> I have a problem with a really near 430Mz transmitter (my own) that is
> breaking though to the TV. The Transmitter isn't off tune and as far as I
> Know or can see, it isn't producing any side transmissions or birdies.
I had similar problems with my 2-meter transmissions affecting all the TVs
in the house.
The root of the problem was the use of a cheap Radio Shack TV signal
amplifier and distribution box. The strong local signal was overdriving
the amplifier (probably a 1- or 2-transistor broadband design) and causing
it to distort every signal passing through it.
You may be running into a similar problem, even if you don't have a
separate distribution amplifier. TV tuner circuits are often optimized
for weak-signal conditions, and may not have a high enough IP3 to hold
off a strong local signal. This "fundamental overload" can't be cured
at the transmitter, as you clearly understand - you have to keep the strong
signal out of the electronics which were being overdriven.
> So, My question is, does anyone know of a really good filter design or
> commercial filer that will block 430.625 so that it dosn't appear in the
> downlead to the TV.
You'll need a notch or bandstop filter, or a set of paralleled bandpass
filters, to allow the TV frequencies through and block the 430 MHz signal.
A notch or bandstop will probably be the best way to go.
The solution I found (after searching around for quite a while) is
a notch filter made by the Tin Lee company in Canada. They specialize
in TV filters of various sorts, and their selection is much richer
than I was able to find via any local supplier.
The one I bought was one of their CR7 models - a band-stop
type which knocks out everything from 130 MHz to 175 MHz.
Mounted right up at the antenna, it does a beautiful job...
the overload/interference problem has gone away entirely.
I suggest that you contact them at www.tinlee.com, and give them
the details of the frequency range(s) you want to pass and which
you wish to notch out. They may have a stock model which will
do what you want, they have some tunable models, and if all
else fails I'm sure they can build you a custom trap against
the frequency you're concerned about.
You could try building a coaxial-trap notch (e.g. a quarter-wave
open section of coax, or a half-wave shorted section of coax,
connected in parallel with the feedline) but this might have
adverse effects on other frequencies.
73,
Dave AE6EO
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