[TowerTalk] infos about relay type.

Jim Lux jimlux at earthlink.net
Tue Feb 13 19:33:09 EST 2007

At 03:02 PM 2/13/2007, VE2RYY wrote:
>We are going to build a 3 element vertical on 160 Meters.
>In each parasitic element we want to put a relay to make a RFL ou a 
>director as the parasitic are t-shaped element...60 ft from the a 
>quarter wave vertical.
>We never use more than 500 watts in the driven.
>Now I was told to put big relays in the parasitic, I mean relays 
>with very good space bewteen the contacts, but I find these relays 
>very big .They look like 30 amps power relays.
>I wonder if anybody could give his advice on this fact....
>What type of relay is recommended. Once again, the relays are in the 
>parasitic elements...not in the driven....

Have you got an EZNEC or NEC model of your antenna?  You can look at 
the element currents to see what the relay will need to handle, current wise.

Then, you need to figure out what the voltage will be that the relay 
needs to withstand.

Put a BIG resistor (100K ohms or so) in series at the feedpoint of 
your parasitic element. Run the model again (the current will be a 
LOT lower in your parasitic).. Look at the voltage across that 
resistor (i.e. segment Current * R) .  That's essentially the open 
circuit voltage your relay has to hold off.  Go for something with 
twice the rating.

As a rule of thumb, the breakdown voltage in a relay contact gap will 
be around 10-15 kV/cm (25-35 kV/inch), if there aren't any sharp 
edges, etc.  (that is, the breakdown voltage might be half the 
uniform field breakdown for air).  It's hard to predict actual 
breakdown voltages.. There's other things to worry about too 
(breakdown from core to contact, etc.)  If the relay is sold as being 
able to pass a 2kV or 5kV HiPot test that tells you something.

>I would appreciate very much any advice
>Thanks a million
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