Bent Wire Radio

Why an EFHW?

Apartment living means I don't have a lot of options. I've got a small balcony and access to a fence post about 40 feet away that I can probably throw a line over. The end-fed half wave appealed to me because you only need one support point at the far end — the other end feeds directly into the transformer at the radio end, so I could mount that near the shack and run coax inside. No need to run the actual antenna wire through a window or anything weird. At least that was the theory.

The core of an EFHW transformer is basically a 49:1 impedance transformer wound on a ferrite toroid. The end-fed half wave antenna presents something in the neighborhood of 2450 ohms at its feed point, and you want to bring that down to 50 ohms to match your coax and radio. The 49:1 ratio does that — 49 times 50 is 2450. That part actually made sense to me once I sat down and thought it through.

Parts and First Attempt

I ordered a couple of FT-140-43 toroids off of an online supplier. The -43 material is what most of the builds I saw recommended for HF frequencies, especially the lower bands. I also grabbed some 26 AWG magnet wire from a local electronics shop — I probably could have gone heavier but that's what they had in stock.

The winding I went with was 3 turns primary and 21 turns secondary, which gives you that 7:1 turns ratio (and 7 squared is 49, so there's your impedance ratio). I followed a diagram I found that showed the primary going right at one end of the core, then the secondary wound over essentially the whole toroid. I also added a 100 pF silver mica capacitor across the primary based on some recommendations I saw — supposedly it helps with matching on the higher bands.

First attempt at winding it went okay but my secondary turns were really uneven and I could tell just looking at it that they weren't spaced well. I also nicked the enamel on a couple of the magnet wire turns with my fingernail while pulling the wire through. Scraped the ends off with sandpaper, soldered it up, and put it in a small weatherproof enclosure I got at the hardware store.

Testing — The Humbling Part

I hooked up a temporary wire I cut for 40 meters — about 66 feet for a half wave. Draped it out the balcony as best I could, thrown over a string line to the fence post. Nothing fancy, lots of sag in the middle. Connected my cheap nanoVNA to the coax port of the transformer and did a sweep.

It was not great. I was seeing an SWR of around 3.5:1 at 7.2 MHz. Definitely not 1:1 and my radio's internal tuner was struggling with it. I posted on a forum and somebody immediately asked if I had a counterpoise. I did not. I had completely forgotten about the counterpoise. End-fed antennas need something to push against — the common solution is a short wire connected to the ground terminal of the transformer, usually around 0.05 wavelength long, so about 6-7 feet for 40 meters.

I added an 8-foot wire to the ground lug of the enclosure, just let it hang down off the balcony. Swept it again on the nanoVNA. SWR dropped to about 1.4:1 right at 7.150 MHz. That I can work with. My radio's built-in tuner brought it right down to 1:1 and I was on the air.

On the Air Results

Made my first contact with the new antenna on 40 meters a couple evenings after I finished it. Worked a station about 600 miles away running 50 watts and got a solid 57 signal report. Not earth-shattering but way better than the cookie sheet whip setup. On 80 meters it's a bit rougher — I'm running it as a non-resonant wire on 80 with the tuner doing more work, and I can tell the transformer gets a little warm after longer transmissions. I'm keeping key-down time short for now.

I did notice some RF in the shack — the kind where you touch the radio and get a little tingle, or the audio gets a weird buzz on transmit. I think I need to add a common mode choke on the coax, which I haven't done yet. That's my next project. Probably 8-10 turns of the coax through another toroid right at the transformer output.

What I'd Do Differently

Honestly, not a ton. The core build was pretty straightforward once I accepted I might have to rewind it. I'd probably use slightly heavier wire next time — the 26 AWG feels fragile when you're winding it around the toroid. I'd also drill the enclosure holes before winding the transformer, not after, because I cracked the lid a little trying to drill near the PL-259 connector hole.

The counterpoise thing was a dumb oversight on my part. Read the instructions, people. Or in this case, read all of the forum posts, not just the winding diagrams.

Total cost was around $25 for the toroids and wire, maybe $8 for the enclosure, and I already had coax and connectors sitting around. Way cheaper than buying a commercial EFHW transformer, and I actually understand how it works now, which feels like kind of a big deal to me honestly.

If you're newer like me and thinking about trying this, I'd say go for it. The math is accessible, the winding is tedious but not hard, and there's a ton of help online. Just remember your counterpoise. Don't be me.

I'll probably post an update once I get the choke sorted out and do some more testing on the higher bands. 73 and thanks for reading.