Topband: TX relays

[Tom W8JI]

> 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