> 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.
Except the sloping wire has the same excitation current as
the tower at the connection point of the feedline. That
means you can have something like an Inverted V dipole with
an apex angle that is extremely sharp when, through bad
luck, the highest current flows downwards in the tower to
something like a top fed sloping vertical if the current in
the tower mostly flows upwards into a large yagi.
What you have depends on the sheer luck of the draw.
>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.
That's true. The current leaving the feedpoint area can
increase or decrease through luck of the draw becuase of
standing waves on the tower structure. It's just like off
center feeding a sharply bent dipole.
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.
Not exactly. Again it's a matter of random luck. You could
have a length of tower below the slopper that is 1/4 wl long
electrically, and electrically very low impedance above the
slopper, say from guylines or a large stack of antennas. In
that case current divides at the shield or feedpoint
connection junction with most of the current flowing
upwards. It's all a matter of luck.
> 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.
The real problem is with a tower that IS resonant and has
physically small antennas on the top.
If the tower is resonant and has only a physically small
antenna at the top and you attach the slopper near the top,
the shield of the coax is working against a VERY high
impedance. Think of it as an Inverted V with one leg stuck
in the mud, and a very tight apex angle. The shield
effectively has a nearly open connection. The feed impedance
will be very high. radiation very poor.
The problem with sloppers (or the N4KG feed) is the height
of the feed, what else is attached to the tower above and
below the attachment point of the feed point of the 160 feed
system, and how cables are routed and grounded all greatly
affect results.
So much affects the results a slopper (or N4KG feed) is
truly just a matter of luck what the system behaves like,
but one thing is clear...it'll never beat a good vertical
for gain. The closest to expect is a tie.
73 Tom
_______________________________________________
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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