[Elecraft] Antenna switches

[brian]

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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