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Re: Mast mounted preamps - I'm a believer!

on 9/18/00 12:51 PM, Clifford Buttschardt at cbuttsch@slonet.org wrote:

> I have some experience with reed relays in RF.  There are a few items of
> concern that will help the situation.  First, make sure that the reeds are
> of differing length.  When they are of the same length as in most cases,
> they stick more frequently.  Secondly, if you can determine either from
> the manufacturer or by observation with a microscope, obtain reeds that
> have mossy Gold on one contact and bright Gold on the other but not of
> the same plating. Lastly, do use a diode across the coil but slide a
> ferrite bead on each side of the diode.  This should prevent RF from
> rectifying and holding the contacts closed.  A small capacitor from each
> side of the diode to ground also helps.  73   Cliff K7RR

Well, I found the solution to my problem.

The way that I activate the reed relays (which then control the RF relays)
in the preamp box is that I connect the ground end of the relay coil to the
PTT line.  So when the box is powered by my supply in the shack, the relays
engage when I pull the one end of the relay coil to ground.

The brick power amplifiers I have put out 12 volts on the keying ports when
not keyed.  When the preamp box is turned off, that voltage ends up being
split down to about 6 volts.  Keep in mind that the voltage path is up to
the box, through the relay coil in the box, back down the positive supply
line and into the power supply for the preamps.  Six volts is not low enough
to key the amps when they are unkeyed.  However, once the amps are keyed, 6
volts isn't high enough to unkey them!  So it's a problem of voltages being
where they weren't expected.

What I've done to solve this is to add a diode in series with the ptt
control lines going up to the preamp.  The diode is placed so that the
conductive direction is from the preamp box to the shack.  This prevents the
13 volts at the ptt port on the amps from going up the control lines.

Works pretty good.

Now, I have an SWR problem on my 2 meter antenna due to that lousy relay
circuit that KLM put on their antennas (since it's all exposed to the WX).
Anyone know a source for those relays or the whole doggone circuit?



Jon Ogden
NA9D (ex: KE9NA)



"A life lived in fear is a life half lived."

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