[TowerTalk] Relay Selection
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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