Author Topic: Passive RF SP12T Switch? Does it exist? Is there a better way?  (Read 1049 times)

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Offline tbw2Topic starter

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How could I select between a dozen 5MHz@400Vpk-pk signals, passively, whilst maintaining good isolation, and 50Ohm Characteristic impedance?

Currently I have a "cat of nine tails" cable which is slow and fragile.

I am imagining a rotary switch with 12 poles, but cannot find a 50Ohm characteristic impedance version. Do they even exist?

What other solutions are out there?

Thank you in advance.
 

Online ejeffrey

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Re: Passive RF SP12T Switch? Does it exist? Is there a better way?
« Reply #1 on: May 07, 2024, 03:26:37 pm »
I would use coaxial relays.  There are 6 pole and 2 pole versions widely available, you can cascade them to make 12.
 
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Offline tbw2Topic starter

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Re: Passive RF SP12T Switch? Does it exist? Is there a better way?
« Reply #2 on: May 08, 2024, 08:36:41 am »
I would use coaxial relays.  There are 6 pole and 2 pole versions widely available, you can cascade them to make 12.

Thanks for the reply. I must be searching the wrong keywords because I am not seeing any 6 pole RF relays, apart from Digikey 3985-PE71S6486-ND which is $$$

I may just have to make something up with a dozen individual relays on a PCB.
 

Offline MartinL

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Re: Passive RF SP12T Switch? Does it exist? Is there a better way?
« Reply #3 on: May 08, 2024, 12:09:37 pm »
A 400Vpk-pk sine wave in a 50 ohm system corresponds to 400W of RF power throughput. That's quite a lot. Those Pasternack coax relays are only rated to 20W input power.

But at 5MHz you are in HF territory. You can look at amateur radio equipment for the HF bands for examples of how to do this. Generally, HF designs do not bother to try to maintain 50 ohm characteristic impedance through the relays. This is a reasonable choice because the relays are electrically extremely short (millimetres) compared to the wavelength (in this case 60 metres), so the effect of the short mismatched section is negligible.

At 400Vpk-pk, the RMS voltage is 141V, and RMS current into a 50 ohm load is 2.82A, so you can use common off the shelf relays rated for 250V AC at 3A or more.
 
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Offline mtwieg

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Re: Passive RF SP12T Switch? Does it exist? Is there a better way?
« Reply #4 on: May 08, 2024, 01:27:55 pm »
I agree with MartinL, at 5MHz fancy "RF" relays are just a waste of money. Common relays used for mains applications should be fine. Many HAM radio builds make use of Omron relays for antenna tuners, bypasses, selectors, etc. Often into the kW power range (so long as there's no hot switching).
 
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Offline tbw2Topic starter

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Re: Passive RF SP12T Switch? Does it exist? Is there a better way?
« Reply #5 on: May 08, 2024, 03:17:56 pm »
Thank you for the responses.
 

Offline vk4ffab

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Re: Passive RF SP12T Switch? Does it exist? Is there a better way?
« Reply #6 on: May 09, 2024, 11:03:40 pm »
Thank you for the responses.

Omron G2RL series of relays are the goto for many ham homebrewers, Good for over 500W to 50Mhz. Depending on where you buy, a couple bucks a unit. Single and double pole only though.
 
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