[TowerTalk] 160-m Inverted L or Sloper?
RadioIR at charter.net
Mon Nov 14 01:39:17 EST 2005
Let me add a little to what Tom is referring to about half slopers. It
is true for the average guy who puts up a half sloper (1/4 wave sloping
wire) on whatever tower he has, tunes it for the lowest SWR he can get,
that he will also get random results (read that as mostly poor).
The reasons are many. The description of a half sloper as being a
method of feeding a tower as a vertical, is not very far off base. The
tower is the main part of this antenna system, it requires a very good
ground radial system to be efficient. Ground currents at the base of
the tower can be large. They can easily be larger than the current in
the sloping wire at its feed point. Which means all the wires from the
tower have to be connected to the tower radial system at the base of the
tower. It also means that all wires exiting from the tower act like a
radial themselves and will have some shield currents (hopefully small if
there is a good radial system). It also means that any guy wires have
to be isolated from ground. If you don't have a good radial system, you
can easily be down many dBs below that of a good quarter wave vertical.
The resonant frequency of the tower is very important also. How many
hams know the resonant frequency of their tower? For a 160M antenna, if
the tower happens to be resonant just below 160M, with the proper choice
of feed point on the tower, you can get a useful amount of front to
back, and a little gain. If the tower resonant frequency is above 2 MHz,
(which is probably most tower systems under 70 feet), you will have a
hard time finding a good low SWR point, assuming you have a good ground
system. Of course, you can add a matching network to solve the SWR
problem, and still feed the system. It should be close to
omnidirectional. If there is no ground system, a low SWR point is easier
to find, but performance will be lousy. Also, with a poor ground system
it is easier to obtain some front to back ratio but the gain will be
The length of wire that you use for the sloper varies a lot depending
upon the resonant frequency of the tower and the feed point height.
It's a lot like feeding an off-center fed dipole. The resonant points
just aren't where you expect them to be.
Tom Rauch wrote:
>>For 160-meter DX work, and a tower height of only 60',
>would you recommend
>>an Inverted L or a 1/4 wave Sloper?
>An Inverted L is very predictable in performance (repeatable
>at different locations), but involves some effort in an
>ground system to be effective. With a good ground system it
>will always be competitive with performance expected from a
>full size vertical at the same location.
>A slopper depends heavily on what is on the tower, and where
>stuff is mounted on the tower. It's a matter of luck how it
>works, or if it works at all. It may be almost as good as a
>full size vertical, or it could be as bad as a total flop.
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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