[CQ-Contest] Bandpass filter
jpklemola at gmail.com
Tue Jul 3 23:56:29 PDT 2012
Now this gets very interesting !
2012/7/4 Tom W8JI <w8ji at w8ji.com>:
>>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
>> 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.
That is, what Ton wrote earlier about, was the harmonic.
But a multi-radio station cannot rely only on harmonic design.
The fundamental frequency cannot exactly be heard despite it can
create a lot of heat.
You just need to prepare for it.
>> A W3NQN BPF on the 20m coax, placed before the wattmeter and dummy load
>> 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 hunderd watts measured at any BPF stopband may destroy the BPF .. be
it ICE, Dunestar, W3NQN or any other.
W3NQN, if done with real good capacitors may be able to take 100W on
stopband for some moments on the stopband.
To tone that hundred watts down, you need that band's TX stub in the
same line at the same time between the antenna and BPF.
Any receiver's internal BPF will go BBQ at ten watt level.
Why? .. the 100 or 200W specification is for the pass band.
The stop band power specification can be like a fiftieth part or so.
The stopband energy becomes heat. More than 95% of it goes to heat.
The passband energy goes through the filter .. a part of it; say 5% or
so would be transferred to heat. 95% goes to the output connector.
Everybody having a 40 or 50W soldering iron can picture the rest of it.
> 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.
15m stub system that rejects 20m provides this feature.
K2TR recipe via K1TTT web says a 23' piece of RG213 as a shorted stub
stops 10&20m; passing 15m.
I can verify this with measured results at OH4A.
>> 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
>> 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
Using a single boom for many antennas used by individual radios is a
little bit more challenging environment than a setup with separate
tower for separate bands/radios.
However, with proper understanding of the system, the usage of the one
boom can be optimized to a level .. higher isolation requirements
between bands as power might be increased just adds to complexity and
There have been successful contest expeditions using a single boom tri
band quad for 10-15-20.. they have used amplifiers and rather simple
band pass filtering.
That tells many things can be done also at home.
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