--IMA.Boundary.643283088
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Don, K5AQ wrote back on Bill, K2TQC's question:
"Bill,
The use of an extender wire at the top of the tower is shown in W1FB's
Antenna Notebook, page 49.
I'm sure there are other references but this is a quick one.
Don K5AQ"
----------
> From: Bill Gibbons <wjgibbon@mailbox.syr.edu>
> To: topband@contesting.com
> Subject: TopBand: Shunt Fed Tower Question
To: <topband@contesting.com>
> Date: Monday, November 24, 1997 5:58 AM
>
> Easy question for the experts. Why isn't there ever any mention of
> extending the electrical length of shunt fed towers with horizontal
> wires off the top? Wouldn't this be quite similar to the inverted L
> idea? I have a 54 foot tower with a WARC yagi and TH6 at the top. I
> presently shunt feed it with an omega match. Would I improve its
> performance by adding a 50 foot or so horizontal wire from the top??
> Thanks, Bill K2TQC
>
Hi All,
I have the following comments:
1. It may be the same drawing, or not, but I saw a sketch
where the shunt feed rod/wire went up to a few feet below
top of tower. In this case about 50 feet, and then the
loading wire went out to add top loading to the tower and
get more current higher on the tower:
o----|o----o <--- yagi
|
| <---mast
o---o----|o-----o---o <--- yagi
X
top loading wire-> X--o--------------------------------O-
X |
X |
tower-----------> X |
X | <- shunt feed rod/wire
~ ~
~ ~
X |
X |
X o <--- omega match connection point
X
------------------------------------------------ <- radials
/////////////////////////////////////////////// & ground
The top loading wire may need to be longer than the 50 feet
mentioned by K2TQC.
2. Simply putting up an inverted L wire, 50 foot up and
78 feet out supported from your tower will give you a
resonant antenna over good radials with a feed point
impedance of 15 to 20 ohms. An UnUn or L - net will get to
50 ohms easily.
Note, adding about 7 or 8 feet to the length of the wire
makes the inductor in the L network. Then a shunt capacitor
across the coax, may be 1500 to 2500 pfd... you need to
experiment or measure feed point impedance and calculate the
L network.. will bring the antenna to 50 ohms.
Good luck, George K8GG
--IMA.Boundary.643283088--
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