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SWR bridge diodes

To: <tentec@contesting.com>
Subject: SWR bridge diodes
From: rohre@arlut.utexas.edu (rohre)
Date: 30 May 1996 14:40:47 -0500
Dave did not mention his 509 models original diode numbers or whether they
were germanium or silicon.  That really is the key to replacing them.

Many diodes will work; I got lucky with a blown B&W tuner's meter and found
that germaniums were used, and replaced the shot diodes with the nearest
industrial germanium I had in stock, (better than 1N34), and it worked well
with the small meter on that unit.

Once you know whether germanium or silicon is needed, and the difference being
one measures 0.25  to 0.3 volts on a diode test of DMM for Germanium, while
Silicon will run 0.5 to 0.7 volts, (usually 0.6 volts);  the next thing is to
get as close to the peak reverse voltage rating of the original, and current
rating as possible.  The SWR circuit often uses small signal diodes, thus
there are many that will come close.  The next issue, is to have two matched
diodes in the metering for forward and reverse.  You can set up a DC biasing,
and see where the diode just starts to conduct, with a DMM monitoring the
barrier voltage.

Pick two diodes that measure the same conduction point, and you should be able
to use them.  Ie, you have to monitor the current starting to flow, and with
perhaps a second meter, measure the voltage that caused current to flow.  Use
DMM for 10 meg input impedance, not the lower ohms per volt of a VOM.  For
silcon, you would want two diodes that both conduct when 0.60 volts is
applied.  However, as long as they match, the exact barrier just affects
overall sensitivity to low power, and can be whatever is close to the

It is puzzling why Ten Tec would not be able to supply exact replacements
unless they used some less common Germanium, which might be in short supply
today.  I would think well matched 1 N 270 diodes would work, then; or even
1N34's from Radio Shack. ( In a bag of 10 from Radio Shack, I have been able
to match a couple before.)

For a high accuracy SWR circuit, there should be calibration pots that allow
one to balance the bridge circuits.  This may be all that is needed to reach
the same accuracy as the original.  If the circuit did not have an internal
pot for this, it likely has fixed resistors that could be replaced by a
combination of pot and resistor for the same total value,
creating a calibration and balancing control.
The same surge that took out the diodes could have caused cheap carbon
resistors to change value, so do not neglect to check ALL components exposed
to the Antenna static charge.  The forward circuit and reverse circuit usually
have mirror copy components, ie same resistor values, caps, etc.
Future insurance:
For any balanced antenna, you could provide across the end of the feed line a
high value of resistance to dissipate static buildup, such as a 150, 000 ohm
resistor, 1/2 watt.  That would protect the diodes, but it is better practice
to disconnect from all antennas when the station is not in use.  In tube days,
we used to use a NE-2 neon lamp, which fires at 65 volts for this.  That would
protect SOME diodes, but not 1n34's.
As a storm approaches, the Neon will fire when lightning is still miles away. 
It will also fire in sustained high winds or dust storms that create static
charge on antenna wires.

Hope this helps,
Stuart K5KVH rohre@arlut.utexas.edu

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