Dave Richards AA7EE

October 5, 2016

An End Fed Halfwave Antenna for Portable Ops

I’m a very casual operator, and an even more casual portable operator. My main reason for not putting much effort into portable operation is that when I go out into nature, I want to enjoy my surroundings and not be distracted by radios. It sounds like an excuse, but it’s true. I spend quite a lot of time hunched over the bench and over my radios at home so when I go out, I don’t want to do the same. I’m more the kind of guy who builds small rigs, then operates them from the comfort of my own home. However, I had to take the SST out at least once simply to prove that I can!

The antenna needed to be compact and lightweight, as did the method of matching it. I just didn’t feel like carrying lots of boxes and interconnecting cables up the hill, and having to fiddle with them all once up there. An end-fed halfwave, often referred to by the acronym EFHW, seemed to be a good choice, as it only requires a support at the far end. I saw photos that Steve WG0AT had posted on Facebook of his little EFHW, with the matching unit built into a dental floss container, for a light and compact solution. I wanted an antenna that small and lightweight! Steve referenced a blog post by TJ W0EA, in which TJ detailed an EFHW matching unit he had made, based on the one in his Par End Fedz antenna. This little matching unit, that transforms the high impedance present at the end of a half-wave length of wire into the much lower impedance of 50 ohm coax, consists of a wideband transformer wound on a ferrite core, and a 150pF fixed capacitor. That’s it. Simple and compact!

What to put it in, was the big question. I spent several weeks looking in stores for suitable small containers, and finally decided on a Carmex lip-balm tube. Here it is with the lip-balm removed –

The remaining tube still has a corkscrew-like central element that needs removing –

It is a fairly simple matter to grasp  the corkscrew with a pair of long and slim needle-nose pliers, and push it until it pops out. You can discard the corkscrew, as it is not needed. The 2 parts on the left of the next picture, the snap-on lid and the main cylinder, are what you want –

The following pictures should show you how it all goes together. A plastic cable tie prevents the RG174 from pulling out of the bottom, and a generous squodge of hot glue keeps the toroid in check. If you have a dual temperature glue gun, use the hotter setting –

This matching unit is designed to work with a half-wavelength wire. Some folk build it so that they can change the wire length for different bands. I decided to make this a permanent 20M antenna, so started with about 36 feet, and continued to trim it down until the center frequency was close to 14060, at which point the SWR was 1.1:1. Not bad! I’ll state the obvious by reminding you that any antenna does need to be reasonably clear of nearby objects, particularly anything conductive, in order to make meaningful measurements. Laying it on the ground isn’t going to cut it – you need to suspend one end up in the air and have the antenna clear of obstructions. This is what my final EFHW looked like, all bundled up and ready for the trail, with a 10 foot length of RG174 –

Interestingly, a few days later, I checked the SWR again, only to find that although the center frequency was the same, the SWR at that point was higher, at about 1.4:1. The only thing that had changed was that the first time I measured the SWR, I was powering my MFJ SWR Analyzer from a “wall wart” transformer while the second time, it was powered from internal batteries. I’m thinking that the first time around, the AC wiring in the house was providing a bigger counterpoise and helping to lower the SWR at resonance. It might be interesting to try connecting a counterpoise wire at the rig to see if it reduces SWR any, but I did like the added simplicity of no counterpoise.

How does it work? I bundled the SST, antenna, small sealed lead acid battery, paddle from QRP Guys, and a few other things into my backpack, and cycled up to Vollmer Peak, a local high spot in the Berkeley Hills. I left rather late, had lunch on the way, and by the time I got up to the top, spent about 30 minutes eating trail mix and looking at the view, before realizing that I didn’t have much time. I didn’t get the antenna very high in the tree, and sat on the ground, listening, finishing off the trail mix, and putting out a few CQ’s before heading back down the hill. End result = no QSO’s, but I did get spots on the Reverse Beacon Network from Colorado, Arizona, and Alberta. The antenna works – it’s the operator who performs better in a cozy indoor shack 🙂

There is really only one more thing to try with my SST, and that is, as I mentioned in this post, to add extra filtering between the TX mixer and the buffer/driver. I think that a lot of harmonic energy is making it to the final and being amplified, before being filtered out by the LPF in the antenna lead. Better to nip all those naughty harmonics earlier in the process, I think. If I do any more work on it, that will be the focus.

Thanks to Ian MW0IAN (great callsign) for clueing me in to this PDF on the G0KYA EFHW.



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