Topband: TX relay assemblies

[Tom W8JI]

> Pi were to directly drive the antenna selection? Then the question becomes 
> whether these relay assemblies are capable of handling 1 KW RF.
>
> These relays assemblies are: inexpensive, SPDT, opto-isolated from the 
> CPU, include reverse diode protection on the coils, rated for 220VAC at 10 
> amps, and the PCBs have been notched to physically isolate the relay 
> common from the two outputs (hard to see that from the photos).
>
> http://www.amazon.com/gp/product/B0057OC5WK
>
> Will these work? How would I test them?
>

Larry,

DC or LF relay ratings do not mean much.

There are five problems (not just one or two):

Ground loops (related to current paths)
Capacitive coupling
SWR changes
Voltage breakdown
RF heating

Arcing is an instantaneous failure on peaks. It is usually a frequency 
independent voltage breakdown.

Heating (usually from current through the switch unless a poor dielectric is 
involved) causes a time and duty cycle failure. Heating and current ratings 
are almost always frequency sensitive.

One obvious compromise in the board link you posted is the relay physical 
layout. The problem is it uses an in-line layout and connects through wire 
terminals. While the connectors are good connectors at low frequencies, they 
are not so good for radio frequencies.

This doesn't mean the system will not work. If the relays are adequate, the 
board might work OK if mounted over a groundplane with connectors passed 
through the groundplane. This is especially true on 160 where layout is 
least critical, but ten meters will almost certainly show problems with an 
in-line layout. My guess would be it has a chance of working OK on lower 
bands, but would likely be falling apart on upper HF.

I have no idea if the relays are good enough or not, my comments are only 
concerning the physical layout of relays pictured at the link you posted.

73 Tom