Larry, I also use "conventional" relays for RF switching.
However, while the relay voltage and current handling of "conventional" relays
may be sufficient for this purpose, I've found that conventional relays usually
couple a great deal of RF into the relay coil leads.
This is the reason that I decouple each of the relay coil leads through
inductors in each of the leads and then followed up with suitable bypass
capacitors. I bypass to the chassis. To me its worth the "hassle" -- $.75 of
decoupling & bypass permits use of very inexpensive relays.
Even some of the vacuum relays can have quite a bit of RF on the coil leads.
I just make it a practice to decouple relay coil leads.
Good Luck,
Don
On Friday, March 09, 2012 09:38:08 am Larry Benko wrote:
> 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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