Jeff, Jerry, and all...
I got a similar result -- my calculated feed impedance at 1825 is around 8 -j55
ohms. There are some fairly significant variables in the quality of the ground
system, exact height and amount of droop, so some adjustment range will be
needed.
The shunt capacitor across the coax connection might be as big as 3900 pF, but
is probably smaller unless you have an extensive radial system. To handle the
current and voltage, parallel several small doorknobs or a bunch of high
voltage ceramic discs -- such as (10) 390 pF/3 kV. (I might even send you some)
The coil in series with the antenna might be anywhere from 5 to 9 uH, depending
on the exact height and any effects of nearby object. Coils are easy to tap, so
just wind about 20 or 25 turns of bare copper, spaced about 1-1/2 wire
diameter, on a 2-1/2" or 3" plastic pipe -- black ABS pipe is electrically
transparent, but PVC is OK at 160M, too. You can also lengthen the horizontal
part of the antenna and make the coil smaller (lower loss). Somewhere around
150 feet, no coil is needed, just the capacitor. Higher is better, too.
73, Gary
K9AY
----- Original Message -----
From: K4SAV
To: secc at contesting.com
Sent: Tuesday, July 17, 2007 10:31 PM
Subject: Re: [SECC] Matching T Vertical for 160 meters
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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