[CQ-Contest] Bandpass filter

Joe nss at mwt.net
Wed Jul 4 05:49:56 PDT 2012


This has me curious.  How did you measure the power coming back down the 
other feedline?  Sounds like a simple question, Is it as simple as a 
wattmeter in line and dummy load?

The Original Rolling Ball Clock
Idle Tyme
On 7/3/2012 5:06 PM, Timothy Coker wrote:
> 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.
> 73,
> 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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