[CQ-Contest] Bandpass filter

Tom W8JI w8ji at w8ji.com
Tue Jul 3 08:00:32 PDT 2012

> My understanding is that the main function of my 40m BPF (when thinking of
> my second radio on 20m) is to reduce the second harmonic on 20m produced 
> by
> my 40m TX.

Nearly all modern radios have excellent harmonic suppression, while external 
power amplifiers do not.

If we have a second harmonic issue, a critically located stub or stubs, or a 
trap, is far more effective and less costly than a high power BPF.

The distance from the source and load, and the characteristics of the 
suppression device, source, and load, usually greatly affect the null depth 
of any suppression. For example optimum stub placement from a pi-L network 
is quite different than optimum placement from a pi network or T network. 
It's different for every antenna, too.

The tradeoff is always cost and work, and immunity to system 
characteristics. A great big expensive filter can work well no matter where 
it is placed in the line. A cheap system, like a stub, might not.

>So if I am on 7.025 running, I will create a very strong second
> harmonic signal on 14.050. Depending on my TX power, antenna proximity, 
> and
> antenna polarization, that signal might be enough to couple enough power 
> on
> my 20m line to fry my second radio front end.

I doubt that would happen. Most amps are -46dBc or more. With 1500 watts, 
that's about 40 milliwatts. 40 milliwatts will never fry a receiver. Added 
to that suppression, we have coupling losses between the antennas that are 
likely 20 dB or more.

The only reason to add a filter or stub is if something is out of spec, or 
someone closeby wants to operate right on top of the second harmonic. For 
example, I can hear, and be bothered by, harmonics that are only 100 
microwatts from hundreds of miles away, **IF** I happen to operate on that 

Of course that level is thousands of times less than a level that could be 
damaging to another local receiver.

The real function of a bandpass on transmitting is mitigating problems from 
someone, local or distant, operating near the harmonic.

If there are issues with TX noise bothering our receivers on an adjacent 
band except near the harmonic, it is worth looking closely at the gear. 
Something is wrong, beyond lacking an extra filter.

At even 30-40 dB of external TX antenna filtering, cabinet and line cord 
leakage will usually dominate harmonic and spurious radiation into the RX 

73 Tom 

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