[TowerTalk] Relay Selection

Tom Rauch W8JI@contesting.com
Fri, 20 Jul 2001 10:01:39 -0400

There is something everyone should keep in mind with RF switches 
that are open, or have open contacts that are held at a different RF 
potential than the input, and connect to a structure up in the air.

There is, besides the RF potential and weather effects on arc 
paths, another often missed effect....sparks from distant lightning 
or precipitation static. The worse possible situation in the world is 
an open contact sectionalizing or isolating areas of an antenna.

If you have a relay in an antenna switch, precipitation static or 
lightning discharges in the distance can trigger a small spark 
across the contacts. If you happen to have RF applied, even 
modest power, the RF will sustain and increase the intensity of the 
plasma in the spark, and the result a few milliseconds later is a 
weakened or ruined relay.

This is one reason why antenna relays that live happily for years at 
many times the power suddenly fail in operation with melted-down 
contacts or carbon tracks or metal deposits across insulation, and 
why antenna relays usually need very large safety margins.

This is why (besides the concerns about creepage across 
insulation due to moisture or dirt) relays that switch antenna 
systems have to be carefully selected.

The procedure I use, besides selecting a component that greatly 
exceeds the expected voltage and current, is intentionally firing a 
tiny weak spark from a HV dc supply across open contacts or from 
contacts to ground while the relay is handling normal RF power. If 
that triggers sustained arc that ruins the relay, the relay will be 
unreliable in service.

As a matter of fact I ran into this last week, testing some new 
relays. I had a relay that handled over 15kW in series with a line 
(contact closed), and nearly 25kW when parallelled with a 50 ohm 
line. Running only 300 peak volts of RF, I fired a tiny spark 
(discharged a .01uF 15kV capacitor) across the contacts. The 
relay immediately sustained an arc and melted the open contacts! 

That's why antenna relays often have to be overkill, and can't 
contain materials that go into vapor or carbon track easily.      
73, Tom W8JI

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