A loaded EFHW Antenna for 40/20/(15)/10

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UPDATE: In a previous revision of this post, I posted results on my VNA that showed this antenna gave a good SWR on virtually all the bands from 40-10. Further experimentation has shown this NOT to be the case. I believe the issue that was leading me to believe the “too good to be true” situation was a faulty connection on one of the connectors for the antenna. To revise – this antenna works as expected on 40/20/10 with it being useable (but not ideal) on 15m.

So, I’m currently waiting on the Australian Maritime College to process my callsign allocation and license application, and I thought I’d use the time while I’m waiting on my license to get an antenna idea off the ground that I’ve had for a while.

Essentially, I started out with the below design for an EFHW on 40/20/15.

https://www.hfkits.com/build-instruction-impedance-transformer-250w-for-end-fed-antennas/

The design is sound, and I’ve built one of these before, but there’s a problem with the design. I want to use it for QRP on SOTA activations and use it as a vertical. It’s 12m long. I wanted it to fit on a 10m fibreglass pole from www.haverford.com.au. How do we do this? Well, people run loaded verticals all the time with ground planes and radials, so why couldn’t the same be applied to an EFHW (or multiples thereof) design?

I did some research and couldn’t find any decent information on shortening EFHW antennas, so I broke it up into a few parts, firstly the unun.

Most EFHW antennas are fed with either a 49:1 or a 64:1 unun at the base point, but they’re usually large and bulky, awkward, use large cores and aren’t very fun things to wrangle when QRP – and heavy. Given lightweight is a mission goal here for use at SOTA activations, I took a leaf out of the qrpguys.com book, and decided to use a pair of stacked FT82-43 ferrites (They actually only use a single) and mount it all inside a nice custom designed 3D printed case. For the capacitor and counterpoise I decided to go a little off-script here, and use a 1m length of RG-174 coax, which provides not only the counterpoise but also the 100pf of capacitance needed to make the unun work.

Some 3d Printing design for a case, and this is what I came up with. A quick check on the NanoVNA showed that it looked reasonable enough with a 3.3k load to try it on the pole. The first step was to cut a 10m length of wire, wrap it around the pole, clip the unun on and see how it performed.

And once it was on the pole, a quick check on the NanoVNA showed it looked pretty good on 20m! Next was to take a step back and look at what I had built. Here’s how it looked while being monitored by one of the two all important supervisors.

I decided that the speaker wire I was using wasn’t going to be suitable for making the antenna with the coils, so I ordered a roll of 22AWG tefzel and waited.

When the tefzel arrived I referred back to the original article for adding the loading coil for 40m. In order to get the length down to about 9.5m overall I had to get creative, so a second loading coil would be needed lower on the pole. For the 40m loading coil I ended up settling on around 100 turns of 20AWG wire on a piece of 20mm conduit from Bunnings with some custom 3d printed end caps, designed to snap around the antenna mast at precisely 1.8m from the end of the pole – as the original article I’m referencing places the 40m coil at this location.

Next was to choose a place for the second loading coil, the location I chose was completely arbitrary, but I ended up going with 1.8m further up the pole from the un-un/feedpoint. Too low and it would end up interacting with the un-un and too high would place too much wind loading on the mast. I ended up winding 28 turns onto a 50mm long piece of the same 20mm conduit, again designing some 3d printed brackets to snap onto the antenna mast.

All in all, I’m pretty happy with it.

A couple of things will change for the next one I build, the Un-Un housing and coil brackets are printed in PLA which is fairly hydroscopic, so I’m not expecting them to last long in the weather and as they absorb moisture it will likely affect the tuning, the next batch will be printed in PETG for a bit more durability. The second will be that the coils are going to be wrapped or coated in a thin expoxy and the brackets glued to the loading coils.

I’m probably going to add the EFHW UnUn clip-on units and coil end caps, and a “complete antenna kit” including the right length of Tefzel wire to build this antenna and complete fully assembled antennas as offerings in the store. The Antenna is designed for a very specific 10m pole from Haverford, but if you already have a squid pole of a suitable length and can supply me the diameter sizings I can make the brackets the right size to clip onto your pole.

Hopefully I’ll see you on air soon, still waiting on my license to be allocated.

73
Josh Mesilane
SWL – for now