What would be a better choice for a stack match?
I'm quite sure that in the heat of a contest these things have been hot
switched on rare occasion with 1500w. There was one time when I
intentionally hot switched one of them with 100w because I thought the
receive signal was down 10dB or so. Problem went away. Anyway I've been
using them for about 6 years or so and only had one that failed after a
big lightening strike.
Stan, K5GO
Hi Stan,
There is a direct opposition between a contact able to take an intense arc
in switching and a contact that is low resistance and maintains low
resistance without "wetting" (wetting is a significant steady voltage that
burns off the micro-thin sulfide and contaminant layer).
I'm sure there is stuff on line by relay manufacturers. Relay contact issues
on receive are probably the number one problem manufacturers face. This
might not show in a stack switch because the receiver will often receive
through some path, even if the stack connection is not what it is on
transmit. The same is true for directional arrays, where an element or two
might drop from the system on receive, and the operator be unaware it is
even happening.
Receive drop out problems like this might be masked in a stack or
directional switch, but they are clearly evident in an antenna selector
switch and in an amplifier antenna transfer relay. Since there are not
multiple receive paths to some portion of the antenna system, they go
completely dead. In a stack or directional array, you just lose pattern. The
operator is just not aware of the problem because it is not hitting him in
the head with a 2x4.
You really don't want to hot switch anything with RF. It is not only rough
on the relay, it is rough on gear.
There are two ways to handle most systems. It is possible to build a small
external box that disallows hot switching. It is also possible in some cases
to build the relay box in a way that does not permit open contact
conditions, which is called a "make before break" system.
Most of the control stuff I'm involved with has hot switch protection. The
DXE 4 square controller, for example, disallows switching while the TX
control line is low. It switches when the TX line is high, but locks off the
amplifier relay control line for a preprogrammed time until the relays have
time to settle. It also cycles the relays repeatedly and rapidly on initial
power-up to wipe contacts.
The RCS 12 Ameritron does the same for hot switch. The RCS12 not only locks
out hot switching, it locks out the wrong band. If an RCS12 is connected to
band data from the radio it could provide automatic band antenna selection
plus stack selection on one box, and include hot switch lockout.
I load my contest station with RCS12's so operators cannot pick the wrong
antenna for the band, and so they cannot hot switch. They can actually
change antennas by stabbing a button during transmit, and the RCS12 holds
off the transfer until the TX drops. When TX drops it opens the amplifier
control line, and then transfers the relay. After the relay transfers it
allows amplifier use again. This way someone cannot pick the wrong antenna,
and they cannot transfer a relay while the amplifier is on line.
When I design a stack box or any relay system, one of my considerations is
to try to not permit wiring or contact conditions that allow open transfer
loads on feeders in the relay box itself. This is called make before break
operation. Sometimes this complicates the relay system, but it improves life
in the field. Often it is not possible, but I generally spend some time
trying to find a way to do this on any new projects. It is sometimes
possible with a stack box and other switching, but it is impossible with an
antenna transfer relay without greatly increasing cost.
The end result of this is usually increased cost, which causes people with
dysthymic disorder to publically vent, but in the long term it is better to
make a system more difficult to break. It is a bigger PITA for me to have to
go out and change a board with 10 or 15 relays because of operator error
than it is to just prevent the problem.
73 Tom
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