Jon Ogden wrote:
>
>Anyhow, back to my current problem....I am trying to do two things.
>First of all, yes I am trying to protect the RX. I do VHF rover
>contesting at least once a year and all of the radios creates some havoc!
> So the filtering is to help there.
As Jon pointed out, his brick contains a preamp, so he wants to protect
that too. However, that STILL doesn't require the filter to be capable
of handling full TX power.
Most commonly, when people add a preamp and run into RX overload
problems, the overloading takes place in the transceiver after the
preamp - because the preamp has increased the level of all signals
coming through it. In that scenario "A", a bandpass filter between the
brick and the transceiver can protect the transceiver where the overload
is occurring.
Scenario B is that the preamp is overloading as well. OK - switch it out
completely, and see if the transceiver gives better copy through the
filter without the preamp.
> I also want to make sure I put out a
>clean signal. I have a neighbor who said I was getting into his business
>band radio. It's likely that his business band HT just has a wide front
>end and it isn't my fault. However, I want to be sure and have the
>filter in place so that I can say that the problem isn't due to me. I
>would like to put out the best signal possible.
If you have the bandpass filter between the transceiver and the PA,
you're driving the PA with the cleanest possible signal. What comes out
of the PA is (bigger clean signal) + (close-in IMD) + (harmonics)...
nothing else.
None of these can benefit from a bandpass filter a few MHz wide.
If you want to reduce the close-in IMD, reduce the drive level, improve
the bias circuit (details on my web site) or get a better brick. If you
want to reduce harmonics, use a lowpass filter or a harmonic notch
filter (again, details on my web site).
I can't ever see a valid case for a QRO bandpass filter (except for the
convenience of filtering the incoming RX signal).
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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