[TowerTalk] Remote Antenna Relay

[Pete Smith]

At 11:43 AM 12/28/00 -0600, Allen Jones wrote:
>I want to control an antenna relay at the base of my tower by feeding
>control voltage down the coax instead of running a separate control cable.
>I know this is usually done by putting blocking caps at each end of the coax
>cable and using chokes to isolate the power supply and relay coil from the
>RF.  Anyone know what value choke I should use?  Is the value of the caps at
>all critical?  I was planning on using 1000 pf.  I know I've seen a project
>like this in print but can't recall where.

Starting from the power supply, my Heathkit switch uses  2 .001 uF disks
and an 8.48 uH RFC in a pi configuration, followed by a 75 ohm 5 watt
resistor and a 1 mH RFC in series.  The resistor's role is not explained,
but all the rest is described as RF isolation.  Transmitter power is
coupled to the feedline by a pair of .02 uF disks in parallel, presumably
for current-carrying capability.

On the remote end, RF is coupled to the antennas through a .02 uF disk, and
the relay voltages are taken off through a 1 mH RFC in series, followed by
a .01 uF disk to ground.  22 mFd electrolytic caps are used at the relay
coils with the proper polarity to smooth the pulsating DC.  Small disk caps
(25 pF) are used to compensate for inductive reactance in the relays and
improve the frequency response.

I would heed N4KG's advice about ratings, particularly for the series RFCs
and capacitors.  Can't help you there, though, because no ratings are given
by Heath.  No doorknobs or vacuum caps in there, for sure.

I've used mine at >1300 watts output, occasionally with up to a 3.5:1 SWR,
for 4 years of contesting with no problems.

73, Pete N4ZR
Happy Holidays!

The World Contest Station Database 
is back up and running at
http://www.qsl.net/n4zr 



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