>> A small neon bulb was used in thousands of ARC-5 Command Set
>> receivers in WW2 for static bleed-off. Similar to an old NE-2 bulb.
>> No need to use a resistor. The neon will conduct somewhere around
>> 70 Volts and shunt any static pickup to ground...then goes into non-
>> conducting state until the next static potential build-up.
>Wow.....70 volts seems a tad high to be protective in my solid state
>portable. I suspect the older vacuum tube sets were far more static
>resistant. I've read the limit for my sony portable should be kept
>below 0.7volts to keep the sensitive front end electronics safe.
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.
Yes, that IS a high voltage, but I encountered it on a (roughly) 200
foot long-wire that the previous apartment owner had put up to a
utility pole prior to 1947. The little bulb in the ARC-5 receiver DID
light. On learning some more about neon bulbs (I was 14 at the time),
I decided it was not a good thing to have the antenna connected during
electrical storm episodes. :-)
>> You can use practically anything modern in the way of diodes there
>> but the high-speed types such as 1N914 and 1N4148 are very cheap
>> and available many places. Varistors could be used (GE "movisters")
>> or even 1N4000 series rectifier diodes. Diodes have a 0.6 to 0.7 VDC
>> forward conduction voltage if silicon.
>> Put them side by side with the anode of one to the cathode of the
>> other at each end. That will limit voltage input to about 1.4 V peak-to-
>> peak. You could put a small series resistor, say 22 Ohms or so,
>> between antenna input and the diodes to limit peak diode current on
>> conduction. Your option...since the series resistor will drop the RF
>> input level slightly.
>> You could also use a high-inductance RF "choke" in place of diodes
>> and neon bulb. 1 to 5 mHy would work at HF bands. That forms a
>> constant low-resistance DC path from antenna to ground and keeps
>> static accumulation bled off immediately.
>> Len Anderson
>> retired (from regular hours) electronic engineer person
>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.
>Guess I might have to just play resistor values and see what doesnt
>hurt signal strenth (another suggestion I read).
>good information though, thanks. - mike
A resistor alone will only serve to bleed off any accumulated voltage
charge. Relatively slowly. It is NOT any sort of protection from a spike
of voltage created by a nearby lightning episode. Those can be anywhere
from a few Volts to 300 Volts peak amplitude, polarity either positive or
negative depending on what Mother Nature decides at that moment...
The "back-to-back" diodes serve as clamps to effect a sudden low-
impedance shunt across the input once they conduct past around 0.7
Volts forward (it's not sudden, but gradual, the Z curve has a lot of
slope steepness until it really begins to conduct). The reason I mentioned
a _series_ resistor between back-to-back diodes and antenna is for three
reasons: It limits the peak current in the diodes; it provides a slight
voltage-divider effect to reduce peaks (even on conduction) at receiver
input; it reduces the rise time of the static peak through a tiny R-C
filter effect using the diodes' junction capacitance.
In truth, NONE of the above is an guarantee of _protection_ of any
receiver input. A slow, gradual charge build-up on an antenna isn't
going anywhere as long as _all_ the components involved have
insulation breakdown voltages that are high. A resistor by itself will
bleed off such slow charge build-up attempts. At around 2.7 KOhms
or so, that resistance isn't going to affect high-impedance values much
at frequencies well away from resonance of the wire antenna.
Since I live in Southern California with a low incidence of electrical
storms, I've not concerned myself with electrostatic charges in wire
antennas. Being raised in northern Illinois, such were quite common
and I've been "bit" by one charge which was probably up around 50
Volts or so on that mentioned long-wire. Lightning storm areas NEED
additional protection for outside antennas.
retired (from regular hours) electronic engineer person
My apologies for the previous posting without content...stupid cat
walked across the keyboard...:-(