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

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

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

In article , Dick Carroll
writes:

Kieren wrote:

Back to back: Take your two diodes and install them in parallel, but
with one 'pointing' in the opposite direction. The idea is that, because
each diode will conduct when the voltage rises above it's threashold, it
doesn't matter if the spike is positive or negative. A radio signal is
highly unlikely to be powerful enough to force either diode to conduct
(and if it did, they'll protect the RX front end).

I don't think they'd help much however! You only have to think about the
kind of potential in a static build-up to decide that you do not want to
rely on a pair of diodes to keep everything calm.

Once the static voltage builds to .7 volt one or the other diode will
conduct and "bleed" it off. Of course that assumes that we're not
talking of lightning-level static charge. In that case all bets are off.

Not quite right.

Semiconductor diodes will BEGIN conducting at lower voltages. Do a
V-I curve of forward conduction polarity to see that. That's a simple
test with a low voltage supply, a pot, a resistor, and a low-range
voltmeter.

Many HF and MF installations use an RF choke of sufficient impedance
placed across the antenna terminals to provide a discharge path for all
static voltages to be immediately shunted to ground without disturbing
the received radio signal in any way. No static ever builds up on the
antenna. All it takes is a small RF choke of sufficient impedance to be
transparent at the frequency of interest.

Not quite right. A reasonably-high value inductance in parallel with any
antenna is "transparent" at DC (limited to DC resistance) but is a VERY
high value of reactance at RF. X_L = 2 pi L. A 2.5 mHy RFC will have
a reactance of 15.7 KOhms at 1 MHz. Even with a long-wire whose
maximum impedance magnitude might reach 5 KOhms, the effect of
paralleling such an RFC is negligible. [in parallel it would be 3.8 KOhms]
At higher frequencies the inductive reactance is proportionally higher.

What MIGHT happen, depending on the particular inductor, is that the
inductor's self-resonance due to distributed capacity would defeat the
high-frequency reactance. Above self-resonance the RFC would appear
as a capacitor.

Len Anderson
retired (from regular hours) electronic engineer person

In article , Dick Carroll
writes:

Kieren wrote:

Back to back: Take your two diodes and install them in parallel, but
with one 'pointing' in the opposite direction. The idea is that, because
each diode will conduct when the voltage rises above it's threashold, it
doesn't matter if the spike is positive or negative. A radio signal is
highly unlikely to be powerful enough to force either diode to conduct
(and if it did, they'll protect the RX front end).

I don't think they'd help much however! You only have to think about the
kind of potential in a static build-up to decide that you do not want to
rely on a pair of diodes to keep everything calm.

Once the static voltage builds to .7 volt one or the other diode will
conduct and "bleed" it off. Of course that assumes that we're not
talking of lightning-level static charge. In that case all bets are off.

Not quite right.

Semiconductor diodes will BEGIN conducting at lower voltages. Do a
V-I curve of forward conduction polarity to see that. That's a simple
test with a low voltage supply, a pot, a resistor, and a low-range
voltmeter.

Many HF and MF installations use an RF choke of sufficient impedance
placed across the antenna terminals to provide a discharge path for all
static voltages to be immediately shunted to ground without disturbing
the received radio signal in any way. No static ever builds up on the
antenna. All it takes is a small RF choke of sufficient impedance to be
transparent at the frequency of interest.

Not quite right. A reasonably-high value inductance in parallel with any
antenna is "transparent" at DC (limited to DC resistance) but is a VERY
high value of reactance at RF. X_L = 2 pi L. A 2.5 mHy RFC will have
a reactance of 15.7 KOhms at 1 MHz. Even with a long-wire whose
maximum impedance magnitude might reach 5 KOhms, the effect of
paralleling such an RFC is negligible. [in parallel it would be 3.8 KOhms]
At higher frequencies the inductive reactance is proportionally higher.

