[TowerTalk] Stubs

[Roger (K8RI) on TT]

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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