[TowerTalk] measuring stubs

[k6ll@juno.com]

On Fri, 22 Jan 1999 07:27:07 -0300 "LU5CW Ernesto Grueneberg"
<lu5cw@geocities.com> writes:
>
>after browsing archives, couldnīt find a clear answer. Can anyone 
>provide a
>good method to cut stubs?
>Thanks. 73

Here's my favorite method:

1. Cut the stub about 2 feet longer than the calculated length.

2. Put a PL-259 on one end.

3. Make sure the stub presents a low impedance at the frequency
   you are trying to measure. In other words, if the stub is
   an ODD multiple of 1/4 wavelength (1/4, 3/4...), leave the
   open end of the coax unshorted. If you are making a stub that
   is an EVEN multiple of 1/4 wavelength (2/4, 4/4...), put a
   temporary short on the open end. It's kind of a pain to keep
   making these temporary shorts as you snip the cable, so another
   approach is choose another frequency and leave the end unshorted.
   For example, if you are making a half-wave stub for 14 mhz, do
   the measurements at 7 mhz (1/4 wavelength) and leave the end
   unshorted. As you get close to the desired frequency, you may
   want to go back to 14 mhz and the temporary shorts.

3. Connect the center pin of the PL-259 to the shell with a
   temporary piece of wire about 1 inch long. Arrange the wire
   so that you have a small one-turn loop formed with the
   center pin, connector shell and piece of wire. This is a
   coupling loop for a grid dip oscillator.

4. Lightly couple the gdo to the one-turn loop and start snipping
   the coax until it's on frequency. Monitor the gdo frequency
   with an accurate receiver.

5. If a gdo is not available, you can use an antenna analyzer in
   the impedance-measurement mode. Instead of steps 3 and 4 above,
   connect the analyzer to the PL-259 and start snipping for
   minimum Z at the desired frequency.

Dave Hachadorian, K6LL
Yuma, AZ
K6LL@juno.com




















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