Topband: Broadcast interfence on a MFJ analyzer

[Tom Rauch]

Sometimes if the BC station is nearby it is impossible to 
get the AM signal
out of the antenna analyzer.>>


That just isn't accurate. It becomes more difficult, but 
isn't imposssible.


 2.    Put a high-pass filter in series with the analizer. 
I did find it
worked almost equally well as the 2 watts to the RX antenna. 
Also it still
gave me some meaningful reactance readings I could not read 
with just 2
watts down the line.  It may take some phase shifting with a 
smith chart to
correct them, but the readings helped me.>>

I'd never do this. A highpass filter has a certain impedance 
distributed along its electrical length, and that transforms 
impedances. Multiple pole bandpass filters always have a lot 
of SWR ripple when the cut off a few percent outside the 
desired band.

The solution is really very simple if we stop and think 
about it. Just use a parallel tuned circuit with reasonable 
LOADED Q across the line. Since the line is 50 ohms the 
reactance could be 10-25 ohms for a Q of 2-5. If unloaded Q 
is reasonably high, even only 50 or 100, there would be 
little resistive loading of the line at resonance.

Now, in parallel with that to ground, add a series circuit 
with high Q. That means high values of reactance. On 160 you 
would have inductive reactance if the circuit was tuned to 
null something in the AM band. Now all you need to do is 
offset the initial parallel tuned circuit to compensate the 
inductive reactance.

The result is at series resonance a very low impedance, and 
at parallel resonance a fairly high impedance. Now you don't 
have any phase shift or transmission line effects to deal 
with.

With large components like air-dux I can get over 50dB notch 
on the upper end of the BC band without hurting 160. With 
cheap small components 20 dB or more.

Try that using a highpass.

73 Tom