[Amps] Relay suggestions please

[Larry Benko]

Steve,

Let's get calibrated:

1.) 1.5kW in a 50 ohm environment is 274Vrms (387V peak) and 5.48Arms.
2.) Add some SWR into the mix (still @ 1500W) and the maximum voltages 
and currents rise by the square root of SWR.
3.) Switching relays generally need to be able to handle maximums since 
line locations can be anywhere along line
4.) With a 3:1 SWR Vpeak max = 670V and Irms max = 9.49A

Past discussions with relay manufacturers when I was using relays in 
commercial equipment indicated that when using conventional relays at HF 
the contact current rating be reduced to 50% of the datasheet values and 
no correction to the datasheet voltage values are needed.  This assumes 
NO hot switching.  I might add that a couple of relay manufacturers said 
no current derating was necessary but most said something around 50% was 
better.

If the relay is used with some contact(s) in a only receive path then 
contact oxidation would be important and either a gold laden contact or 
using a relay with bifurcated contacts would be better.   I have 
simulated bifurcated contacts by using double pole relays with the 
contacts paralleled.  There are numerous small DPDT relays with 8-10A 
contacts which work well at HF.  Breakdown voltage is not a concern at 
these power levels with reasonable SWRs.  Any wiring you probably do 
will be slightly inductive (compared to 50 ohms) so some small 
capacitance to ground will maintain a good SWR through the relay box.  
Generally 5-20pF is about correct.

Hope this helps.

Larry, W0QE


On 3/9/2012 1:38 AM, Steve Thompson wrote:
> I'm looking at a lower cost alternative to a switching system that
> currently uses coaxial relays. Frequencies up to 30MHz, powers in
> the region of 1kW and continuous carrier has to be catered for,
> but no hot switching.
>
> Any suggestions for open frame or pcb types with a track record in
>    amplifier products?
>
> Thanks, Steve
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