On something like a C31XR, just outright grounding of the 20m
reflector and director will keep the others from breaking down.
The arcing occurs because the gap between the boom and element is the
"end of the line" for the exciting signal, hence the maximum voltage.
If the 20m extreme elements are grounded, then the distance out the
booms and the longest elements significantly reduces the voltage at
the other gaps. At 1.5 kw on 160m, that's a reduction from a
theoretical maximum of over 20 kv to a "mere" 1000v or less anywhere
on the boom. Drops it a good deal more on 80m.
So probably Tom's idea at ONLY the 20m ref and dir on a C31XR will do
just fine. That cuts the number of coils from 11 down to 2. (14
elements in all.) From what I can tell, those two elements effectively
control the size of the "top hat" effect anyway.
73, Guy
Homework exercise for EZNEC'rs and other model users:
Single wire 0,0,0 - 0,0,131 (feet), perfect ground, .5 in conductor,
131 segments, frequency 1.825 MHz. Set source to w1e2, V. Observe feed
voltage in Src Dat panel (very high at the top of 1/4 wave).
Set source to 87.02% of length from E1 (segment 114) which will be 17
feet from the top (or 1/2 the length of a typical 20m reflector).
Which voltage point would you rather insulate?
Not absolute figures, of course, but good for order of magnitude.
Tom's coil will be effectively invisible at 160 meters, a few ohms
reactance in series with the 650-j123 Z at segment 114 in the model at
segment.
From: "Tom Rauch" <w8ji@contesting.com>
To: "Towertalk@Contesting. Com" <towertalk@contesting.com>; "Henrik
Weiss" <ozling@mail.dk>
Sent: Saturday, July 06, 2002 8:23 PM
Subject: Re: [Towertalk] Tower shunt feed for 160 M and modified
force12 antennas
> > The thing is a friend of mine burned the pvc insulator that
seperates
> > the force12 elements from the boom due to high voltages at the top
of
> > the tower when running high power.
> >
>
> That is a common problem.
>
> Ground each of the elements to the boom near the element to
> insulation junction. Use a choke coil of about 15-20 turns of #16 or
> heavier insulated wire about 1" or 2" in diameter wound in a single
> layer solenoid clamped with stainless hose clamps between the
element
> and the boom.
>
> That will allow the elements to float for higher bands, and be a
> virtual short circuit on 160. It will likely be a bit better for
> lighting and other problems, and stop worries about trying to beef
up
> insulation on the elements.
>
> It is a much better fix to do this than anything else.
>
> 73, Tom W8JI
> W8JI@contesting.com
>
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