How Does the Speaker Wire “No Counterpoise Antenna” Work?

Fellow blogger Craig WB3GCK recently posted an account of his build of the speaker wire “No Counterpoise Antenna” and I was immediately intrigued. In fact I rushed out to the store and bought myself a couple of 50ft rolls of 18AWG speaker wire to experiment.

After a couple of days of intense backyard wire whispering I have concluded that the name “No Counterpoise Antenna” may be a misnomer. Some sources refer to it as the “no counterpoise on the ground antenna” which is probably more accurate.

What is a “No Counterpoise Antenna”?

The name is applied to a length of twinlead 25ft or 50ft long, with one conductor stripped away for half its length.

Does it Work?

To this question I can only give a qualified “yes”. I connected the twinlead end of the the 50ft version of the antenna to the balanced line input of my “Super T” antenna tuner and was easily able to find a match on the three bands of interest for my QRP Labs QMX: 20m, 30m and 40m.

What Kind of Antenna is it?

The No Counterpoise Antenna resembles a Zepp or a W3EDP. But it could also be a very strange interpretation of an Off-Center Fed Dipole. I read several sources online and none of them has come to a firm conclusion on that question.

Is it a Zepp Antenna?

The first question I asked myself was: “why 25 or 50ft?” The best answer may be: “because that’s the way speaker wire is sold”. Next question: “why strip one wire half way? Is twelve and a half, or twenty-five feet a significant length for the twinlead feedline?” Then I considered a third question: “is this antenna related to the Zepp?”

The original “Zepp” or Zeppelin antenna comprised a half-wavelength radiator fed through a quarter-wavelength transmission line. The transmission line acted as an impedance transformer to bring the very high impedance of an end-fed half wave wire down to something a transceiver could handle. So, if the “No Counterpoise Antenna” is a variation of the Zepp, then the twinlead part of the antenna would act as an impedance transformer.

Problem #1 with this theory is that this would make it a single band antenna whereas the speaker wire No Counterpoise Antenna is a multiband antenna.

Problem #2; Jose VA3PCJ has built and successfully used a full-sized W3EDP, a half-size W3EDP and even a quarter-size W3EDP. [A W3EDP is a modern interpretation of the Zepp in which a 17ft feedline feeds a 67ft radiator and tunes 160m to 6m]. The full-size version uses a 17ft feedline, the half-size uses an 8.5ft feedline and the quarter-size uses a 4.25ft feedline – and they all work.

Problem #3; Speaker wire is designed for audio frequencies; let’s say from DC up to 30KHz. Can it handle Radio Frequencies? Is the dielectric effective at RF? Maybe for QRP but I wouldn’t trust speaker wire as a feedline at or beyond 100 watts.

Is a Quarter Wave Speaker Wire Feedline an Impedance Transformer?

Speaker wire has a nominal characteristic impedance of around 100 ohms but that may not remain constant since it was never intended to be used for RF. I measured the velocity factor, with my RigExpert AA-55 Zoom antenna analyzer, to be 0.95 then trimmed a feedline to make it an electrical quarter wavelength at the bottom end of the 20m band. Then a halfwave wire was soldered to one of the conductors of the feedline, leaving the other conductor open.

A halfwave wire will have a very high impedance at its end – typically around 2500 ohms. If the radiating element is shortened to a random length, say 29ft, its end-point impedance should drop to around 200 ohms. So we can easily make some simple calculations to see if an impedance transformation is taking place by comparing a half-wave radiator with a random length radiator.

Lets do the math:

Parameters: Zo=Transmission line characteristic impedance | ZL=Load impedance (e.g. halfwave end-fed wire) | Zin=50 ohms (desired impedance)

Formulas: Zin=Zo^2/ZL | Zo=(Zin x ZL)^0.5

Case #1: Match 2500 ohm EFHW to 50 ohms
Zo=(2500×50)^0.5 = 353 ohms

Case #2: Match 200 ohm EFRW to 50 ohms
Zo=(200×50)^0.5 = 100 ohms

• In case #1 – matching an End-Fed Half Wave wire to 50 ohms requires a transmission line with characteristic impedance of approximately 350 ohms. Our speaker wire has an assumed characteristic impedance of just 100 ohms so we would expect a 3:1 mismatch.
• Case #1 actual measurements: SWR is 1.43 (after tuned for best SWR). Resonance is at 14009 KHz R=69 ohms, X=5.54 ohms
• In case #2 – matching an End-Fed Random Wire to 50 ohms requires a transmission line with characteristic impedance of 100 ohms. Our speaker wire has an assumed characteristic impedance of 100 ohms so we would expect a perfect match.
• Case #2 actual measurements: SWR is 1.34 (no further tuner adjustments). Resonance is at 14047 KHz, R=67 ohms, X=2.41 ohms

Clearly, the actual measurements do not conform to the calculated values so it is very unlikely that the twinlead feedline is acting as an impedance transformer.

So how does the antenna measure up on 20m, 30m and 40m?

The radiating element was restored to 33ft while maintaining the twinlead portion at the same length (15 feet 8 inches). Each of the three bands of interest (20m, 30m, 40m) were then measured on my RigExpert AA-55 Zoom antenna analyzer with the following results:

• 20m (14065KHz): Resonance (i.e. X=0) = 14096 | SWR = 1.17 | R = 58 | X = -2.94
• 30m (10125KHz): Resonance (i.e. X=0) = 10132 | SWR = 1.12 | R = 44 | X = -0.13
• 40m (7060KHz): Resonance (i.e. X=0) = 7063 | SWR = 1.36 | R = 37.8 | X = -0.37
• * all R, X measurements in ohms

Conclusion

I was finally able to get a QSO completed on the speaker wire antenna, on 40m. The wire was erected as a low (about 20ft) lazy inverted-L. I responded to a POTA operator just south of Lake Ontario in upper New York State from my QTH on the shore of Lake Huron in Ontario. The distance between us was only about 250km which under present conditions was a little too close even for 40m. The other op was coming in at 599, but with significant QSB. He gave me a RST report of 229 (I was using my QMX radio at 5 watts CW). So I can give the speaker wire antenna a qualified thumbs up.

How does it work? My belief is that the short wire is in fact a counterpoise. If it were not for VA3PCJ’s good results with using very short length’s of feedline I would recommend using the original dimensions reported by Craig WB3GCK.

It’s fun to experiment with oddball antennas and that is what ham radio is all about for me. If you have any opinions about this antenna let me know in the comments.

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