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…
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.
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!
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.
