> I don't know much about the alpha delta coax devices but I
don't think
> that they have a capacitor to isolate the equipment end.
If not that
> means that the lightning voltage on the line would first
have to burn
> through, for example, the receiver front end coil in order
for the
> voltage on the cable to rise high enough to fire the gas
tube. Not what
> you want.
Gary,
I don't believe a capacitor would make any difference at all
at HF. I think it is mostly picking nits, but it could
easily make things worse!!
Think about the energy content of lightning energy. It has a
peak in hundreds of kilohertz, and with a resonant HF
antenna most energy would be passed at higher frequencies.
Now look at a capacitor. Any series capacitor would have to
be at least .02 uF to not affect 160 meter SWR seriously.
That's about 4.5 ohms Xc on 160 meters, and about 50 ohms on
the peak energy frequency of lightning.
The problem comes in where most receivers look like 30-500
ohms over HF. Low Z near the desired band and higher Z as
you move up frequency. Below pass they look like a very high
Z also. The idea that sticking another 50 ohms in series
with something that is largely hundreds or thousands of ohms
makes a significant change is not very sound technically.
A potential problem is this. When the series cap charges to
allow the gas tube or clamp to fire, you now discharge the
capacitor into the front end! The receiver not only had the
charging currents while the cap was charging into the front
end, it now has to take a reversed polarity hit as the
capacitor discharges.
While I think this is all very small, I can tell you from
doing CE testing on receivers all of the equipment I tested
was not helped by a series capacitor. Only a tuned bandpass
filter with diode clamps helped. I actually found no
difference at all using "dc blocked" protectors or dc
passive protectors in HF systems.
By the way, capacitors in T network tuners actually
contribute to directional coupler failures by the above mech
anism. The antenna capacitor charges and stores energy.
When voltage reaches the breakdown of the feedline or some
other component, the antenna capacitor dumps all the stored
energy on that side into the shunt inductor. This creates in
effect a "spark transmitter" or dampened oscillation as the
inductor-capacitor combo in the tuner rings, and this can
wipe out diodes in the directional coupler.
I'm not a big fan of lightning protectors in active high
power transmitting lines. In the course of working for
Ameritron and Heathkit I saw dozens of cases where
bandswitches failed from lightning protection device
flashover. The problem is largely that peak RF plus dc
voltage across the feedline cannot exceed firing voltage of
the protector. That means the device would have to hold off
550 volts peak of ac plus and superimposed dc just at 1500
watts and a 2:1 SWR. gas tubes are normally 20% tolerance.
You'd need a kilovolt breakdown for any headroom at all.
If a shunt device lets a kilovolt out, it lets a kilovolt
in. Most rigs won't like that, especially if the pulse is
extended by adding a series capacitor between the protected
device and the shunting device.
By the way, a substantial failure mode of relays in
transmitting antenna switching systems can be traced to dc
or pulsed voltage ramp up while RF is applied. Just the
smallest momentary arc sets up an ionized path that converts
a major portion of transmitter energy into heat in the
plasma formed in the area of relay contacts. This either
pits the contacts or carbonizes the insulation.
The best idea in threatening weather is to stop using the
radio. In my station EVERY cable remains connected, some
20-30 cables. Some are over 1/2 mile long. The only thing
that happens is a few DXE antenna switches open up the
center conductors of cables to the transmitting antennas.
Shields all remain attached. All of the control lines are
attached, all of the receiving antennas are attached. Power
line connections are attached. My TV sets are attached also,
as are computer modems.
I'll take a isolation type switch with good center conductor
lightning isolation over a clamp device that allows at least
a kilovolt back into my radio any day of the week.
73 Tom
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