[R-390] Tube receivers and long wire antennas

[Sheldon Daitch]

Far too many years ago, when I worked at the VOA
receiver site in Greenville, we had a fair number of
rhombic antennas that were used for our receiving systems.

Lightning protection for the antennas and equipment were
not really exotic, but was effective.

With perhaps one or two exceptions, all of the RF feedlines from
the antennas to the inner compound fence line were balanced
feedlines, and at the inner compound fence line, we had a
series of TMC BALUN boxes, 30 or so, with a matching transformer
from the nominal 516 ohms balanced line to the 50 ohm coaxial line.

Each of the balun units had fuse clips, so the two inputs to the balun
were fused, I think we used 2AMP fast blow glass fuses, and also
the coaxial output was fused.  Also, each of the balanced legs had a
TMC plug-in spark gap unit, part number I have long since forgotten,
but it was had a large brown fiber tube, with metal ends, and some type of
glass insert filled with some power material.  Basically, if the thing didn't
rattle it was still good.  Later, we were replacing these with modified units
that had some discrete spark gap unit installed in the fiber tube.  In the
event of a hard strike to or near the antenna, sometimes the energy would
shatter the glass in this spark gap unit.

After a lightning storm, we'd go out to all the balun units with a box full of
2 amp fuses, a handful of the spark gaps and replace all the defective
items found.  Every now and then, an extremely close strike would open
up a winding on the balun and we'd have to replace them, and finally, I suppose
it was the early to mid- 1980s, no more baluns were available from TMC.
We then started making our own baluns, winding the coils in the shop and
fabricating new base plates.  We also experiments with graphite ball
spark gaps rather than the plug in spark gap units.

Also, each of the four steel towers supporting the rhombic antennas were
well grounded, as well as the balun boxes at the inner compound perimeter.

I can't ever remember that we had any equipment damage inside the building
from outside lightning strikes, so, between the fuses and the spark gap units,
the system was effective in keeping lightning headed into the ground.

73
Sheldon
WA4MZZ




Mark Huss wrote:

> Yep, been there, done that!
> first, get a TV Coax Lightning Arrester.  Place it in series with the
> coax lead-in where it enters the house.  Drive a ground rod at that
> point and connect with at least #12 wire.  Bigger is better.  Shorter is
> also better, since you want to short the high-frequency components if
> the lightning strike.  This will not protect your radio, but will
> probibily save the house!
> Second, you need to provide a way for static build-up and induced
> currents (from nearby strikes up to half a mile away) to drain to ground
> without going through your receiver.  This depends on the type of
> receiver, whether solid state or tube.
> Tube receivers are rugged devices.  Heck, where a Nuke will take out
> every solid-state receiver for hundreds of miles, a tube receiver won't
> even notice!  Protection for these often consists of nothing more than
> an NE-2 neon tube.  if voltage on the antenna exceeds 82 volts, it will
> conduct and drain the charge off the antenna.  It also makes a sobering
> display during thunder and snowstorms.  There is usually a one-meg
> resistor across the neon tube which handles slower buildup.
> Transistors, and espically FET's are more delicate.  Normal static
> buildup can take out a solid state front end easily if no protection for
> the input is provided by the manufacturer.  Sony 2010's are famous for
> this.  RX-320's are a bit better, but I have seen it happen.  Protection
> for these must be provided if you have an outside antenna.  two 1N914
> diodes wired back-to-back in parallel (cathode of D1 to anode of D2 to
> shield, cathode of D2 to Anode of D1 to center conductor) is a common
> solution.  Problem with this is local radio stations.  They can cause
> the diodes to conduct, and you get birdies.  if this is a problem, you
> can stack them, i.e. wire several diodes in series before wiring them in
> parallel.  Also add a ten k resistor in parallel with the diodes to
> drain slow buildup of static.  you also need to replace this simple
> circuit annually, as a nearby strike can take out the diodes.  MOV's can
> also be used, and are a bit more rugged, if more expensive.  You will
> need this even if you impliment solution number three.
> Solution Three, which i have put on all my antennas, is to use an
> isolation transformer at the point the feedline from the antenna reaches
> the ground.  Drive another ground stake at that point.  Then isolate the
> antenna from the feedline to the house using an isolation transformer.
> This provides a low impedence to ground at lower frequencies, preventing
> static build-up and grounding the higher power components of a
> lightning-induced charge.  I build my own, not only because it is
> cheaper, but because i always use 12 to 14 gauge wire and a big torid
> (FT-43 material) rather than the 28-30 gauge wire commonily used for
> receiving isolation transformers.  First, the bigger Torid reduces
> saturation from local AM radio stations.  Second, athe larger gauge wire
> will handle a nearby strike without burning out.  The third thing the
> isolation transformer does is to cut down significantly on interference
> generated in the house!  It is worth installing just for that.  Note
> that this does NOT replace solution number two, the diodes,.  The diodes
> will handle the high frequency components of the strike, and also handle
> the leakage into the coax from the strike.
> It all sounds complicated and expensive, but with an investment in a
> drill, a cheap Radio Shack Soldering Iron, and $20-30 for parts, you
> will save at least the $100 a repair WILL cost you, or trying to explain
> to the Insurance company why you did not have a lightning arrester on
> you antenna after your house is burned to the ground!
>
> Brad Huff wrote:
>
> >I recently put up a long wire antenna approx 125' long and was amazed at the static buildup and arcover as I started to put a coax connector on the end of the feedline during a storm. I am told that the static during a snowstorm is amazing as well. Now to my question for the group-The arc between the center conductor and the ragged end of the just cut braid was probably a kilovolt or so during that storm, a storm that was a couple of miles away. Now I don't intend to leave the antenna hooked up to the radio when it is not in use but I'm still concerned about front end damage while I'm using it. What does one do about this? I don't know if an in line lightning arrester would do the trick or possibly a neon bulb from the center conductor to ground or both. The schematic shows a neon bulb across the unbalanced input but nothing across the balanced one. I don't think that a solid state rig would survive. I've asked a few vendors of antenna supplies about this and they don't have an
> answer, since I didn't invent the long wire antenna I'm sure someone has dealt with this before. Any help would be appreciated.-Brad
> >_____________________________________________________________
> >