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<DIV><FONT face=Arial size=2>Jeff, Jerry, and all...</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>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.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>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)</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>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. </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>73, Gary</FONT></DIV>
<DIV><FONT face=Arial size=2>K9AY</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<BLOCKQUOTE
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=RadioIR@charter.net href="mailto:RadioIR@charter.net">K4SAV</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=secc@contesting.com
href="mailto:secc@contesting.com">secc@contesting.com</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Tuesday, July 17, 2007 10:31
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [SECC] Matching T Vertical
for 160 meters</DIV>
<DIV><BR></DIV><FONT face="Helvetica, Arial, sans-serif">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. <BR><BR>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. <BR><BR>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.<BR><BR>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. <BR><BR>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.<BR><BR>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).<BR><BR>Jerry, K4SAV</FONT><BR><BR><A
class=moz-txt-link-abbreviated
href="mailto:ku8e@bellsouth.net">ku8e@bellsouth.net</A> wrote:
<BLOCKQUOTE
cite=mid20070717180016.OBBU7952.ibm68aec.bellsouth.net@mail.bellsouth.net
type="cite"><PRE wrap=""> 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 <A class=moz-txt-link-freetext href="http://www.angelfire.com/md/k3ky/page40.html">http://www.angelfire.com/md/k3ky/page40.html</A> .
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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