Ed,
Let's compare the 90' top loaded vertical against a 90 foot vertical without
top loading wires.
Mechanically it might be nice to not have the top loading wires. In that
case the radiation resistance drops from 22 ohms to 14 ohms and the base
loading coil, which was 5 uH for the case of four 20' top loading wires,
rises to 15 uH. With a 20 ohm ground resistance (twelve 40' radials) the
radiation efficiency drops from 52% to 41% for a decrease in gain of 1 dB.
How about two top loading wires as recommended by Clive, GM3POI? Two 28' top
loading wires sloping at 45 degrees resonates the antenna at 1.83 MHz given
a base loading inductance of 5 uH. The radiation resistance is 21 ohms vs 22
ohms for the four wire top loading. The two wire arrangement provides
performance about equal to the four wire.
How about a self-supporting top hat? For 12', 1" diameter spokes requires a
base loading inductance of 5 uH. The radiation resistance is 23 ohms. The
radiation resistance is lower with the sloping wires because the magnetic
field resulting from the downward current in the wires partially cancels the
magnetic field resulting from the upward current in the vertical. We now
have a good electrical arrangement with a good mechanical arrangement.
Dave WX7G
Sat, Aug 29, 2009 at 6:04 PM, DAVID CUTHBERT <telegrapher9@gmail.com> wrote:
> Ed,
>
> here are some dimensions derived using EZNEC:
>
> Height 90', 4" diameter
> four #14 top hat wires 20'
> Base coil 5 uH (make coil 10 uH and tap it)
> Radiation resistance 22 ohms
>
> This article by K3LC will help you chose radial lengths numbers.
>
> http://www.ncjweb.com/k3lcmaxgainradials.pdf
>
> A starting place is to use 500' of wire to make twelve 40' radials as shown
> in Fig. 3. Being that your vertical is shorter than a 1/4 wavelength these
> K3LC graphs are not optimum but I beleive they are more than good enough.
>
> If we say the 32,000 radial system is close to 0 ohms, the 500' radial
> system (12 radials) is close to 20 ohms (based on the gain figures). Given
> that the radiation resistance your antenna is 22 ohms this gives an input
> impedance of 42 ohms, a VSWR at resonance of 1.1:1, a 2:1 VSWR bandwidth of
> 200 kHz, and a radiation efficiency of 50%.
>
> Install 2000' of radial wire to obtain a 10 ohm ground and a signal
> increase of 1.2 dB.
>
> Dave WX7G
>
>
>
>
>
>
> On Sat, Aug 29, 2009 at 3:32 PM, Ed Stallman <n5dg@airmail.net> wrote:
>
>> Hello Towertalkers
>>
>> My plan is to construct a 160m vertical using 4 inch irrigation
>> tubing 90 ft tall, with 4 top loading wires spaced 90 degree and
>> sloping down at 45 degrees . My target freq is 1.830 but would like
>> to make the top-loading a bit short so that the antenna was resonant
>> about 2 MHz. Then put a small (a few uH) loading coil at the base to
>> bring the antenna to resonance at the desired frequency. I am
>> placing the antenna 300ft away from any other structure , hoping for
>> no interaction . I'll start with 32 ground radials and add more
>> later, not sure on the length ? 90ft or 130ft ?
>>
>> two questions
>> how long will the top-loading wires be for resonant at about 2 MHz
>> using # 14 copper insulated wire?
>> how long should I make the ground radials ?
>>
>> Thanks Ed N5DG
>>
>> _______________________________________________
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>> Topband@contesting.com
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>>
>
>
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