[R-390] Tube receivers and long wire antennas

[Mark Huss]

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
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