[CQ-Contest] Bandpass filter

Keith Dutson kdutson at sbcglobal.net
Wed Jul 4 07:52:50 PDT 2012

Does not take rocket science to question your measurement of 135 watts
feedback power.  About the only way this could happen is if the two antennas
were physically attached together.  And that is even questionable based on
different bands being measured.  I think there is a flaw or
misrepresentation in your measurement.

Keith NM5G

-----Original Message-----
From: cq-contest-bounces at contesting.com
[mailto:cq-contest-bounces at contesting.com] On Behalf Of Timothy Coker
Sent: Tuesday, July 03, 2012 5:07 PM
To: Tom W8JI
Cc: cq-contest at contesting.com
Subject: Re: [CQ-Contest] Bandpass filter

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...

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

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


Tim /N6WIN
On Jul 3, 2012 8:02 AM, "Tom W8JI" <w8ji at w8ji.com> wrote:

> > 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 harmonic.
> 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
> 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 antennas.
> 73 Tom
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