On Tue, 23 Sep 2003 12:40:22 GMT, mike wrote:



A side effect I noticed after installing the 1N914 diodes was images
scattered across the bands. For example, wwcr on 3200 was also on
2300. Another gentleman posted me link in the antenna group where he
found the same thing happening.

Might the resistor in series with the diodes reduce this side effect?



hmmm... might be my impedance mismatch causing reflections of imcoming
signals back and forth along the antenna, thus causing images.

probable?

mike

Roy,

I recall as a kid making a "Decision Maker" project that used two neons
to indicate Yes and No. My father thought it was pretty cool, especially
when I mentioned that I had noticed it came up on Yes more often. I told
him I'd fix it but he seemed insistent that I leave it the way it was.

I didn't see it much after that, but suspect it played a part in some
gambling game in which he held a decided advantage while being able to
claim a totally impartial device. ;-)

Rob


Roy Lewallen wrote:

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. Strike voltages
varied from 55 minimum to 135 maximum, and sustaining specs went from a
minimum of 46 to a maximum of 78. They also exhibited a "dark effect",
which I believe was that the strike voltage was dependent on the ambient
light level. I recall that a trace radioactive material was added to
some -- to reduce the "dark effect", I think, by keeping the gas close
to ionization. I imagine the sustaining voltage was controlled by the
mixture and pressure of gas.

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.

I came in just as their day was ending.

Roy Lewallen, W7EL

Avery Fineman wrote:

Allow me to correct a number. The strike voltage of a typical small
neon bulb is high but once struck, and a resistor is in series with it,
the bulb potential is around 50 VDC.
. . .

Roy,

I recall as a kid making a "Decision Maker" project that used two neons
to indicate Yes and No. My father thought it was pretty cool, especially
when I mentioned that I had noticed it came up on Yes more often. I told
him I'd fix it but he seemed insistent that I leave it the way it was.

I didn't see it much after that, but suspect it played a part in some
gambling game in which he held a decided advantage while being able to
claim a totally impartial device. ;-)

Rob


Roy Lewallen wrote:

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. Strike voltages
varied from 55 minimum to 135 maximum, and sustaining specs went from a
minimum of 46 to a maximum of 78. They also exhibited a "dark effect",
which I believe was that the strike voltage was dependent on the ambient
light level. I recall that a trace radioactive material was added to
some -- to reduce the "dark effect", I think, by keeping the gas close
to ionization. I imagine the sustaining voltage was controlled by the
mixture and pressure of gas.

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.

I came in just as their day was ending.

Roy Lewallen, W7EL

Avery Fineman wrote:

Allow me to correct a number. The strike voltage of a typical small
neon bulb is high but once struck, and a resistor is in series with it,
the bulb potential is around 50 VDC.
. . .

....[snip]....
I've read suggestions for resistors ranging from 2.2 k ohms to 56 k
ohms all the way up to 100 k ohms. The most recent information being
the lowest value resistors. From the schematics I have seen, the
resistors were placed in parralel between the antenna input and ground
input. Or in the case of a two wire unbalanced input, between each
wire and the case of the tuner which is grounded.

It just depends on how much power you want to "waste" in the paralleled
resistor. Think of it this way: IF you had a 50-ohm dipole antenna and
you used a 50-ohm resistor across the antenna (and ignoring the effect
of a 25-ohm load on your xmtr), half of your power go into the antenna
proper and half would be dissipated in the resistor.

That's probably too much, so try a 500-ohm resistor: now about 90% of
your xmtr power goes into the antenna and 10% into the resistor.

One more try: 5K-omn resistor: now about 99% goes into the antenna and
1% is wasted in the resistor.

This leads to one of the "rules of thumb" from a beginning EE class oh-
so- many years ago: the power loss from paralleling a 100*R-ohm resistor
with an R-ohm resistor is essentially negligible.

There is another GOOD effect of using any resistor across your feed line:
a simple ohm-meter check from inside the shack can tell you if your feed
line is intact!

--Myron, W0PBV.
--
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

....[snip]....
I've read suggestions for resistors ranging from 2.2 k ohms to 56 k
ohms all the way up to 100 k ohms. The most recent information being
the lowest value resistors. From the schematics I have seen, the
resistors were placed in parralel between the antenna input and ground
input. Or in the case of a two wire unbalanced input, between each
wire and the case of the tuner which is grounded.

It just depends on how much power you want to "waste" in the paralleled
resistor. Think of it this way: IF you had a 50-ohm dipole antenna and
you used a 50-ohm resistor across the antenna (and ignoring the effect
of a 25-ohm load on your xmtr), half of your power go into the antenna
proper and half would be dissipated in the resistor.

That's probably too much, so try a 500-ohm resistor: now about 90% of
your xmtr power goes into the antenna and 10% into the resistor.

One more try: 5K-omn resistor: now about 99% goes into the antenna and
1% is wasted in the resistor.

This leads to one of the "rules of thumb" from a beginning EE class oh-
so- many years ago: the power loss from paralleling a 100*R-ohm resistor
with an R-ohm resistor is essentially negligible.

There is another GOOD effect of using any resistor across your feed line:
a simple ohm-meter check from inside the shack can tell you if your feed
line is intact!

--Myron, W0PBV.
--
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

In article , mike
writes:

On 23 Sep 2003 05:09:01 GMT, (Avery Fineman)
wrote:


Silly me, I put them all in parallel.grin. I am a mechanic with some
electrical knowledge, but not much electronics. So the resistor should
be in series with the diodes to limit current.

A side effect I noticed after installing the 1N914 diodes was images
scattered across the bands. For example, wwcr on 3200 was also on
2300. Another gentleman posted me link in the antenna group where he
found the same thing happening.

Might the resistor in series with the diodes reduce this side effect?

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 might be possible if you have some Local Oscillator energy leaking
out to the antenna connection, but even that is unlikely given "modern"
(in the last couple of decades) receiver design.

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.

It might be possible that some unusual circuitry in your receiver presents
a DC Voltage at the antenna port. If so, it might cause one of the diodes
to conduct. It would be better then to AC-couple the back-to-back diodes
to the receiver through a capacitor of 0.001 to 0.01 uFd to eliminate that
possibility. You should be able to measure any DC potential at the
antenna port with a high impedance multimeter.

Len Anderson
retired (from regular hours) electronic engineer person

In article , mike
writes:

On 23 Sep 2003 05:09:01 GMT, (Avery Fineman)
wrote:


Silly me, I put them all in parallel.grin. I am a mechanic with some
electrical knowledge, but not much electronics. So the resistor should
be in series with the diodes to limit current.

A side effect I noticed after installing the 1N914 diodes was images
scattered across the bands. For example, wwcr on 3200 was also on
2300. Another gentleman posted me link in the antenna group where he
found the same thing happening.

Might the resistor in series with the diodes reduce this side effect?

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 might be possible if you have some Local Oscillator energy leaking
out to the antenna connection, but even that is unlikely given "modern"
(in the last couple of decades) receiver design.

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.

It might be possible that some unusual circuitry in your receiver presents
a DC Voltage at the antenna port. If so, it might cause one of the diodes
to conduct. It would be better then to AC-couple the back-to-back diodes
to the receiver through a capacitor of 0.001 to 0.01 uFd to eliminate that
possibility. You should be able to measure any DC potential at the
antenna port with a high impedance multimeter.

Len Anderson
retired (from regular hours) electronic engineer person

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

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

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