My first couple of SOTA activations used a SOTABeams Band Hopper III linked dipole. This worked well, but with two wires, a 10m coax and an extra guy, it took a bit of time to set up and it was easy to get all the wires tangled if the wind got up. I started reading about alternatives and decided an End-Fed Half Wave (EFHW) looked like a good option.The benefits are:
- One matching unit could feed a range of elements that were just simple wires (or could be multi-band with links or traps)
- The matching unit was low to the ground in an inverted V configuration and hence could be fed with a short coax
- The antenna could be used as either an inverted V or a half wave vertical on the higher bands
The challenge with the EFHW is feeding it as the end is a high impedance point (apparently between 2000 – 6000 ohms depending on the surrounding environment). I explored a range of matching unit designs. They seem to fall into two main categories:
- A transformer with a tuned circuit for the secondary
- A simple impedance transformer with a fixed capacitor across the primary
The first model is well described by AA5TB (Steve Yates) along with several example designs. The main limitation of this design is the need to tune the tank circuit for the operating frequency in use. Designing a single matching unit that will cover a broad frequency range is challenging. It also requires a tuning capacitor that is capable of quite high voltages that appear at the end of a half wave antenna. The typical construction of such a matcher uses a Polyvaricon capacitor which limits the power to QRP levels only (noting the high voltages present at the end of the antenna).
The second design is that used by PAR EndFed antennas. It is a broadband match design with no tuning required and this appealed to me. The trade off is that you can’t get the SWR down to 1:1 except by luck, but it is possible to get a SWR below 2:1 for most frequencies of interest with a typical half wave wire. I found a couple of good references to such designs by Dutch hams here and here. They include details of creating multi-band EFHW antennas using this matching unit.
I followed the Dutch designs quite closely, but experimented a bit to see if the 150pF capacitor is the right size. In my case, I used a FT140-43 toroid (wound with a layer of Teflon tape for protection of the toroid and enamel coating) with a 3:24 turns ratio wound using 1 mm enameled copper wire. Here’s a photo of the competed unit:
I experimented by attaching a load resistor between the output and ground for a range of possible antenna impedances from 2200 to 6800 ohms. Here’s a series of SWR plots across the HF bands from 80m to 10m with first no capacitor and then a range of capacitor values as detailed on the images.
As you can see, the capacitor is clearly necessary in this design and 100 pF allows a better than 2:1 match across most HF bands and impedance values. Whilst 150 pF provides a better match at some frequencies, the 100 pF seems to provide a more even performance over a wider range of loads and frequencies. Values above 150 pF resulted in much worse plots and weren’t recorded.
It would be possible to replace the fixed capacitor with a variable capacitor (such as a Polyvaricon). This may allow the SWR to be reduced through tuning for the specific frequency and antenna impedance at the time. I haven’t tried this as the goal was to get adequate performance with no tuning needed. The advantage of this design with a variable capacitor is that the voltage across the capacitor is much lower than if the capacitor is on the high impedance side of the transformer. Hence a Polyvaricon in this position would likely be fine with 50W or possibly even 100W (with the sections wired in series).
You’ll see that there is no counterpoise with this matcher and the coax to the rig (and the rig and operator) performs that function. If this were a permanent high power set up, it may be necessary to add a counterpoise of some kind, but the Dutch amateurs seem to indicate that this does not appear to be necessary in practice even at high power.
The 3:24 turns ratio is better for the lower frequency bands and if I was building for the upper bands, I would probably have tried a 2:16 ratio instead. As you can see this design should be OK for 80m to 12m bands (which is what I wanted for SOTA activations).
In terms of power handling, I expect that the FT140-43 should be good to at least 50W. Above that, the core will likely saturate and losses will grow rapidly. I haven’t tested the loss of the unit, but hope to get round to that at some point. In practice it seems to work well with no problems making contacts with my 20W SSB transceiver. I’ve also driven it with my IC7000 in a field day contest (JMMFD) with up to about 70W with no obvious problems.
In practice, I’ve used this matcher mainly for 80m-20m to date and it has worked very well and makes antenna setup a breeze with no tuning necessary. I’ve also now tried it with my 40m-15m trapped EFHW and it seems to provide an acceptable SWR for all 5 bands.