For a 130 ft long Tee antenna, 35 ft high and not allowing anything for
droop, it should resonate at 2.07 MHz. Radiation resistance is about
8.2 ohms. With a ground radial system consisting of 32 quarter wave
radials, the feedpoint impedance at 2.07 MHz should be about 16 ohms.
At 1.85 MHz the impedance should be about 70 -j79.
One way to match this is with an LC, the C across the coax input and the
L in series with the antenna. C = 2200 pf, L = 9 uH. At 1500W, the
voltage across the C is 203V and current is 5A. Voltage across the L is
895V and current is 8.7A. Power loss in the inductor is 45 watts for an
inductor Q of 200. Two to 1 SWR bandwidth is 50 kHz. This is a large
value of cap, so maybe another type matching network would be better.
Including the matching network and ground losses, antenna gain is -3.4
dBi at a take-off angle of 26 degrees. Ground loss is 607 watts for
1500 watts input.
The matching network is only an estimate. There are a lot a factors
that cause the actual required values to be different from the
calculated ones. Actual loss in the radial system versus the assumed
value is a big factor. I assumed an equivalent ground loss of 8.5
ohms. Actual wire lengths used for hooking up the matching network is
another. Antenna droop was also not included in the calculations.
The way I do these matching networks is by measurement. I have a small
variable cap and a variable inductor. I take those and an antenna
analyzer and sit down at the base of the antenna and vary the LC
configuration and the values until I get a match. Then I measure the
values. Then I replace them with the appropriate size L and C. I do go
back to the simulation to calculate voltages and currents on the LC to
see what those ratings need to be.
A measurement of the antenna impedance using an analyzer might work OK
if there are no strong radio stations close to you. Usually with a big
antenna like this, AM radio stations cause big problem with the analyzer
on 160. I have a 1 KW station on 800 kHz, 8 miles away, that messes up
my analyzer on 160. If you get a good reading of the impedance, you can
use that to calculate a matching network. The readings you get won't
directly translate into matching network values (unless the R reading is
50 ohms).
Jerry, K4SAV
ku8e at bellsouth.net wrote:
> I recently switched my center fed zepp from one cut for 40 meters to one
> that is 130 feet long.
> It is about 35 feet high (35 feet of 450 ohm line) in the trees and a flat
> top.
>
> I was thinking by shorting the feeders to use it on 160 meters as a T
> vertical. Problem is that I don't
> have an extra antenna tuner lying around to stick out in the yard at the
> bottom to tune it. Does anyone
> have an idea what the feedpoint impedence is for this type of antenna ?
>
> Also, I wonder if anyone has built a matching network using the method
> described in K3KY's article
> on tuning bobtails and half squares ? See
> http://www.angelfire.com/md/k3ky/page40.html .
> He basically uses a length of RG8 or RG213 for the capacitor and winds a
> coil for the inductance.
>
> I bought a MFJ 259 B Antenna analyzer in Dayton this year. It measures
> inductance and
> capacitance. If I adjust for the lowest SWR by temporarily putting an
> antenna tuner at the feed point
> or the antenna and then measure the C and uH using the 259B will that give
> me the exact values I
> need for my capacitor and inductor ?
>
> Jeff
>
>
>
>
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>
>
>
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