate the approximate RF resistance including skin effect - the tendency for RF to flow on the outside skin of a conductor rather than through its cross-section - versus frequency:

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

If I make a very broad assumption that the feedpoint resistance at each coil is about 25 ohms (the two in series being around 50 ohms) we can see that in this hypothetical situation about a third of the total resistance could be due to the coil, and since P = I²R - and if we presume that the current is consistent throughout the coil (it probably is not) we can roughly estimate that the total power loss will be proportional to the resistance implying that about 1/3rd of the total power is lost in the coil.  In practical terms, a 33% power loss is around 4.8dB - still less than one "S" unit, so this loss may go unnoticed under typical conditions.

In operation, we would be unlikely to need all - or even most of the turns of the coil for operating on the higher bands, so the overall coil losses are likely to go down as the need for loading inductance at these frequencies is also significantly reduced:  Since we actually use only about 2/3 of the turns of the coil on 40 meters, the loss is more likely to be something on the order of 5 ohms rather than 7.2, reducing the loss even more.

Note:  K6STI's "coil" program - Link - calculates the loss for this coil as being closer to 8 than 5 ohms - a bit higher than the simple loss calculation of Owen Duffy's wire calculation and likely more representative of in-situ measurements.

When operating on 40 meters with 100 watts of CW or SSB, the coils definitely do get quite warm - but not dangerously so and thus I would presume that the very rough estimates above are likely in the ballpark:  If you operate heavy duty-cycle modes like RTTY or FT-8 and insist on running 100 watts key-down I would occasionally check the coils to be sure that they aren't getting too hot.

By comparison, the calculated DC resistance of  the same length of 18 AWG bare copper wire is under 0.5 ohms, but the RF resistance due to skin effect at 28 MHz is around 2 ohms and about an ohm at 7 MHz - roughly a 7:1 difference meaning that if the above analysis is in any way close to being correct, our losses at 7 MHz when using the full coil (again, we don't!) and presuming that the feedpoint of the individual coil stayed at 25 ohms (it probably won't) our losses would drop from about 30% to less than 5%.

As a consequence, if wound with copper/silver plated I would expect that the not only would the antenna become narrower than the 40 meter 2:1 bandwidth of about 120 kHz - which would make it slightly trickier to tune - I would also expect the feedpoint resistance to drop, possibly increasing the VSWR at the feedpoint.  From a practical standpoint, even a modest antenna tuner capable of handling only 3:1 mismatch should be able to cope with this, but it is likely that some of the gains from using lower-loss wire might be offset by the increase in losses caused by feedline mismatch and the losses within a tuner - both of which could easily exceed 3dB in a portable set-up with moderately-long, small-diameter coax.

Note:  I have since rewound a coil with 18 AWG silver-plated copper jewelry wire and have done direct comparisons with it and the original coil wound with stainless-steel wire - you may read about those results in the blog entry Rewinding the Stainless Steel Coils with silver-plated copper wire on the JPC-7 and JPC-12 antennas.

Final comments

Figure 10:
Operating 20 meter CW from POTA entity K-6085, with the
Conger mountains and the JPC-7 dipole in the background.
Click on the image for a larger version.

Is this an antenna that is worth getting?  I would have to say "yes".

Remembering that you will also need to supply a suitable tripod mount (e.g. an inexpensive "light stand" ) this antenna is quite portable and, if you have a bit of practice, quick to set up and adjust.  Unlike a vertical antenna, it doesn't need a set of ground radials and it is likely that the antenna itself will be up and above everyone's heads when it is deployed.

Best used on the higher bands (20 and higher) its efficiency will be quite good - certainly equal to or better than a typical mobile antenna.   As this is a large-ish antenna on a tripod, be sure to weigh down the legs and/or attach simple guying to it to prevent it from blowing over in the wind or being knocked over by tripping over the coax:  I can attest personally that the latter can easily happen!

* * *

I also have the JPC-12 vertical (discussed in the link below) and I find this antenna (the JPC-7 loaded dipole, that is) to be far more convenient to use than the vertical (e.g. no radial system), particularly if you plan to change bands several times during the operation - something that is quite likely to happen on the higher bands as propagation varies over the course of a few hours.  For the vertical, best performance requires adjusting the radials as well as the antenna itself, although it would probably work "just fine" if the radials are left at maximum length.  Another advantage of the JPC-7 loaded dipole being a (largely) horizontally-polarized antenna is that in an urban environment it is likely to intercept less noise on receive than a vertical - and it can be inconspicuous in its deployment as compared to a taller vertical.

For the lower bands (40 and 30 meters) the JPC-7 works quite well - particularly if one operates CW or digital modes.  As mentioned, it can also work competently on 60 meters as well with the addition of extra length of the elements by the purchasing of extra rods and/or simply attaching "drooping" wires to the ends of the telescoping rods.

Over the course of several POTA and related activations I have made about 500 contacts with this antenna on the bands 60 through 15 meters - on CW and voice:  I'm sure that the antenna works well on 12, 10 and 6 meters as well, but I just haven't tried it on those bands.

Overwhelmingly, the sense has been "If I can hear them, they can hear me" and with this antenna as I have worked quite a few QRP and DX stations that I could barely copy above the band's natural QRN level.  Admittedly, some of these times I was on the receiving end of the frenzy - being the activator during POTA operation - but there were many times when I had to stop operating not because I ran out of people to work, but because I ran out of time.

Addendum:

Figure 11:
This is the configuration of the balun.  It is, in fact, a 1:1
balun and is symmetrical, but it isn't very "balanced".  It's
recommended that a common-mode choke be used instead.

After a bit more thinking I was able to determine how the 1:1 balun was wired - and it's not very balanced as the drawing in Figure 11 shows.

While symmetrical, this is NOT "balanced" - but it is not completely "un-balanced" either.  The tell-tale sign that this isn't a truly "balanced" balun is the fact that - especially at higher frequencies - you can grasp the coaxial cable near the antenna and affect its tuning:  A properly balanced antenna would have no current on the feedline at all and would not be thus affected.

 

What problems can this cause?  Perhaps none - but since the feedline would be part of the antenna, any noise - such as from electronics (e.g. charger, PV/solar system, etc.) connected on the same power source as the radio (e.g. mains power, other device on battery) - or to the radio (e.g. computer) could be conducted to the antenna on the coax.  Similarly, it's possible that you may end up with a "hot" chassis with RF which can cause issues with computers (e.g. USB devices freaking out, RF in audio) or get interference from/to other devices - not to mention the possibility of RF burns.

 

The easiest "fix" to this is include a "current choke" in the antenna kit which is simply 8-12 turns of small-diameter coaxial cable on a ferrite core such as an FT140-43, FT240-31 or FT240-43:  Any of these will suffice in a portable environment and go a long way toward choking the RF from the feedline.

* * * * *

Related articles:

• Silver-plated coils:  Since this article was posted I have added an article describing the effects of the stainless-steel coils in the JPC-7 and JPC-12 and how to rewind with silver-plated copper, found HERE.  This article also discusses the effects of stainless-steel versus copper loading coils in general.
• I posted an article that gives a description of the JPC-12 loaded vertical - and ways to improve it - and it may be found HERE.
  

This page stolen from ka7oei.blogspot.com

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