RadioBanter - View Single Post

September 23rd 03, 01:40 PM

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

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.

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?

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.

I chose 2.2k ohm 1/4 watt resistors. Couldnt find any higher wattages
at Radio Shack. This was the latest resistance value recommended by
Arnie Coro at Radio Habana.

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.

I live in Vermont. We get some lightning but not that much. My main
concern is static buildup due to wind and the elements.

mike