1/4 wave stub lesson learned.
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 1/8”.
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…
Bob, WB4APR
When we installed the VOR in the Gold Coast I was moving the center pin in and out at 113 MHz !
Sent from my iPhone
On 26 Jul 2017, at 8:26 am, Robert Bruninga bruninga@usna.edu wrote:
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 1/8”.
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…
Bob, WB4APR _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
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Robert Bruninga