Hi Paul

Yes it looks like you have a real problem here..

- Yes running ribbon up a power duct is not a good idea. Generally
speaking ribbon is pretty good in rejecting noise when there is it least
a few inches of air spacing between it and metal objects. The nature of
ribbon (or balanced feeder) is that any noise induced in one wire will
also appear in the other and they will add out of phase in the receiver.

- Yes coax is a better idea, but be aware that the length of coax will
also receive in its own right if both the radio and antenna ends arent
terminated in its surge impedence (ie 75 ohms). There are some
variations on this statement but I wont go deeper.

- The wire coming out the back of the radio is probably loosely
referenced against its metal chassis. Unless you want to go inside the
radio your only real option is to join the coax outer to the chassis
near where the single wire exits, and the centre conductor to the wire
itself (after cutting it short or removing/replacing it)

- The above of course gives you an unknown RX load impedence.

- The existing power twin and earth might be usable except for the
possibility of induced noise. I assume you mounted a dipole at the top
of it? I would guess that it would be maybe 100-200 ohms impedence but I
dont know the effect of the earth wire to that. If you had nothing
better to do you could always make it into a tuned feeder length or
multiples of 1/2 wavelengths. Dunno if its worth the effort though.
(Tuned feeder installations allow you to have any feeder impedence
provided the source and load are the same impedence. Doubt its any point
doing in an RX system but worth mentioning out of interest)

- If patience was in huge supply I'd try matching at the receiver input
for a 75 ohm cable. I'd go inside the radio to do this.

- If the signal is still way too low it might be worthwhile
buying/building a masthead preamp to go near/on the antenna. Then noise
induced by the power duct and computer would then be lower. You may also
want to notch other strong stations at the input of the preamp with a
series of 1/4 wave stubs so the preamp doesnt overload.

Hope this helps - or at least doesnt confuse!

Cheers Bob (Sydney Australia)



Paul Davies wrote:
A supid question from someone who thought he knew a bit about
electronics...