On 11/13/2013 1:39 PM, Jim Brown wrote:
On 11/13/2013 10:22 AM, Tom Osborne wrote:
Do I make the stub for the correct length, hooked to the tuner on the
output
of the amp, or do I need to subtract the length from the output of the
exciter to the output of the tuner?
The length of the stub is to the point where it is Teed with the main
coax. But that's not all -- it is VERY important to tune the stub at
the frequency you want the stub to kill, using something like an
antenna analyzer. The procedure is pretty simple -- if you want the
stub to kill the 2nd harmonic, tweak the length of the stub until the
analyzer sees a short at the second harmonic. If you want the stub to
kill the fundamental of the band below you, tweak that stub at that
lower frequency.
As a quick check, if you have a length (the longer, the better) of the
same coax, same lot. Hook that to your analyzer with the proper velocity
factor entered and see if it come up with the same length. If not,
adjust the VF to get the proper length and use that VF when figuring
lengths. It should be very close.
You might find it worth the effort and you might not.
73
Roger (K8RI)
Another very important point -- if you are going to switch in stubs
for multiple bands, you must make the measurement through the switch
and the short length of coax between the switch and the Tee.
More detail about this in k9yc.com/Coax-Stubs.pdf
Before cutting your first stub, figure out what the Vf is for the coax
you're using, and don't count on the published value being right.
Most solid dielectric coax is close to 0.66, and most foam is in the
range of 0.75 - 0.85.
73, Jim K9YC
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