[TowerTalk] RF Relay QUestion

[Jim Lux]

Here's some notes I collected a while ago:
http://home.earthlink.net/~w6rmk/relay.htm

& comments interspersed below.

-----Original Message-----
>From: David Gilbert <xdavid at cis-broadband.com>
>Sent: Dec 15, 2007 9:50 PM
>To: "Dr. James C. Garland" <4cx250b at muohio.edu>
>Cc: TowerTalk at contesting.com
>Subject: Re: [TowerTalk] RF Relay QUestion
>
>
>It depends upon what your requirements are. If you don't care about 
>isolation between selections and are switching at a point that always 
>has low voltage (maybe at the base of a vertical or something), almost 
>anything will work. I've used inexpensive high current automotive relays 
>from places like Auto Zone for that sort of thing and they work fine. 
>Most aren't really sealed, but they keep the dust out.

Any non-RF relay you get will have some parasitic C and L, but in most applications, that just means that whatever you're switching needs to get adjusted a bit.

Watch out for the coupling from the switched circuit to the coil and frame.  It will be different when the relay is open and closed.


>
>On the other hand, if you want a relay rated for high voltage and good 
>isolation, they don't come cheap from anyone. The ones that go into the 
>better remote switches fall into that category.

The president of Max Gain Systems has some nice vacuum relays at decent prices.. Check out http://www.mgs4u.com/ 

>Dr. James C. Garland wrote:
>> Does anybody know what kind of open frame relays Array Solutions (or other
>> companies that make remote RF switches) are using?  I’d like to find a
>> relatively inexpensive open frame relay that can handle at least 1.5kW (cold
>> switched only).  For long-term reliabilty, something rated at 3KW would be
>> better.
>> Tnx and 73,
>> Jim W8ZR
>>


Most mfrs use relays that are NOT rated for RF duty.  Rather, they choose conventional DC/60Hz style relays and try a bunch to find a model that has decent RF properties.  For HF purposes, most relays are small in terms of wavelengths, so it's not like you need carefully designed moving striplines (check out the inside of a Dow-Key or other high quality UHF or microwave relay sometime.. it's very slick).  You need something that doesn't have some fatal flaw for RF (e.g. tiny wires in some bad place in the circuit) and can handle the RF current and voltage (which almost ANY relay can do..)

The other issues are if you are switching receive signals, some power relays aren't all that hot, because they get an oxide film on the contacts. No problem switching 10Amps at 120VAC 60Hz, but not so great for 1 microvolt at 30MHz.

The other problem has to do with RF coupling into the coil and frame and causing internal shorting/breakdown.


That said, if you look at a sufficiently high resolution photo of the insides of a variety of commercial antenna switches, you can probably identify the make and model of the relays that the mfr has chosen, even if they have sanded off the numbers or used a house numbering.  It's not like relay technology is so exotic that only a few companies have the "recipe".

Normally, I'd say this little bit of reverse engineering isn't playing fair (the mfrs have spent time and money finding a relay that works), but, I've also found that once you do the reverse engineering, you'll find that the manufacturer is actually giving you a fair deal. They buy the relays cheaper (in quantity), so by the time you figure out what you're paying for the relay and your time to order and assemble it, they're cheaper overall.

They also deal with the ever present problem that the model that happened to be RF suitable (remember the mfr of the relay doesn't guarantee this) yesterday doesn't work today, because the mfr decided to change their design slightly.

Jim, W6RMK