On 12/21/2011 1:23 PM, Jim Thomson wrote:
> Date: Wed, 21 Dec 2011 12:46:24 -0500
> From: K8RI<K8RI-on-TowerTalk@tm.net>
> Subject: Re: [TowerTalk] SWR Problem Balun or lightning protection?
>
> On 12/21/2011 10:30 AM, Chuck wrote:
>> I like gamma matched beams because you don't need a balun and there is no DC
>> short to ground at the antenna. You can check for shorts and high
>> resistance paths to ground from the shack.
> That DC short to ground is one of the best things about them. IE, no
> static build up.
>
>
> 73
>
> Roger (K8RI)
>
> ## whoa ! On my F-12 6-el 15M yagi, it's a Direct 50 ohm feed set up,
> like
> an OWA yagi. Just a 17" long bead balun, made from large type 43 beads,
> slid
> over RG-393 teflon coax. There is NO hair pin used on these types of yagis,
> so
> it's NOT dc grnded. The way to get around that is to either [A] install a
> 35 uh
> choke across the feedpoint, [like 12 ga magnet wire on a torroid, like ICE
> uses
> inside their lightning arrestors] of [B] install a 35 uh choke between
> hot and chassis on your amp output, right at the coax output. [dunno why amp
> maker's don't do this already, I do it on all my hb amps]
You just jogged my memory. IIRC Back in the old days virtually all
transmitters and amps had one of those chokes right across the output
coax connector and receivers had a neon (NE51 I think) across the coax
(or input) connector(s).
> . All amps
> already have a safety choke across the load cap, but it's only in the circuit
> on
> TX. Alternately, an ICE lightning arrestor, or just a 35-50uh choke, from
> hot side of coax to grnd , any where on the feedline, will do the trick.
> You could even install the 35-50uh choke inside a remote switch box, on the
> input.
>
> On my AS remote switch box, they use a mess of DPDT open frame relays
> with 10 kv stand off. It's configured, such that all de-energized ant ports
> have their center conductor's DC grnded..
All of my remote antenna switches are configured like this as well
> Gamma match's are great..except
> when the DE is a mile out from the tower. Even on those, you can still use
> a
> 35-50 uh choke. The 12 ga magnet wire on a torroid type is nice, since 12 ga
> wire is pretty robust. You can also get 10 + 8+ 6 ga magnet wire from planet
> wire.
>
> ## One fellow I know abt 400 miles east of me... had static build up so bad
> on his yagi
> that it was arcing across, a real mess.
Years ago...as in about 1966 we were living in a mobile home (house
trailer) while building a new home on the corner of the old family farm.
I had a 1/4 wave, 40 meter vertical with probably no more than 8 or 10
radials bout 100 feet from the trailer...er house. The coax came up in
a small closet just off the kitchen. One morning I was setting there
eating breakfast when I heard a loud "pop". They got louder and more
frequent until there was a really loud snap (or bang) about every 5 to
10 seconds. It was windy, snowing, and very cold. IE what we call a "dry
snow" which makes one whale of a static electric generator and every bit
as good as a Van der Graaf generator.
I opened the closet door to see this big, fat, blue arc off the end of
the PL-259 that would have done any ignition system proud. I pulled the
end of the coax out onto the kitchen floor and well away from anything
flammable using a broom handle. The field from the arc was so strong the
arc extended out from the PL259 by well over an inch when the shortest
and direct path was less than half an inch.
Of course there was no type of lightning or static protection in that
system. Fortunately it wasn't still connected to the transceiver.
73
Roger (K8RI)
>
> Later........ Jim VE7RF
>
>
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