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SWR sweep of different EFHW 49:1 transformers

Updated: May 5, 2023

I had a client contact me a couple of days ago with an issue with one of my transformers a 2xFT240-43 transformer. To help them out, I completed a sweep of my EFHW I use at home, it's the same spec as the one posted to my client.

It's a simple thing to if you own or can borrow a VNA or Antenna Analyser. All you do is hookup a 3.2kohm resister from the antenna connection to ground and then sweep the transformer. See below the sweep setup a RigExpert AA54 and a scan of my 2xFT240-43 antenna.

This is the AntScope sweep from the above setup, I dangled the transformer off the desk to simulate it in free space (sorry bad joke).

Then I though, hang on lets do some more sweeps. The next one is an Ultra Light EFHW with a Fair-Rite T50-43.

Then a mid size PL239 with a Fair-Rite FT140-43. The T140-43 is my choice for SOTA EFHW (it may not be the most efficient, but its light).

All the above transformers are 49:1 windings and I use them all for various activations and at my home QTH, except a 56:1 winding for the mid size core.

You will see they are all different. I know through experiance that the large 2xFT240-43 cores do handle more power at lower frequencies than the smaller cores. But at the expense of SWR on the higher bands, and this also works in reverse for some of the other cores. Also, your windings change the SWR on EFHW. So the nack is to pick a core/winding for what you would like to acheive.

At home I use a 49:1 2xFT240-43 for my 80-10m antenna for 80-20m.

Under test I have a 56:1 widing using a Fair-Rite Mix 43 - 1.54" by .875" I think these toroids will give a good balance of medium power handling, efficiency on the higher bands. But for now I'm trying them out in different configurations.

The last thought was, well I might as well pop the transformer back in situ and sweep my main antenna. Warts and all here it is 2xT240-43.

It's been up now for two years and I think the antenna wire has stretched a little looking at the 80m band. If you wondering 80m is a little high, but this is due to the height of the antenna and whats below it. All the other bands are acceptable, 80m tunes ok but it is a little high. But I'm not that worried about it at the moment. I do not have a compensation coil for phone on 10m or a trap in situ for 60m bands. Also, during the sweep my common mode choke was out of line, which is not ideal.

I will pop some of the other transformers onto my antenna wire soon to see what happens when the inductance of the transformer windings changes v's SWR in a following post.

But for now 73's


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