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mike luther · February 19th 09, 03:35 PM posted to rec.radio.amateur.antenna

Suggestion.

wrote:

I have a Rohn 30 foot telescoping mast that I'd like to use as the
vertical element of a 40 meter ground plane. My idea is to add 3 feet
of pipe to the top of it to make it the required 33 ft. long.
I want to elevate the bottom of it about 8 feet of the ground and run
a couple of quarter wave wire radials out from it. This way they can
be walked under.

I've been using elevated quarter wavelength verticals in 4 square vertical
array operations for years like this. But there is a way to do this which is
FAR safer for you relative to lightning strike issues as well.

In my case, the vertical element is always a solid metal pole/tower which goes
all the way down to and into the ground at the base. This works even without
the elevated model with elevated radials as well.

Feed the vertical element at the feed point height, ground level, 8 feet up for
you - whatever, with a gamma match section attached directly to your coax feed
line. I make my gamma match sections with the little six inch TV feed line
holders through which I thread copper wire in the little brown insulators that
are out away from the pole by their length. You tie the feed point up at the
top of a run of several of these. Then at the bottom at the feed point for the
coax, you bond your coax braid to the actual tower element.

You run the hot tip wire for it to, at first, a variable capacitor which you
adjust for the desired flat SWR match for the feed. Of course if you are going
to use that variable permanently for the required capacitor in the gamma match
section, it will need to be decent enough spacing for whatever power level you
intend to use with the array.

But there is another trick! You can use a small receiving spaced variable
capacitor to do the experimentation for the match with one of these little
antenna analyzer units, Or .. even just and SWR meter in a crude sense. Then
when you get the nice perfect flat match with the variable, you can figure out
the capacitance needed for how many feet of your feed coax that would be!
Usually it is only a few feet or so on 40 meters for RG-8 for example. You cut
a piece of it that will have MORE than the required capacitance. You tie the
braid to your hot tip point feed wire from the feeder coax. You then tie the
tip point of the little piece of feed coax that is being used as a capacitor in
this case to your gamma match input wire tip.

At that point, you take a cutter, and start snipping off little bits of the
short coax piece you use for the capacitor! When the SWR comes down to the
intended bottom nicest value, you simply strip a little more of the braid off
the free end of the capacitor coax chunk, leaving the insulation and center
wire free in the air for an inch or so. And tape over it with tape to protect
the end from the weather if you like.

Presto! Absolutely flat SWR match to your vertical element that will stay out
there in the weather for years with no trouble. Since the mast is always hard
flat grounded to the earth, in concrete for my towers, or with a few little six
foot radials around the ground point for poles, to distribute the lightning
strike into the surface, POOF no lightning damage to the place!

Sure .. you bring the feed coax down to ground level. You go under the surface
to the shack, with a gas discharge protector plan on it as normal.

My 80 meter 4 square built like this .. four gamma match sections, one on each
element, gets hit directly at my QTH at least once a year here. Absolutely no
damage to the shack stuff from this ever for almost two decades of this now.
And it is connected and up 24X7 just like the broadcast tower stuff.

W5WQN

--

-- Sleep well; OS2's still awake!

Mike Luther