----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Jim Lux" <jimlux@earthlink.net>; "RCARIELLO" <RCARIELLO@si.rr.com>;
"Towertalk" <Towertalk@contesting.com>
Sent: Tuesday, January 04, 2005 11:02 PM
Subject: Re: [TowerTalk] Help need Vacuum Relay Isolation 1.8 to 30 MHz
> > Even though the relay mfr only guarantees 40dB, you have
> seen 60 dB?
>
> Sure. I measure it all the time with accurate instruments.
Useful to know... Thanks.
>
> RatPak, etc.
>
> The Rat Pack I measured had -51.7 dB crosstalk on 14 MHz,
> and -45.3dB on 30 MHz.
Did you happen to measure the impedance characteristics of the cross talk?
That is, given that the isolation decreases with increasing frequency, it
would appear to be primarily capacitive, as opposed to, say, transformer
style inductive coupling.
Mostly, I'm just curious, but not curious enough to go out and hook up a
relay to a network analyzer, especially if someone else has already done it.
>
> > But even with 60dB, you're still putting 0dBm into the
> second receiver. If
> > you're expecting same band operation, that's a pretty
> challenging dynamic
> > range requirement if you're looking for narrow band
> signals at -150 dBm
> > close to the noise floor.
>
> I don't understand why anyone would worry about that.
Here's a scenario..
If you had several fairly directional antennas, pointed different
directions, with good isolation (say they were on opposite sides of a hill..
a situation mentioned on the list recently), and you wanted to hook a
transceiver to one antenna, and use a receiver to monitor the other at the
same time, on the same band, and then be able to switch roles quickly.
(Hey, maybe you're looking for your own signal after going all the way
around the world?)
>
> Not only is that clearly NOT how he intended to use the
> switch, the path loss between antennas and other things
> external to the switch would be an even bigger problem.
I don't recall exactly what the original post was (it's on another
computer), but wasn't he switching multiple antennas to either of two
radios? Antenna 1 goes to radio A or B, Antenna 2 goes to Radio A or B,
etc. Antenna to the moving arm, radio A on one relay contact, radio B on
the other relay contact.
And, yes, antenna to antenna isolation would be an additional concern.
>
> Typical receivers will easily handle 30dBm without ill
> effect when you are not duplexing on the same band.
Here's an interesting question... Does it matter if the interfering signal
is higher or lower than the desired signal? Lots of radios have a low pass
filter (not a bandpass), for harmonic suppression, and don't have any thing
else for preselection. A lower frequency signal will blast right on through.
I'd find it sort of amazing that the receiver front end can take a watt out
of band without blocking a lower level signal. That would be a pretty fancy
mixer unless there's some preselection in front of it.
>
Jim
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