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[TowerTalk] RE: [YCCC] Relay Contact rating for RF useage?

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Subject: [TowerTalk] RE: [YCCC] Relay Contact rating for RF useage?
From: (Jordan, David)
Date: Fri Aug 8 11:15:39 2003
There is no straight conversion for AC/DC to RF use
General spec can lead to some insight.
Breakdown voltage across the contacts should be above 1.5KV
for a safety factor of 2 above a 4:1 VSWR into a 20 ohm load at 1.5KW
for KW power 10 A is a minimum I would use.
Since it is next to impossible to know the installation details, i.e. Zin,
cables, power and location
use a big relay w/ low contact resistance and high breakdown voltage.  
Depending on the relay design, a compensation circuit or capacitor may need
to shunted across the contacts at 30mhz.
Relays that have a long thin wire running from the socket to the contact
generally need to be compensated.
There are modifications that can be made to improve the relay for RF.
Take a DPDT relay and cut off each wire connecting the contact to the base
pin.  Solder a jumper bar across the 2 metal 
contacts and convert the relay to a SPST or SPDT relay.  It will reduce the
inductance and contact resistance.
Best method is to hook up the relay in the circuit and measure the VSWR at
10m connecting it to a 50 ohm dummy load.
Add small Pf's until the swr is flat at 10m.   It also helps to short out un
used relays to prevent rf pickup.
Also, Use relays that can be plugged it because you will blow one up
If your going to the trouble of designing a switch using relays do it right.
Use a 062" thick FR4 PCB and use microstrip to make all
connections.  That will help maintain the phase and cut down on unwanted
062" 50 ohms is .110 wide at 30 mhz VP=54%, 75 ohms is 50 mils @vp =56%
.125" 50 ohms = .220" @ 54% at 30 mhz, 75 = .105" @56%

For short runs you can use 100 mils or larger and ignore impedance.  I would
not use 50 mils in a 1.5KW system.

Leakage.  In SO2R or multiops, it take very little power to cause major out
of band noise.  It takes less than 1/4W to smoke some receiver front ends.
That why it is best to design a switching circuit that terminates all unused
antenna to ground.  Make sure you have a very good ground connection in the
relay box to
your ground system.  

Some others may want to chime in on this subject

Dave KC1Q

David Jordan, Pr. Antenna Engineer
> Automotive Telematics and Commercial Antenna 
> 1011 Pawtucket Blvd. Lowell, MA 01853, MS 158
> ph: 978-442-4190  fx: 978-442-5419
> -----Original Message-----
> From: RCARIELLO []
> Sent: Wednesday, August 06, 2003 7:53 PM
> To:   Towertalk; YCCC; FCG
> Subject:      [YCCC] Relay Contact rating for RF useage?
> Hello to all,
> How is the rating of a relay converted to RF usage?
> A company normally gives the voltage and current  rating for the relay
> contacts:
> 250 v ac @ 2 amps  (assumed at 60 CPS)
> 30 v dc @ 3 amps
> Also in some cases the maximum voltage from contacts to relay coil.
> How would this example rate for RF usage up to 30 MHz?
> What is the formula to be applied to other relays?
> What is the formula to be used for power in watts?
> Will there be an impedance bump at the relay?
> If there is an impedance bump should it be considered a problem ?
> Should SWR in the line be a consideration?
> Thanks
> Rich AA2MF
> _______________________________________________
> YCCC mailing list
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