That sounds more like a mismatch issue than dielectric loss. Standard
PVC tape has a dielectric constant (Er) about the same as the
polyethylene insulator in the cable. So as your joint must have
increased the diameter of the inner conductor, its Zo will have reduced
and given you a small section of mismatched line at 50cm (cable)
wavelength. However, as the foamed is part air the resultant lower Er
would partly compensate for the increased conductor diameter. Mind
you, PVC is a lot more lossy than polyethylene so still some element of
real loss in this.
Andy G4JNT
I used to use plastic sticky tape (any colour) to insulate
between inner
and outer of RG-213 coax cable where I'd joined it (typical
amateur using
odd lengths!) . Then I found I was losing 1-2 dB at each
joint at 70 cms so
abandoned it and used foamed polyethylene instead which is
fine. Don't
know if there are different types of plastic tape with
different RF properties.
Walter G3JKV.
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