I've got a finished design, that I tuned the matching network using bare boards.
Say for example, my tuned TX board has an over-the-air 3 meter reading on my spectrum analyzer at -50dbm. (This is at VSWR 1.2:1).
When I put the TX board, into the transmitter enclosure I see significant signal attenuation. My tuned TX board now has an over-the-air 3 meter reading on my spectrum analyzer at -75dbm. (At unknown VSWR).
Unfortunately, in the 4 months since I tuned these boards, my USB VNA is not working, so I had to re-order another one, so I can't measure the impedance in the enclosure quite yet.
I have chalked it up more to the plastic attenuating the signal, as opposed to a massive impedance change.
Is it possible there is that much change to the impedance when a perfectly tuned TX board is put into an enclosure?
UPDATE: It's definently not the plastic per se, weakening the signal. The TX signal gets weak when the LiPo battery is positioned in a certain way in the enclosure. Not sure if that is a change in the impedance or some other electro-phenomenon, until I get a working VNA to make the measurements.
RESULTS:
TX bare boards in your hand, battery hanging in free space: -50dbm.
TX in the enclosure, with battery inside (10mm away from the board – huge space): -75dbm.
TX in the enclosure, with battery hanging outside the enclosure: -48dbm
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(I tried shielding the battery with copper tape, it doesn't seem like it is some LiPo magic anti-RF waves are being emitted and messing with the transmitter IC or something else).
Best Answer
This sounds not just possible but likely, especially once you are working with decently high frequencies. This is also a common problem with antennas where their center frequency will shift (sometimes so much it no longer covers the band of interest). Depending on the material, it could be the plastic being very lossy (PVC is for instance very lossy at certain frequencies due the it being polarized) or the fact that your plastic is a dielectric that changes the impedance of your lines.