Coax Switching

[JF]

Bernie Arcand wrote:
 >I have a Heathkit Coax switch that I built decades ago, and now that I
 >have two rigs, I have the output of both of these rigs into the switch,
 >with the common port the output to the tuner/antenna.  Question:  Is
 >there a possibility or potential or likelyhood that by transmitting on
 >one radio, that I can damage the receive on the other radio when it is
 >turned off?


Are you talking about the HD-1234 manual coax switch, a hexagonal 3"
box with a knob in the center? I made some limited tests on one of
these:
on 75 meters the RF VOLTAGE difference between active and grounded
ports was about -69 db, but on 40 meters it dropped to about -63db
(on 40 M the RF leakage voltage on the grounded port near doubled
from that at 75 M, to 135 mv p-p with 100 watts through the active
port), so the isolation probably continues to decrease in this
fashion as the frequency goes up. I guess this is to be expected
with this rotary wafer switch design, as opposed to cavity types.
How much RF it would take to damage a rcvr front end would depend
on the individual design - you might write the manufacturer to ask
about yours.

At any rate, for 2 xcvrs into one antenna, I'd limit the output
power on the HF bands to whatever your xcvr maker says is safe,
and use some other switch arrangement for the higher bands. Some
time ago I recall seeing a commercial outfit using a Heathkit
HD-1234 to switch antennas to a VHF business radio, and the RF
power loss through the active port to the antenna was substantial.
I don't know how much the isolation dropped between the active and
grounded ports at that VHF frequency, but it must have been a good
bit.

73   KE5ZB

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