[TowerTalk] T Antenna

[T A RUSSELL]

.........................

"I went out today and removed the coax from an 80M dipole. I added, in
its
place, about 60 feet of vertical wire, ran six elevated radials, put in a
small ground rod, and hooked it all up. So I now have a "T antenna" for
160M to use as a vertical for the ARRL 160M contest."


"My problem is that I have about a 3 to one SWR at resonance. I suspect
this
is so because my feed point Z is probably around 15-20 ohms. I put a 3KW
MFJ tuner on it and it wants to arc at about 1100W output. (I know, I
know
- but it is the only tuner I have for 160M)"

<snip>

de   K5NA
............................


Try a 1/4 wavelength transmission line matching transformer.

	Rin  X  Rout  =  Zo  X  Zo  

 where Zo is the characteristic impedance of the 1/4 WL line.

Intermediate impedance lines can be made by paralleling two (or more)
lines.
I use two parallel lines of RG-59 to match my toploaded 80M elevated GP
This gives me a 36 ohm line which transfroms the 17 ohm feedpoint 
impedance up to 80 ohms.  I then run a piece of 75 ohm CATV line to
the house and feed it directly on CW, with a tuner on SSB.  No problems.
If you want a flat SWR, you can use the tuner on CW as well.  It is
generally
easier to match a higher impedance than a lower one with a tuner at the
transmitter.

Two parallel 50 ohm lines gives a  Zo of  25 ohms.
Two parallel 75 ohm lines gives a  Zo of 37.5 ohms.
A combination of 50 and 75 ohm lines gives a Zo of 30 ohms.
Each line MUST be an ELECTRICAL  1/4 wavelength.

(see Simple and Effective Elevated GP Antennas in June 1994 QST)

de  Tom   N4KG




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