[TowerTalk] Stubs and Reactances in parallel

[Jim Brown]

Steve,

Wes's question and your reply yields the key. You're switching the 
resonance of the antenna, but not the stub. Take a look at the plots in 
http://k9yc.com/Coax-Stubs.pdf  which I did by point-by-point 
measurements before I had a VNA. The stub is only a very high impedance 
at resonance, and the Q is too high to cover as much of the band as 
you're attempting to cover. Note from my app note that the location of 
the stub along the line matters. You can probably get away with a 
compromise of the location between CW and SSB, but you will need to 
switch the stub, either by switching in added length or switching 
between two stubs, and you'll need a very good coax relay.

And the Z of that stub is only resistive at resonance -- either side 
it's L or C.

SimSmith is a good tool for doing this design. You'll need to measure 
the 80M antenna using a good analyzer with both CW and SSB settings and 
with only the coax, no stubs, export data from both measurements to 
SimSmith, and design from there. Note in the earlier reference I posted 
that it's possible to stagger-tune two stubs to cover greater bandwidth. 
This is a very tricky design problem because of the bandwidth.

73, Jim K9YC

  On 6/24/2020 5:10 PM, Jim Brown wrote:
> On 6/24/2020 4:10 PM, Steve London wrote:
>> The stub on the 80 meter dipole does kill the interaction to the 40 
>> meter beam. However, the Z of the stub is around 1000 ohms 80 meters, 
>> so it affects the 80 meter dipole. Read my earlier writeup carefully 
>> for details.
> 
> Then it's the wrong stub. See the link I posted.