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| 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 much 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!
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| Figure 2: The original stainless steel coil (rear) for the JPC-7 (and JPC-12) with the coil rewound with silver-plated "jewelry" wire in the front. Click on the image for a larger version. |
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) where a fair bit of inductance is required.
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| 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 "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 - 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 (35-70 Ohms, depending on the antenna and its environment) to provide a decent load to a modern radio with 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. With the lower-loss silver plated coil we (mostly) eliminate it as a lossy component - but end up with a different problem.
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. This heating seemed to be more severe at the low impedances (below 20 Ohms) than at 50 Ohms.
Eliminating the tuner
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| 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. |
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: 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, called an autotransformer: 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.
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 and the Swan MMBX, both of which have a number of low-impedance taps.
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| 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 (XL ) 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 XL 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 : A small amount of this inductance would lower the resonance frequency slightly, but not disastrously so.
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| Figure 7: 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. |
As can be seen in Figure 7, 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 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:
Ztap = (Zsys) / ((Turnstotal/Turnstap)2)
Where:
Ztap = Impedance of the autotransformer tap
Zsys = System impedance (typically 50 Ohms)
Turnstotal = Total number of turns on the autotransformer (13 turns in our example)
Turnstap = Number of turns from the bottom (ground) end of the autotransformer to the tap
In other words:
Ztap = (50) / ((Turnstotal/Turnstap)2)
Taking our 13 turn autotransformer as an example, we can calculate the impedance at any turn. Taking the 8th turn as an example:
Ztap = (50) / ((13/8)2) therefore,
Ztap = 18.9 Ohms
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.
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| Figure 8: 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. |
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.
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| Figure 9: The auto transformer with the common mode choke on the JPC-7's feed. Click on the image for a larger version. |
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 8 and 9 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: Transmitting 100 watts on 40 meters also resulted in only very slight heating of the auto transformer - certainly a much lower amount than that which resulted in the heating and discoloring of the toroid in the antenna tuner pictured in Figure 4.
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| Figure 10: 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. |
Does it work?
I have put this configuration pictured in Figure 9 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. 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.
The common-mode choke also seemed to be very effective: While touching the "ground" side (bottom lead) of the autotransformer slightly affected tuning - indicating the imbalance inherent to any dipole fed with coaxial cable - the same action on the coax shield distal side of the choke did not cause any detectable change on a NanoVNA or other antenna analyzer indicating a useful degree of isolation.
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.
* * * * * * *
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
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