[TowerTalk] Stubs

Jim Brown jim at audiosystemsgroup.com
Wed Nov 13 13:39:46 EST 2013

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.

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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