High(er) EFHW SWR, PVC-U puzzles

I thought I was smart fitting my 49:1 unun + cap into an left over piece of PVC-U tubing I had kicking around… its does great at protecting it from my abuse.


I built another unun using the same model ferrite core and cap to help a friend, enclosed using a PLA container and used heat shrink to protect it. I get consistent and significantly lower SWR when this pipe is not involved (using the same wire and coax) on all bands.

I’m a bit stumped as to why - maybe I wasn’t so smart, I’m beginning to suspect that maybe the PVC-U has a conductive additive that I have overlooked ? or is something else being a gremlin…

1 Like

PVC is quite a lossy dielectric, that may be a factor if the torroid windings are close to the pipe…?

1 Like

I didn’t know that. Torroid is right up-against the PVC, it a tight fit, so well within the strong fields.

It might be worth a test by removing the unun and testing without. But i fear i kept the tails short and may have to rewind this or solder stubs…

A good indicator of whether a plastic is suitable to use in RF fields is to place a chunk of it in a microwave oven and run it up. If the plastic gets noticeably warm (or melts!) then it’s very lossy in RF fields. You will have to use your skill and judgement to ensure there is enough load in the oven to keep the Magnetron happy, I always place a large mug of cold water in at the same time.


LOL, I was worried about risking the wrath of my wife with dinner tasting of PVC……

I’ll give it a try with the pipe

1 Like

I should have said run it up for maybe 30secs max.

I can report Significant warmth…. 25seconds

And generated an oily sheen:

I just hope thats not last nights dinner


I would definitely change the plastic raindome but only after more tests on the prospective tube.



There are a lot of different plastic boxes (e.g. on eBay), most of which are made of PP. They are intended for storing, collecting, sorting,… and they come in all sorts of colors, transparent, shapes and sizes… for little money! Some are even rainproof. - There shouldn’t be any such problems with this!

73 Armin

1 Like

Thanks everyone. I got rid of it and replaced with a mix of PLA + heat shrink. Much better results with SWR over the weekend…


This now looks very similar to my 2016 Slim EFHW Coupler version (also used by HB9BIN on the 4 week SOTA Balkan trip with HB9DST in autumn 2017 …).

BTW, when installed in a small housing or when shrink-fitting, the permittivity of the (near or directly adjacent) housing material results in an undesirable influence on the coupler characteristics.
This usually results in overcompensation of the coupler measured before installation in a housing, which is advantageous in terms of SWR on the lower bands but disadvantageous on the higher bands. The value of the compensation capacitor can/should therefore be chosen 10-20 pF lower (result to be checked after boxing).

In any case, after changing or replacing the impedance coupler, it is recommended to check and, if necessary, adjust the antenna length to the desired resonance frequencies.