[amsat-bb] 1/4 wave stub lesson learned.

Robert Bruninga bruninga at usna.edu
Tue Jul 25 22:26:15 UTC 2017

1/4 Wave stub lesson learned.

I always wondered exacly where one measured a ¼ wave coax stub.. the
shield, or the total connection length to the center coax?

ANSWER: The length of the center wire makes a difference.

We had poor second harmonics from our Cubesat VHF transmitter. Carefully
built a prototype shorted ¼ wave stub and tweaked it to over 26 dB of a
notch at the second harmonic.  But then to install it on the cubesat
transmitter board, we couldn’t quite get the prototype coax to fit right at
the transmitter connection and had to extend the center of the coax about

This detuned the stub by over 5 MHz at the 145 MHz operating frequency
though the length of the shield remained the same.

Now I know. Everything counts!

Fortunately it detuned it lower so I have two more cuts available on the
flight unit to bring it in…


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