Although 0.7 volts is a good rule of thumb for diode conduction in
moderate impedance environments, it's important to keep in mind that
diodes don't abruptly "turn on" at a fixed voltage. They conduct *some*
current at *all* forward voltages. It just happens to be an exponential
function, so it looks kind of like a sharp knee when viewed on a linear
I-V scale. What's important here is how well the diode conducts compared
to the circuit it's across. On a lab test bench, that's 25 ohms (50 ohm
source and 50 ohm load), but it could be more or less when connected
across an antenna.

A quick check with a couple of signal generators and a good combiner
showed that cross modulation 60 dB below either signal was present when
there was about 0.7 volts p-p (peak envelope, that is, at the peaks
where both signals are in phase) across a back-to-back pair of 1N914
type diodes with 50 ohms source and load. That's only about 0.35 volts
of forward diode voltage. At about 0.8 volts p-p, the cross modulation
product was only about 40 dB below either signal. So appreciable cross
modulation can occur at least several dB below the 0.7 volt level often
considered to be the diode's conduction knee.

Paul's caution about multiple signals is important to note. Also
remember that under some circumstances (for example, high Z looking into
the antenna, and also looking into a tuner or receiver, from where the
diodes are mounted), the diode's Z environment can be higher than 25
ohms, resulting in appreciable cross modulation at lower signal levels.

Roy Lewallen, W7EL

Paul Keinanen wrote:
On 24 Sep 2003 22:04:07 GMT, (Avery Fineman)
wrote:



The effect should not be there with or without diodes, with or without
any resistors...unless there is some VERY big RF source out of the
receiver's tuning range that is supplying energy to the diodes and
thus causing the "mixer" effect.


It should also be noted that when several quite strong out of band
signals are present at the antenna, say ten signals, each with S9+60
dB, which is 50 mVrms (71 mVpeak) into 50 ohms and -13 dBm. On
average, these signals produce a combined signal ten times as large at
-3 dB and the rms voltage is about 150 mV. However, from time to time,
the vectors for each individual signal add up, so you have to add the
_voltages_ for that moment, so the maximum theoretical peak amplitude
is 700 mV (10x71 mV), thus, a single silicon diode starts to conduct,
causing all kinds of mixing products.

Using two (or more) 1N4148 type diodes in series instead of a single
diode in the each back to back pair, will prevent any diode conduction
as long as the peak voltage is larger than 1,4 V in either direction.

The maximum number of diodes in series is determined by the amount of
voltage the following stages will tolerate without disintegrating.
Since most likely there will be some selectivity between this diode
clipper and the first amplifier stage, the amplifier stage will never
see voltages as the limiting voltages in normal operation, but the
diodes will still cut out some abnormal peaks e.g. induced by
lightnings.


The diodes should not have any effect on anything but a few millivolts
of any signal arriving on your antenna. A non-conducting diode simply
shows a junction capacitance to the rest of the world. That's a minor
reactive discontinuity to the antenna connection.


Putting multiple diodes in series in the back to back combination also
reduce the capacitances, since the capacitances in each string are in
series.

Paul OH3LWR