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Author Topic: Antenna Switches using Industrial Relays.  (Read 1176 times)
K9WJL
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Posts: 183




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« on: January 05, 2008, 05:39:18 PM »

Assuming a properly filtered DC power supply, and an RF tight enclosure, Do the Potter & Brumfield plug in relays work for antenna switching? I would like to do a few things here that I can't do with a typical antenna switch.

Thanks!
Bill


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W8JI
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« Reply #1 on: January 05, 2008, 05:51:36 PM »

Depends on the particular relay and the particular application. In general though they are NOT a preferred relay for high power RF switching.
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AA4PB
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« Reply #2 on: January 05, 2008, 06:49:30 PM »

Some of the HF remote antenna switches use the PC mount version of them. They are probably good for a couple hundred watts in a 50 ohm system.
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W7ETA
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« Reply #3 on: January 05, 2008, 07:58:22 PM »

A local ham used relays for the motor in refrigerators.

Relays from AC units?

Bob
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WB0MCO
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Posts: 82




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« Reply #4 on: January 05, 2008, 09:24:36 PM »

I've been using them for years, ones that have 30amp
rating.The ones out of old AC compressors will also
work.I run a 1000 watts through mine with no problem.
Just don't switch them with RF applied.
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HA6SST
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« Reply #5 on: January 06, 2008, 05:20:30 AM »

They work fine on HF but struggle from 6m upwards. I replaced a damaged relay with a P&B unit in our club linear six months ago and it's still handling 600W of RF without complaining.

HA6SST
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W8JI
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« Reply #6 on: January 06, 2008, 06:08:57 AM »

It is IMPOSSIBLE to say if they will work or not when no one knows:

1.) The particular type of relay he is talking about.

2.) The application he is talking about.

What works OK in one application can absolutely fail to work in another even when the relays are the same type. In this case we don't have the faintest idea what relay he is talking about or the actual application, so any answer is nothing but a wild guess.

73 Tom
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SSB
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« Reply #7 on: January 06, 2008, 07:07:02 AM »

Finding relays good for rf use is useless at this point since I have not seen a relay in 30 years that was advertised as suitable for rf use.  Vacuum relays work well into the HF region but are expensive when bought new.  Ebay always seems to have a few but nobody seems to want to buy them.  I don't know what hams generally use for rf switching other than cheap plastic power relays.

I bought a boat load of vacuum relays a few years ago for a very good price but if I didn't have them I don't know what I would use in amps or high power antenna switching.

Alex....
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K9WJL
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Posts: 183




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« Reply #8 on: January 06, 2008, 07:19:09 AM »

OK, Thanks So far,
 I was just wondering if these P&B relays are RF capable for 1.8 to 30 Mhz, at about a max of 1350 watts. I would be using them to switch Rigs and the amp and tuner from one antenna to another, while being able to use the rig not in line with the amp and tuner to continue to monitor the opposite antenna.
 I would use the system to switch the keying circuit to the amp as well, but with another relay.
 There are thousands of these relays available in many different configurations so it's just a general question, but it would just be for general switching in the example above.
 Thanks
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K8GU
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« Reply #9 on: January 06, 2008, 07:50:35 AM »

Tom makes an important point...we'd need to know exactly which relay model number you're talking about to really say anything about it.  Just because a relay isn't rated for RF use doesn't mean it won't do the job.  But, there are tests to be done...
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W8JI
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« Reply #10 on: January 06, 2008, 08:43:14 AM »

No one knows:

What relay

What the exact application is

How critical the application is

What the environment is

What you expect overall

In general with relays things just have to be tested.








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N3OX
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« Reply #11 on: January 06, 2008, 12:56:25 PM »

"are RF capable for 1.8 to 30 Mhz, at about a max of 1350 watts"

Relays aren't good to a power level.

Even if you knew their specs exactly in the 1.8 to 30MHz range they'd have a current limit and a voltage limit.

Depending on the SWR on the line, you could exceed one or the other or both of these at all different sorts of power levels.

I think the answer to your question is:

Many relays that don't have the letters "RF" mentioned in any specification or literature work just fine to switch RF.  It is just electricity after all, and they're just switches.

However, the exact breakdown voltages, allowable currents, contact resistances, package inductances and capacitances, all that kind of stuff has probably never been tested for many relays.

There are also relays that are better and worse for receiving applications... they tend to need some minimum current to achieve lowest contact resistance..

It sounds like you have some relays and it sounds like you have some watts to pump through them.  Try it, measure the SWR and the loss through one and if it don't arc over or melt and gives decent SWR and low loss, you can use it.

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K4DPK
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« Reply #12 on: January 06, 2008, 01:31:21 PM »

One of the big concerns with using open-frame power relays in RF applications is the insulation.

Many power relays have molded plastic headers that hold the contacts in position, and many of these are black in color.  Often the molder uses carbon black to pigment the plastic, and while carbon black/resin combinations might be good insulators at DC and LFAC, it could easily cause a breakdown at RF.

Some power relays have G-10 circuit board material (or similar) for headers.  These are the ones that are least likely to have or cause problems.

Since you can't test the relay for dielectric loss in a microwave oven (because of the presence of the metal) the best idea is to stay away from black insulation.

73
Phil C. Sr.
k4dpk

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K9WJL
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« Reply #13 on: January 06, 2008, 02:59:04 PM »

Thanks for all the help guys!
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WA3SKN
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Posts: 5554




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« Reply #14 on: January 06, 2008, 03:05:55 PM »

Not familiar with the relays you mentioned.
But, non-coaxial relays were used in the "Good Old days" for HF up to about 20 Mhz, or so... then problems arose due to stray capacitance.
I see you are talking HF, and High power.  The current, voltage, and power rating for the contacts are usually not specified for RF, just DC or 60 Hz.
If you already have the relays, you could try it.  But, I do see a high probability for problems.
Good Luck!

-Mike.
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