[MRCA] Antenna "protection?"

Mon May 26 16:41:37 EDT 2014

Thanks, Al. Will take your advice.

73 Eugene W2HX

From: MRCA [mailto:mrca-bounces at mailman.qth.net] On Behalf Of Al Klase
Sent: Saturday, May 24, 2014 9:23 PM
To: mrca at mailman.qth.net
Subject: Re: [MRCA] Antenna "protection?"

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!

[cid:image001.png at 01CF7901.5CCB28A0]

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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