[MRCA] Antenna "protection?"

[Gene Smar]

Eugene:

      You are correct - this device (the P model at least) is a spark 
gap that is intended for use with twisted-pair telco drop wires into a 
customer premises.  It won't fire until the voltage at the terminals 
reaches about 5000 VDC, depending on atmospheric humidity, at the time 
of the lightning strike.  If your tuner is made with QRO components then 
this device might not cause damage to the tuner. However, if your tuner 
is an automatic tuner, even QRO, I would hesitate to rely on this device 
to protect the innards.

      It won't bleed off static electricity during approaching thunder 
storms.  Static is bled off in many Ham surge arresters with either a 
high resistance or inductance shunting the antenna terminals to ground, 
as is done with Polyphaser or Alpha Delta equipment.  If you're worried 
about static build up, you can try shunting across the antenna feed 
point to ground a 100 k Ohm resistor of some high power rating or a 
wound-wire inductance that presents a high impedance at the lowest 
operating freq of the antenna element.  (At higher antenna freqs the 
impedance only increases.)

      Tying two or more together, presumably in parallel, won't help, 
either.  Invariably only on of the parallel gaps is likely to fire 
during a strike, effectively shunting the other devices and preventing 
them from firing.

73 de
Gene Smar  AD3F


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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