QRP EFHW antenna (Richard VA7AA/XE1)

I read the blogs and news on the site from time to time… the blog on the QRP EFHW caught my eye.

I too purchased one of those fake QRPGuys autotransformer assemblies (Why Amazon allows this??)… they are indeed pure junk… the material the cores are made of is anyone’s guess. I knew they were fake and I just wanted the PC board to experiment with.. I wouldn’t want to ruin a genuine one. 

.. there is a tradeoff in going super small for aesthetics, cuteness and utility… but in general, efficiency goes down the tube. 

Recently a few guys discovered the Fair Rite 2643251002 cable core of 52 material. With that core using a wind format to traditional linear 14:2 for a 49:1 ratio, along with a high quality 100pf TDK cap will give you near 90% across most of the HF bands. If all your playing with is 5 or 10 watts you need all the help you can get.

So what’s different from the so called ‘traditional’ method?

The core is wound in 1 continuous winding 2:14 tapped 2 turns up from the ground point… there is NO crossover, who came up with that one I have no idea, it’s not needed and never was…. it was probably started for aesthetic purposes.  

The gauge of the enameled wire is not critical…. #18, even #20 is fine. remember @ 100 watts the RF current is less than 1.5 amps @ the 50 ohm feed point… you don’t need #14 wire for that. 

It’s important to use a quality 100pf cap… those cheapies you got off of Amazon 250 for $10 will not cut it… TDK from Dig ikey…you get the 2643251002 from Digi Key as well… and neither are expensive.  If you really are qrp even a 500v silver mica would work… they are of high quality. @100 watts the voltage is about 70V @ that 50 ohm point. 

The 2643251002 is more compact yet far superior to the FT 240 43… why? It’s about form factor… there is more meat to the core right where you want it… and where you put the windings on. 

Photos and supplementary info below.

(Richard backs up his work with research. Check out the videos below.)

… the Evil Lair… now this guy is off the wall weird… but he’s pretty much got it figured…

YouTube video – EFHW ham radio HF antenna transformer tests

YouTube video – EFHW xfmr geometry STILL matters!

Note I have not put my core in a container…at  least in the experimentation short term…. I have found there is no need as it is passive, as an experiment I let it sit out in a tropical rainstorm… made no difference. Works fine.. and much of the DX I hear….on a 130′ EFHW 80-10

Richard VA7AA/XE1

1 reply
  1. Alan
    Alan says:

    I enjoyed this article – thanks! I use a 66′ EFHW antenna on my city lot and am super pleased with it. The big advantage of an EFHW is how well it works on all the major ham bands – as good or better than dipoles.
    The critical component (as you note) is the 49:1 “unun” transformer. I made the mistake of buying one from China, which caused enormous frustration as it did not work. So I cut open the sealed container to find – surprise – a small ferrite bar wound with fine wire. The kind used in old transistor radios. Buyer Beware!
    I then ‘bit the bullet’ and wound my own toroid following the design instructions published by K1RF at http://gnarc.org/k1rfs-presentation-on-end-fed-half-wave-antennas. This was not as hard as I thought, but if people want to buy a ready-made 49:1 “unun” I recommend the ones from Balun Designs.

    Reply

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