[CQ-Contest] Bandpass filter

Timothy Coker n6win73 at gmail.com
Tue Jul 3 15:06:53 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...

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 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
> antennas.
> 73 Tom
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