What MIGHT happen, depending on the particular inductor, is that the
inductor's self-resonance due to distributed capacity would defeat the
high-frequency reactance. Above self-resonance the RFC would appear
as a capacitor.

Len Anderson
retired (from regular hours) electronic engineer person

In article , Roy Lewallen
writes:

Neon bulbs are curious critters. As you say, they have hysteresis -- a
higher strike voltage than sustaining voltage. The company I worked for
once used them as low current regulators here and there, as well as for
static protection, so they bought or selected them to various
specifications for strike and sustaining voltages.

Tektronix. :-) I'm thoroughly familiar with the 53n and 54n Tek scopes
and their "seriesed" power supplies. A rather good design concept in
my later opinion. Used to calibrate them at Ramo-Wooldridge Standards
Lab 1959-1961.

According to the parts descriptions they were controlled-characteristic
miniature neon pilot bulbs. That worked out rather well since I only had
one problem among about 300 or so scopes at R-W...and that was due
to the error amplifier (tube circuit), not the voltage reference of the
neon.

Much smaller than the common "high grade" VR tube, a 5651.


The bulbs were commonly used as pilot lamps, but not when the supply was
DC. (This lesson was learned the hard way, judging by company documents
and app notes.) Depending on the supply impedance, the pilot bulb could
become a relaxation oscillator, interfering with sensitive circuitry.

Heh, Tektronix and several other manufacturers of the 1950-1960 period.

General Electric had that problem in one piece of broadcast TV thing.
Encountered that at WREX-TV in 1956, where it was messing about
with the local color sub-carrier generator.

I came in just as their day was ending.

I'm glad those are nearly gone. Neons are a nice AC pilot bulb or night
light where the minor heat and supply current is not a problem.

Today is a whole different ballgame with logic supply voltage dropping
to 3.3 VDC and rail supplies for op-amps down to 1.5 VDC. LEDs are
now cheap, take less power, and have different colors. Neon lamps are
rather fixed at orange.

ESD built into many MOS ICs makes it much easier on designers and
users and repair folks. Gotta love it now! :-)

Len Anderson
retired (from regular hours) electronic engineer person

In article , Roy Lewallen
writes:

Neon bulbs are curious critters. As you say, they have hysteresis -- a
higher strike voltage than sustaining voltage. The company I worked for
once used them as low current regulators here and there, as well as for
static protection, so they bought or selected them to various
specifications for strike and sustaining voltages.

Tektronix. :-) I'm thoroughly familiar with the 53n and 54n Tek scopes
and their "seriesed" power supplies. A rather good design concept in
my later opinion. Used to calibrate them at Ramo-Wooldridge Standards
Lab 1959-1961.

According to the parts descriptions they were controlled-characteristic
miniature neon pilot bulbs. That worked out rather well since I only had
one problem among about 300 or so scopes at R-W...and that was due
to the error amplifier (tube circuit), not the voltage reference of the
neon.

Much smaller than the common "high grade" VR tube, a 5651.


The bulbs were commonly used as pilot lamps, but not when the supply was
DC. (This lesson was learned the hard way, judging by company documents
and app notes.) Depending on the supply impedance, the pilot bulb could
become a relaxation oscillator, interfering with sensitive circuitry.

Heh, Tektronix and several other manufacturers of the 1950-1960 period.

General Electric had that problem in one piece of broadcast TV thing.
Encountered that at WREX-TV in 1956, where it was messing about
with the local color sub-carrier generator.

I came in just as their day was ending.

I'm glad those are nearly gone. Neons are a nice AC pilot bulb or night
light where the minor heat and supply current is not a problem.

Today is a whole different ballgame with logic supply voltage dropping
to 3.3 VDC and rail supplies for op-amps down to 1.5 VDC. LEDs are
now cheap, take less power, and have different colors. Neon lamps are
rather fixed at orange.

ESD built into many MOS ICs makes it much easier on designers and
users and repair folks. Gotta love it now! :-)

Len Anderson
retired (from regular hours) electronic engineer person