Topband: Relays in *TX* array

[Ford Peterson]

Eric asks:

> QUESTION: What sort of relay to scrounge/purchase? There's a hamfest
> happening this weekend I may be able to attend, so any quick, *specific*
> response is appreciated.
> 
> Thanks,
> 
> Eric W3DQ
> Washington, DC
>

And Steve ponders:

> Ford and Tom,
>
> Can you provide some examples of "good" relays you have found, say, from 
> the Mouser or Digikey catalog ?  Frankly, it's a little daunting to go 
> through 65 pages of relays, trying to surmise which ones might be good 
> for topband and then study the spec sheets.
>
> Thanks,
> Steve, N2IC


Ok guys, first of all, I'm a bean counter--hardly in the same league as Tom when it comes to RF, so I defer 100% to the experts.

But, I did do some research on the subject using the web pages of many popular manufacturers of relays.  There is a wealth of knowledge to be gleaned by doing your mousework.  Volumes are available on-line if you were interested.

With that being said, I found that most manufacturer's data sheets did rate their contacts for current handling, insulating properties between open contacts, and between contacts and the frame of the relay.  Some generalizations can be made after looking at dozens and dozens of potential candidates.  While these are generally the 60Hz ratings, they (in my humble opinion) do provide some guidance as to the current handling capability at RF.  Finding data on surplus relays is only possible about 25% of the time.  Some surplus has been on the shelves since the '70s--no kidding.

Voltage ratings on DPDT designs were found to be, virtually without exception, 1/4th that of SPDT or SPST designs.  Why?  I have no clue but can speculate that there is just so much packed into a DPDT design that the normal air gaps are significantly reduced.  Somewhere I saw an "engineering approximation" for the reliable insulating properties of air to be approximately 21v / thousandth of an inch.  I just looked for the reference and could not locate it quickly.  There was no 'caveat' about whether this was the cobweb laden, high or low humidity, etc. rating.  So take it for what it is--a SWAG.  This suggests a 0.1" gap to be good to 2100V. A 0.025" gap to be about 525v.  Visual inspection can give you a general idea of the gaps provided.  So a paper-thin gap cannot be expected to insulate a QRO signal.  As in all things RF, your mileage may vary.

SPDT designs were widely available in the 2000v variety, with DPDT designs at 500v to 600v.  I looked at several very crude 12v relays and wanted to sample and test them.  These are automotive quality relays that handle, at times, very high DC current.  And cheaper than dirt.  But I did not evaluate any of these and none of the manufacturers cared enough to even measure voltage handling.  From the sounds of the "CLUNK" that they make, I speculate some of the air gaps in these clunkers are massive.

Contactor materials were all over the map.  Gold, Gold Alloy of every variety, Silver, Silver Alloy of every variety, Beryllium(sp?) copper, etc.  There is no standard and no direct comparability from one manufacturer to the next.  Each manufacturer has it's own special mix of rocket fuel that makes their relay better than the rest.  Go read and giggle to yourself because NOBODY rates this stuff at RF unless you are buying an RF relay--huge bucks by the way.

Generally, a relay that is designed to self-clean by 'wiping' itself clean is not going to be a high current relay (e.g. >2A).  Good reasons for this I might add.  Relays are manufactured to handle current at 60Hz and switch under load.  Relays switched under load do not need to wipe--they clean themselves by arcing.  Relays that wipe under hot switched loads are a very low life relay.  Why the low life?  Because during intended use (110v or 220v AC) the hot switched action causes an arc.  Said arc while wiping would be destructive to the contactor and result in huge transients as it wiped.  So you will not find a wiping high current relay in nature.  My travels confirmed that.  You may be able to look at the mean time before failure to determine wiping characteristics--with lower MTBF being a wiper and high MTBF being a non-wiper--but that's counter-intuitive pure speculation on my part.

Some relays use the frame (typically sheet metal) of the relay as part of the signal path.  I concluded that this was bad.  I also noted that some relays had no jumper wire connecting the fingers to the frame and rely 100% on a friction contact between the finger and the frame.  I concluded this was bad.

I went to the local surplus emporium in Minneapolis and examined literally hundreds of different styles and variety of relays on the shelves at ABC.  My conclusion?  Find a relay with a good gap, with good conductivity from in-to-out, and then plan to run some DC through the connection as you will NOT be hot switching RF.  By the way, you will NOT be able to get the speed out of it for normal QSK.  Antenna selection is fine, but QSK requires blazing fast speeds and hot switched vacuum varieties, or low power sealed designs.