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.
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