Date: Wed, 5 Oct 2022 04:05:27 -0500
From: Rob Atkinson <ranchorobbo@gmail.com>
To: towertalk <towertalk@contesting.com>
Subject: Re: [TowerTalk] Peak Voltage at the tips of ants ?
<You can't go wrong with good insulators at the ends of a dipole or the
<top of a hanging monopole. Long ceramics or glass, dogbones, rope a
<couple or more in series, help minimize loss when the support rope is
<wet. Take a look at long guy insulators on 50 KW AM stations--some
<have corona rings--that's wild overkill for ham but the concept is
<valid: more insulation, less loss. If the dipole is one lambda then
<it's voltage fed so the center insulator should be more than a single
<dinky dogbone, _in my opinion_.
<It might be interesting to run a neon bulb along a dipole to see if it
<dims along the way. It has to be made sufficiently insensitive I
<think.
<73
<Rob
<K5UJ
Back in the mid 70's, I had a 204BA sitting in the backyard, on a pair of
wooden sawhorses. With 600w cxr applied, to a BN-86 balun no less, I could
easily light up a 4' long fluorescent light tube, when held parallel to the
DE tip. I assumed the V is wicked at the tips of the DE.
Be careful when using dacron rope, in the wet, at HV points. On my oem F12
75m rotary dipole, the original LL wires are brought together at the
8'aluminum cross bar..at the mast. F12 used 2 x short pieces of Dacron
rope...per side. Problem is, when one side is say -10 kv, the other side
is + 10 kv. The fix per N6BT, was to use 4 x egg insulators, problem solved.
In the new config, the LL alumoweld wires were tossed and replaced with T
bar capacity hats. BUT the single 5/16" Dacron rope used to truss the 68'
long ele, has to be insulated where it meets the ele on each side. Same
deal on the 5/16" dacron rope used to truss each of the 40m eles...one per
ele half. Ideally, a serrated insulator is better in the wet, vs a smooth
long one. But use whatever is readily available.
Buddy terminated the 4 x radials of his 160M GP into lower tree branches,
using just insulated wire, and no insulators. One night, he set one of the
trees on fire. Garden hose would not reach it. 10' ladder used to haul
buckets of water up there..at just after midnight. The peak V on the ends
of the radials was also wicked. It just gets worse with more power used.
That was a lesson learned the hard way.
I was going to suggest for testing, perhaps a full sized 144 mhz rotary
dipole, say up 10' above the grnd, then try and measure the tip voltage,
with 10-50 watts applied. The test gear would screw up the ant, so dumped
that idea.
Laporte’s Radio Antenna Engineering. He gives the formula:
Vm = Vo / cos G
Where:
Vm is the end voltage
Vo is the feedpoint voltage
G is the length of the radiator in degrees (half of a dipole length)
However, he cautions this is not accurate when
G is between 75 and 105 degrees, which would
include our standard dipole case.
Cos of 90 = 0. Well that won't work. Cos of 89 = .0174
Assume 1.5 kw at the feedpoint, and say 389 V peak.
We get 389 / .0174 = a whopping 22,356 Volts (peak). That might just be
close to reality.
Funny thing is, small birds would sit on the tips of the DE, with one
helluva lotta power going into the 20m yagi.... and they don't even
flinch...go figure.
Jim VE7RF
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