Let's add some real numbers.
I have a mechanical A/B switch DIAWA Model CS-201. Fairly inexpensive.
Internally, it has a cavity design to provide isolation between the ports.
Using an HP signal generator and an HP-3586C selective level meter the
isolation of the switch was measured. The used port was terminated in a
50 ohm dummy load. The "open" port went into the selective level meter
which had a input Z of 50 ohms. The HP-3586C can measure RF power
levels accurately within a few tenths of a dBm. Differences are at
least as accurate.
Here are some numbers:
160M/80M 80 dB isolation
20M 69 dB isolation
10M 60 dB isolation
6M (by extrapolation) 46 dB.
YMMV
Note the frequency dependence.
This switch provides the 40 dB isolation necessary.
I have no connection with DIAWA but was interested in this topic.
I suspect there is some real junk out there. Obtaining 60+ dB isolation
isn't easy.
The slide switch "designs" described by some are likely in that junk
category.
If in doubt measure. You have two rigs and an S meter. Input .1 or 1
watt to the switch terminated as above and look at the S meter for the
rig on the open port. Scale the results to 100 W. If it shows S9+80 dB
or less, you are likely OK. Keep in mind many S meters are horribly
inaccurate. They tend to be too generous at around S9.
Not knowing could be expensive.
73 de Brian/K3KO
P.S. In the past, I did the same measurements with coax relays. My
recollection: Obtaining 40 dB isolation at 10M was not possible with
some of them. Their internal design was not as good from an isolation
standpoint. Lower power coax relays provided better isolation (smaller
contact size?) than high power relays. So be careful with coax relays.
On 1/3/2017 3:23 AM, Don Wilhelm wrote:
Jim,
The most important factor when talking about connecting multiple
transceivers to antennas is isolation.
The real question is just how much RF energy will leak back into the
non-transmit transceiver.
Yes, there is the possibility of damage to the receiving circuits and
T/R switch of the receive transceive that is just supposed to be
listening if it gets too much RF at its receive input.
That is what the real discussion is all about. The amount of isolation
required in the switch depends on the power level. If you are talking
about 100 watts power from the transmitter, >40 dB isolation is likely
OK, but at higher powers, you will need more isolation.
73,
Don W3FPR
On 1/2/2017 9:53 PM, Jim Miller wrote:
Off topic. Sorry. What are the functional disadvantages that make it
apparently undesirable to build an antenna switch that is a crossover 2
radios x 2 antennas? What if you only used one path at a time but could
switch them easily? Crosstalk? Is it so strong as to damage the other
radio? Is it not possible to achieve acceptable compatibility with 2
radios
and 2 antennas? Or to swap amplifiers between two radios and two
antennas
by placing one on the input and another on the output of the
amplifiers? Or
to swap in the better antenna for a short contact that you just can't
seem
to make on the current one?
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