Hi. This may get a little long, but I'll try to be as accurate as possible....
I am refurbing my M2 6M7JHV. I had a failure at the end of one of the elements in the 'T' match. M2 uses a press fitting of a piece of copper into a slit in the end of the 'T' elements. One element connection was tight, the other was quite loose.
While checking the rest of the system, I noticed that water appeared to have infiltrated the balun. This despite the waterproof 'F' connectors and multiple layers of good electrical tape and several coats of clear lacquer.
Here's my observation / question:
M2's balun is a 1/2 wave (obviously) made from what appears to be RG6 cable.
Using my trusty MFJ 259B, I measured the resonant frequency to be 48.130 MHz. (X=0 on the MFJ). Since this is a 1/2 wavelength section, the far end was shorted. I used my 756PRO to double check the frequency on the 259B. They were spot on.
Next, I measured the capacity of the line (unshorted, natch) using my LCR bridge which was calibrated to NIST specs. The capacity was 350 pf.
I then made my own balun using RG6 quad. It is EXACTLY the same length as M2's.
However, it measures 50.150 MHz (X=0 on the MFJ).
When I measured it's capacity, it measures 135 pf. Quite a difference, eh?
So, I'm unsure if M2's balun was cut to that frequency on purpose in order to work correctly with the rest of the matching system, or could the original coax have degraded to the point the capacity went up 3X?
I have professional RG6 quad connector attaching tools, so I am confident in my balun being well sealed.
M2 has been pretty helpful, but I think I've bothered them enough! I do NOT want to spend $75 for new parts, when my new mechanical/electrical connections are MUCH more positive.
If you perfer to correspond directly, to save everyone else the agony, here's my e-mails:email@example.com
thanks in advance!