[TowerTalk] Re: Half - Slopers and elevated GP antennas

[T A RUSSELL]

Question and answer below....N4KG

On Wed, 4 Feb 1998 04:46:26 EST CP2235@aol.com writes:
>In your eMail from 03.02.98  14:13:37, you wroteie:
>
><< That is the origin of the N4KG Reverse-Fed Elevated 
> Ground Plane Antenna  >>
>
>
>Hi Tom,
>
>thanks for your description. Only one question: What is the benefit of
>reverse-feeding this elevated GP? Why would it be worse to connect  
>the braid
>to the (insulated) radials and the center pin to the (grounded) tower?
>Is there a difference if the tower is not grounded?
>
>73 Con DF4SA           Cornelius Paul             CP2235@aol.com
>

Good questions Cornelius!

The reason I chose to reverse the feed and connect the braid to the
tower is that there will then be ZERO voltage difference between
the braid and the leg since the coax does run up the leg to the 
connection point.  The breakdown voltage of the JACKET is only
a few hundred volts whereas the breakdown voltage of the
SOLID dielectric is a few thousand volts.  Admittedly, the
voltage at the feedpoint will be low, but I choose to be safe.

With the braid connected to the radials and the center conductor
connected to the tower, the CAPACITANCE between the braid
and the tower leg is then connected in parallel with the feed point.
With a low feedpoint impedance, this is probably not very significant.
But again, using reverse connections eliminates this problem.

Yes, there is a difference if the base of the tower is not grounded.
This is probably a better configuration but the difference in performance
seems to be small.  I am conducting comparison tests with a separate
full size vertical and ground radials.  The results will be published
later.
(Early results indicate imperceptible difference on skip signals.)

The beauty of the system is that it provides an almost free antenna
that performs well.  It presents a quick and easy way for visiting 
DXpeditioners to put an existing station on the low bands.

 Another benefit versus using a separate vertical in close proximity 
to the tower is that it eliminates parasitic interaction.  I once had a 
sloping wire 80M vertical, 50 ft west of a 50 ft tower with a TH3 on top.
 
It was a KILLER to the Pacific to the west but useless to Africa to the
east.

73,   Tom  N4KG


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