[MRCA] Antenna "protection?"

[Al Klase]

Eugene and the Gang,

I'll take a shot at this from my limited experience.  Imagine a 33-foot 
vertical, half-wave at 14 MHz, fed at it's base by a tuner, running 
ORO.  The RF voltage at the output of the tuner is going to be 
astronomical, think Tesla coil.  So it's going to be difficult to 
provide a spark gap that won't arc over under "normal" operation, 
especially if you're contemplating all-band operation..

Most of my experience has been with an SGC-230 200-watt auto tuner.  At 
our last house, a 20-foot-wide brownstone here in Jersey City, this was 
at the base of a 25-foot vertical strapped to the chimney on the flat 
roof.  A modest radial system was deployed on the roof and down the 
sides of the building, and included a earth ground.  We were at the top 
of what's left of the Palisades ridge, and the antenna was quite 
exposed.  It was up for two years without incident.  OK, that's 
anecdotal, so it isn't a complete answer.

The topology of the SGC tuners is a pi network fed with a broadband 
transformer from the coax from the rig.  So there is a DC path from the 
antenna to ground under all conditions, eliminating static buildup.  In 
the power-down state, the pi-network capacitors are disconnected, and 
the inductor elements are shorted, so it can survive minor "discharge" 
events.

Of course, there's no sure protection from a direct lightning strike, no 
matter what you do.  SGC recommends that if you're really concerned 
about lightning, you should power down the tuner, and disconnect it from 
the antenna, and ground the antenna when the system is not in use.  I 
can envision some sort of Frankenstein dead-man switch that would 
disconnect and ground the antenna when the station was powered down.

However, if I wasn't in a lightning-prone area, especially if there were 
some trees or other structures to give me some cover, I'd just put a 
proper tuner at the base of the antenna, and earth everything in the 
prescribed way, and provide a cutout and grounding switch on the coax in 
the shack.  This is not an unusual situation.

My two cents,
Al

Al Klase -- N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/

On 5/24/2014 4:25 PM, W2HX wrote:
>
> Ok, first -- I know this is a fraught topic. Big topic. Volumes have 
> been written. I have a basic question.  Today at a hamfest, I picked 
> up this little terminal block (see picture below). It is basically a 
> spark gap terminal block. As you can see the studs on the bottom are 
> intended to be bolted to a grounded block.  Then the terminals have a 
> gap between a fixed washer and a finger that connects to the grounded 
> block.  The model is Marconi 1648P and the manual says the following:
>
> "provide protection in a range of 5,000 volts DC and approximately 
> 10KV or less on a surge front of 5KV/msec."
>
> My intention is to use between a vertical antenna I am planning and 
> the antenna coupler that will be sitting at the base of this antenna.  
> What I started to think about is whether this might be of any use in 
> protecting the antenna coupler and anything else down stream from it.  
> I was also thinking that if the antenna produces 5000V during normal 
> use (perhaps depending on how the impedance works out for a given 
> working frequency), this could actually hamper its performance.
>
> If it would help, I guess I could tie more than one of these terminals 
> together before going to the antenna to give more gap area.  Now, I 
> know that a direct lightning hit will simply melt everything. But 
> would this terminal block add any beneficial protection for other 
> kinds of atmospheric activity? The whole thing cost a few bucks so I 
> don't mind throwing it out if it has no value. Thanks!
>
> A link to the datasheet
>
> http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCYQFjAA&url=http%3A%2F%2Fwww.emersonnetworkpower.com%2Fdocumentation%2Fen-us%2Fproducts%2Foutsideplant%2Fensys_terminalblocks%2Fdocuments%2Fospds-061900_3-6-pair-blocks.pdf&ei=G_2AU-zeB-jjsASt0IHgDA&usg=AFQjCNE9KIIGnMdho1mx8WVMjmPDB36vzg&bvm=bv.67720277,d.cWc&cad=rja
>
> 73 Eugene W2HX
>
>
>
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