Antenna tuning
12/08/2026
The counterpoise compromise
Before I installed my common mode choke, every SWR sweep I did was different from the last. That's the classic signature of common mode current on the shield: the coax itself becomes part of the radiating system, so whatever you're measuring includes the antenna, the feedline, and your house wiring all mixed together. Not exactly a repeatable measurement.
With the choke doing its job, I still needed a proper counterpoise for the EFHW.
That'ss why I'm using about 6 meters of coax between the 49:1 EFHW balun and the
common mode choke, and letting the shield of that coax section act as the
counterpoise.
Yes, that piece of coax radiates too — it's not a "clean" counterpoise in the textbook sense. But hey, maybe one day it'll help me work some unexpected DX rather than hurt me ;-)
About the antenna
A quick word on the antenna itself:
- It's an EFHW with 20 meters of wire, resonant on 40m.
- At the far end there's a loading coil that brings 80m into play.
- Because of that loading coil, the antenna is very narrowband on 80m.
Since most of my operating will be CW, I focused the tuning specifically on the CW portion of the band rather than the whole thing.
The sweeps
| Frequency | SWR |
|---|---|
| 3.500 MHz | 1.35 |
| 3.519 MHz | 1.24 |
| 3.590 MHz | 2.07 |
Since 80m is extended using a loading coil, the bandwidth there is narrow. I managed to place the resonant frequency close to the center of the CW band, which puts the 2:1 SWR point at 3.590 MHz — as a result, the entire CW band falls within that bandwidth.
| Frequency | SWR |
|---|---|
| 7.000 MHz | 1.19 |
| 7.116 MHz | 1.05 |
| 7.200 MHz | 1.13 |
On 40m, I tuned the resonant frequency slightly higher instead. That brings 20m out a bit better, yet the SWR across the entire band still stays below 1.2.
| Frequency | SWR |
|---|---|
| 13.999 MHz | 1.61 |
| 14.350 MHz | 2.06 |
Because I set the resonance point on 40m right in the middle of the band, the SWR on the full 20m band just barely fails to stay within 2:1.
| Frequency | SWR |
|---|---|
| 20.635 MHz | 1.10 |
| 20.998 MHz | 1.22 |
| 21.450 MHz | 1.63 |
On 15m, the resonance sits slightly below the band. As a result, the CW portion is reasonably good, though toward the top end of the band the SWR climbs to 1.63:1. Certainly usable for CW.
| Frequency | SWR |
|---|---|
| 28.000 MHz | 1.19 |
| 28.217 MHz | 1.05 |
| 28.700 MHz | 1.13 |
The 10m band is a wide band; I only measured the first section of it. The CW portion looks very good, comfortably under 1.2:1.
** A small disclaimer on the data:** right now I'm literally taking photos of the nanoVNA screen, which is not exactly the cleanest way to document results. I'm planning to look into NanoVNA Saver soon so I can log and plot this properly instead of pointing a phone camera at a tiny display. For now, though, this gets the job done and tells me what I need to know.
Comments