My base insulated tower is 127' of Rohn 25G. An 80m half-wave dipole is
attached at the 119' level, the ends of which droop down to about 100' at each
end, with runs of Phillystran from the ends of the dipole to wooden poles in
opposite directions, each about 350' away from the tower. End insulators over
300' long! Extensive buried radial ground system consists of several thousand
feet of #12 copper. The dipole is fed with open wire tuned feeders, and used
on 80m and 40m, but works quite well when tuned to 160m as a quarter-wave
dipole. When using the tower as a vertical, the end of the OWL near the base
of the tower is opened with a knife switch and left floating. The OWL runs up
to the dipole through the interior of the tower, fixed in position with
plexiglass spacers every 10 feet, maintaining the line conductors symmetrically
about the geometric centre of the triangle for the entire length of the
feedline.
Strictly speaking, this antenna is closer to a vertical tee than to a simple
quarter-wave vertical. Due to the proximity of the feed line to the tower
along the full length, the dipole is close-coupled to the tower even though
there is no metallic connection between the dipole or feeders to the tower at
any point. The measured base impedance is about 180 ohms resistive and a
little more than 300 ohms inductively reactive, as opposed to the expected
36-plus ohms and negligible reactance of a simple quarter-wave vertical.
Interestingly, with the bottom end of the OWL connected to the tower (which is
accomplished using a knife switch when the antenna is not in use, for any
lightning protection it might offer), the base impedance of the tower drops
much lower, exactly 50 ohms at 1812 kHz as indicated at 1:1 on a SWR bridge. I
once tried grounding the bottom end of the OWL directly to the radial system,
and the measured base impedance of the tower dropped even lower, between 10 and
20 ohms IIRC. I run the antenna with the OWL floating, since that's what my
ATU is designed for, but have never tried comparing field strengths between the
OWL floating and bonded to the tower at the base. The system is usable all
the way from 1800 to 2000 kHz by adjusting the single resonating capacitor in
the ATU. The measured base impedance varies across the band but remains within
the range of the tuner, which consists of a simple parallel tuned circuit, one
end grounded to the radial system and a tap on th
e coil leading to the base of the tower. The number of turns on the coupling
coil were carefully adjusted by trial-and-error for the best match to a 450-ohm
untuned OWL running from the shack to the dog-house at the tower, with no
additional variable capacitors or inductors between the OWL and the coupling
coil. The resonant frequency of the ATU at the base of the tower is adjusted
using a reversible DC motor and worm drive, controlled from the shack.
The prototype of the ATU was built with whatever scrap pieces of coil stock I
could find around the shack. Some of it consisted of much-degraded pieces of
air-core coil stock with corroded wire and deteriorated plastic insulation.
When I got it working to my satisfaction, I replaced the junky coils in the
prototype with top-grade silver plated edge-wound coil stock and proper coil
clips, salvaged from discarded broadcast equipment. To my surprise (and
disappointment?), the final version of the tuner with the good quality coil
stock worked exactly the same as the prototype made of pieces of junk coil.
With identical DC input to the transmitter final for each measurement, the
measured base current at the tower was exactly the same with either tuner.
Don k4kyv
________________________________________
From: donovanf@starpower.net <donovanf@starpower.net>
Sent: 06 December 2016 05:28
To: REFLECTOR: Topband
Subject: Re: Topband: Inv L Config
A very important caution about the performance of T-verticals
vs. Inverted-L verticals.
If the performance of your 40 meter antennas is important to
you and your T-vertical is within 300 feet of your 40 meter
antenna, its important that the top of your T should be less
than 55 feet long or more than 80 feet long.
Why? If the T-top is 55-66 feet long it will act as a 40 meter
director. If its 66-80 feet long it will behave as a 40 meter
reflector. Don't ask me how I discovered this...
If the top of a T-vertical needs to be 55-80 feet long and within
300 feet of a 40 meter antenna that you don't want to degrade,
its better to use an inverted-L vertical, which has little or no
affect on nearby higher frequency antennas.
73
Frank
W3LPL
----- Original Message -----
From: "Guy Olinger K2AV" <k2av.guy@gmail.com>
To: "Jerry Keller" <k3bz@verizon.net>
Cc: "REFLECTOR: Topband" <topband@contesting.com>
Sent: Tuesday, December 6, 2016 5:05:37 AM
Subject: Re: Topband: Inv L Config
The horizontal section also radiates, more or less than the vertical
depends on the specifics. Easy to see in a very simple NEC model. If you
are opposed to radiation from the horizontal on principle, then put up a T.
But the radiation from an L's horizontal fills in the doughnut hole in the
pattern, essentially getting the energy for that by taking it away from
ground losses. Assuming that on 160 one has RX antennas because TX antennas
are notoriously noisy, then you only care about what happens to TX. Filling
in the doughnut hole helps to minimize or eliminate skip zones, and help
keep a run frequency running low power.
The effect of a particular change to wires applies more to where the
current is more. Given that, doubling the vertical wire is what you do. But
I would model that and see what it buys you. Do the change both in free
space and over ugly dirt.
73, Guy K2AV
On Mon, Dec 5, 2016 at 11:37 PM, Jerry Keller <k3bz@verizon.net> wrote:
> Is it advantageous to make both the vertical and the horizontal sections
> "fat" (for improved bandwidth), or is it enough to "fatten" the vertical
> (radiating) section ? How much BW will 3" diameter spacers give me?
>
> 73, K3BZ
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
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