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Old 26-May-2020, 6:09 AM   #4
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 153
Quote:
Originally Posted by bobsgarage View Post
I'm just wondering if my screenshots are showing in the post above? ...they were all there
They?

See one image. It is open combiner case.

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There could be other difficulties, but a noticable one is:

A difficulty with combining UHF with Q-wave matching is the union of the three cables.
All parameters are more critical at UHF, than VHF.

Inside the open case, the impedance is no longer 50 Ohms or 75 Ohms.
Therefore the correct impedance match of 37.5 Ohms (parallel combo)
to 75 Ohms via the Q-line is not occurring.

Since at VHF the dimensions (wavelength related) are about three times larger,
then the mismatch caused by impedance bump at the joint has less effect (although it is still there).

Best to join all cable as closely as possible (including shields).
Maybe could do that by drilling the case and running shields all in.
That means very short distances of cable that are not fully shielded.
Even this approach is considerably less than perfect.

There could be other factors not considered.

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