[CQ-Contest] Bandpass filter

Jukka Klemola jpklemola at gmail.com
Tue Jul 3 10:16:02 PDT 2012

>>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.

Correct for fully functioning and tuned amplifier.

In case a glitch happens or when tuning the amp, the harmonic emission
level may exceed the presented figures.

> to that suppression, we have coupling losses between the antennas that are
> likely 20 dB or more.

Likely more than 20dB yes.
It saves from small glitches and amp mistunings.

But does not fully prevent the 20m RX from frying.

> 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.

Considering the others is important.

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

Regarding a far-away receiver, correct.
An immediate next property neighbor might get a frying shot into the front end.

At your own station the 40m fundamental signal may go in your receiver.

I mean, if you have a good harmonic suppression, it does not prevent
your 20m RX from getting exposed to the 40m fundamental.
You can test this phenomena.
Put a watt meter to your 20m antenna and a dummy load after the watt meter.

Do not play surprised if the power meter shows some tens of watts if
the antenna separation is less than a tenth of wave on the lower band.

It would certainly destroy .. not only fry your 20m receiver.

> 73 Tom

The real recipe for a real multiband setup with two or more radios is two fold:

1. to have a harmonic rejecting system after the active amplifier.
That may be stubs or traps or other kind of band reject or some band
pass construction.

2. To have a band reject or band pass system for the active receiver.
..regardless the band combo..
For example a TX being on 20m might destroy a RX on 15m just with the
power of the fundamental 20m frequency propagating to the RX on 15.

In case you want to go for more challenging designs, like two radios
on one band, build up the attenuation between the two radios somehow.
Building that attenuation usually requires some level creativity.
Like Tom W8JI pointed out a vertical and a horizontal antenna null
each other when the dipole faces the vertical .. not when the dipole
has it's end towards the vertical.

I think I will add some of this on www.oh4a.fi.

Jukka OH6LI

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