A 10m end-fed halfwave antenna
January 1, 2025 (published in QRP quarterly 2025)
For lightweight, low power portable operation, an obvious
choice was an end-fed halfwave (EFHW) antenna cut for 10m and built with
lightweight wire. I had a stash of #26 AWG Teflon coated wire purchased
at discount from eBay that would serve well. The remaining questions were the
details of the matching transformer, the feed line, and counterpoise.
There are plenty of on-line articles and videos with design details for
multi-band EFHWs, usually built for 40 through 10m, but I could not find anything
for a dedicated 10m version. Specifically, I needed to know what number of
turns to use on the matching transformer.
The antenna consists of a coax feed/counterpoise section, a
matching transformer, and a radiating wire. The overall design is shown
in Figure 1. I have previously had good experience using a short piece of
RG-316 coax as both a short feed line and as a counterpoise, so I decided to go
that route. Such configurations can be susceptible to noise pickup, but
on remote summits this is almost never a problem. The usuaI advice is to
make the counterpoise for an EFHW about 1/20 of a wavelength long on its intended
band of operation. I used about 20 inches of coax to go from a BNC connector
(where it would attach to the radio) to the matching transformer. Many
people have had success at QRP levels with foregoing a feed/counterpoise
section like this and simply relying on the natural capacitance to ground of
the radio itself to serve as a virtual counterpoise. I decided to use
the feed/counterpoise section because I think the SWR of the system is a little
less susceptible to variations in antenna deployment, terrain, and operating
conditions. It also allows one to more easily elevate the end of the
antenna wire a bit off the ground, which I imagine can help with efficiency in
some cases.
Figure 1. Design diagram of 10m EFHM antenna. |
For the matching transformer, I decided to use my nanoVNA
and test a few different windings to find something that looked like it would
work. I was aware that an EFHW antenna presents a high impedance at the feed
point, of order a few thousand ohms, and so during this trial-and-error process
I substituted a 2450 ohm resistor in place of the antenna radiating wire.
This made testing a variety of windings quicker and easier than attaching
to a radiating element and raising it on a mast for each iteration.
The toroid that I had available for the transformer was an
FT50-43 (Fair-Rite part number 5943001101) ordered from Mouser. I tested
several different variations for the number of turns for the primary and
secondary windings. For each test, I used the nanoVNA to measure SWR and
return loss to the S11 port and used the substitute resistor in place of the
antenna wire. I found that I could get a pretty good match with 2 turns on the
primary and 10 turns on the secondary. I used #24 AWG enamel wire
for the windings and did not use a crossover winding or anything fancy. The primary and secondary wires are twisted
together. Unlike many multi-band EFHWs, there is no additional capacitor
across either the primary or secondary turns.
An image of the transformer attached to the coax is shown in Figure 2.
I do not have a good explanation for why the turns ratio is relatively
low compared to usual multi-band EFHWs, but the result was effective.
Figure 2. Transformer used for the antenna.
To complete the antenna, I removed the substitute resistor and attached a 17 foot long #26 AWG radiating wire. I deployed the antenna on a mast in my yard and trimmed the wire to achieve a good SWR at 28.100 MHz. This took a couple of iterations, and I wound up trimming a couple inches in total. I measured the antenna in a couple of different deployment configurations. When erected as a 45-degree sloper to a mast with the BNC end attached to the nanoVNA on the ground, the minimum SWR was better than 1.2:1. When raised completely vertically it was around 1.4:1, and it was somewhat better than that if it was raised high enough to put the nanoVNA at waist level.
We've now used the antenna on several occasions with good
success. One notable experience was on Antimony Peak, a 6800’ peak in
Kern County, California. Both W6LOR and I worked a number of Midwest
stations with this antenna and the Rockmite running 250mW. It’s also
nice that it makes for a very compact kit in one’s pack. A photo of the
completed antenna, wound up on a kite string winder, is shown in Figure 3.
The photo is from a hike and SOTA activation in Wyoming.
I hope this short article is useful as a starting point for building a 10m EFHW for anyone who is interested.
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