[CQ-Contest] Bandpass filter

Tom W8JI w8ji at w8ji.com
Tue Jul 3 18:37:28 PDT 2012

>I recently measured the power coming back down my 20m line while
> transmitting on 15m. It's over 135 watts of power when using my K3 and 
> Acom
> 2000 at 1500w forward output. Not milliwatts...

Fundamental power. That's no surprise if the antennas are real close in 
terms of wavelength. If you look at 160 meters, two dipoles 1/4 wave apart 
can have well over 20% of applied power into the RX, so it pretty reasonable 
to expect that with a 20M antenna near a 15M antenna.

> A W3NQN BPF on the 20m coax, placed before the wattmeter and dummy load 
> was
> able to knock the measured power down to 1w of back fed power.

That's the proper thing to do, if you can't move the antennas. You added a 
20M RX bandpass filter to the 20M system, to keep the 15M out.

A TX stub
> then placed on my 15m TX line then resulted in less than 1W being back fed
> on the 20m line.

Maybe from added 15M loss, or changing common mode currents, but certainly 
not from removing non-existent 20M energy from a 15 M transmitter.

> I know you're a smart guy Tom, but not sure where your numbers come from.

Nothing you said disagrees with a single thing that I said.

> My station QTH has room for one tower and my 20-10m antenna is a C31XR 
> with
> individual feedlines. I cannot go without good filters and expect my K3
> front ends to survive.

So you feed all that power to an element just a few feet from the second 
radio's antenna element. I'd expect to have major problems with fundamental 
signal levels. Certainly nothing I said contradicts that.

A 15M bandpass filter would not do a thing to help reduce your strong 20M 
antenna's signal from the 15M transmitter, by further filtering the 15M 
signal. We filter the receivers for the band they are on, not the 
transmitter for the band it is on, unless we operate near harmonics.

73 Tom 

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