In article , Paul Keinanen
writes:

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.

True enough. Some friends of mine live near the transmitter site of
AM broadcast transmitter of KMPC in the San Fernando Valley section
of Los Angeles, CA. KMPC is the only high power station in the "Valley"
at 50 KW _into_ the antenna. :-)

Within a few blocks of the KMPC transmitter, ANYTHING is possible
insofar as IMD products, from tube type to solid-state receivers,
some telephones, a few computers, intercoms, etc. :-(

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.

Good point.

The maximum number of diodes in series is determined by the amount of
voltage the following stages will tolerate without disintegrating.

:-) I once had a zener that became a sort of LED on a breadboard. For
about 5 seconds or so.

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.

True enough, but an electrostatic pickup during a storm MAY reach
as high as 200 Volts or so. That's a static charge effect during the
build-up period for lightning.

A friend of mine living in the mountains decided he would "scientifically"
measure the electrostatic charge build-up with VTVM connected to a
small strip-chart recorder on a long-wire antenna. Lived at a 4000 foot
elevation. Observation resulted in the "200 V" value. Unfortunately, he
had so much trouble with the cheap strip-chart recorder that storm
season was over by the time he got the recorder working. :-(

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.

2 pFd at 10 MHz is only an 8 KOhm reactance.

Len Anderson
retired (from regular hours) electronic engineer person