Martin Sole wrote:
> Thanks Jim,
>
> My original question, or one of them anyway, was do I need more isolation
> than a single relay will likely provide? From your answer I'd say the answer
> was no, not necessary. Now this is (slightly) at odds with what others have
> either hinted at or in the case of DX Engineering it seems outright stated.
> The thinking being that all connected wires pickup signals, the question in
> my mind is how relevant they are likely to be in the face of wanted signals
> from a resonant antenna. So my 40m dipole provides more of the broadcast
> signal that's on 7.4MHz than does my 20m quad, but with 40dB of isolation
> will that be something that needs caring about?
Not unless you're worried about 40dB down sidelobes<grin>?
>
> Just writing this down as I think about it, let's say I want to copy a
> signal at around S1 or about -100 as I seem to measure it, this on 20m with
> a resonant antenna. The big broadcast signal on 7.35MHz registers about
> S9(-73dBm) on the 20m quad and with the front end filters in the radio is of
> no real concern. With the 40m dipole however the same signal is 60 over 9,
> about -13dBm, with the 40dB through the out of circuit relay it will be at
> about -53dBm, 20 over nine. In fact this is clearly still within the ability
> of most modern radios to reject so should not impact anything much.
That's an excellent practical analysis. It's the same as if your
antenna had a -40dB sidelobe pointed at the interfering signal. Your
regular antenna probably doesn't suppress the signal that well.
Except,
> does it impact the noise floor of the radio? Does it add, together with all
> the other big signals in and around that area of the spectrum something that
> could possibly degrade my already weak 20m signal? What about all the other
> connected antennas adding their bit of noise from the area they are resonant
> on, or nearly so.
Not really... think of them as contributing their little sidelobes to
the overall pattern.
> The real question remains, will it be noticeable, will it make a difference?
> Maybe this is not quantifiable and I just need to build it anyway, maybe
> both ways and compare. Since both AS and DXE take pains to strive for better
> performance in his area than a single relay switchbox might provide I
> suppose we should be thinking that it has an impact at some level.
Their isolation requirement is driven by Tx into one port, and Rx hooked
to another port. Then, you need that 100dB kind of isolation. Or, if
you have a multi op setup, where you have fairly strong signals coupling
into the unused signal (much larger than your example of the SW
broadcast)...
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