TopBand: Shunt matching tower for 160M

Eric Gustafson
Wed, 3 Dec 1997 16:03:27 -0700


I have received a few responses that reveal that I wasn't as
clear as I should have been with my description for an
alternative method to shunt feeding (gamma or omega) a tower for
use as a vertical antenna.  Actually, what I was attempting to
describe was this:

	||   ||
	||   ||
	||   ||              loading wire                |
	||   || coil                                     |
	||   |*//////+-----------------------------------|
	||   ||                                          |
	||   ||                                          |
	||   ||                                         hat
 tower> ||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
	||   ||
        ||   ||
            /  \          X  is connection point of feedline
  radials> /    \            shield, tower leg and the radials

                          *  is the connection point of the
                             feedline shield and one end of
                             the shunt coil.  The location of
                             this point on the tower is selected
                             so that the resistive component of
                             the feedpoint impedance is less
                             50 ohms (25 to 35 ohms)

                          +  is the connection point of the
                             feedline center conductor, the
                             loading wire, and the other end
                             of the coil

Feedline runs from base to feedpoint inside enclosed triangular
(or square or whatever) area of tower as directly as possible.  I
normally tape and tiewrap it to one of the legs.

Power handling capacity will be determined by selection of
feedline and connectors to be used (so long as the coil is
adequately robust).  RG213 and UHF connectors are good for up to
40KW at the resonant frequency.

The coil can be wound on schedule 40 PVC pipe (3 or 4 inch
diameter) or suported on a rectangular slab of 1/2 inch
polycarbonate sheet.  This support can also be the insulator for
the feedpoint.

The Loading Wire (or "resonator") need not be perfectly
horizontal.  But it must be pulled away from the tower.  It can
be thought of as a 1/4 wave (approximately) sloper.  However, the
_VAST_ majority of the radiation from the system is from the
tower not the wire.

The open end of the wire is a high voltage point.  So it is a
good idea to keep it at least 12 feet in the air.  Also keep the
entire length of the loading wire clear of nearby objects.  This
is especially true if high power operation is contemplated.  If
the hat is used, the voltage at the end will be lower but it can
still be quite high.

Hope this is useful.

73  Eric  N7CL

Eric Gustafson  N7CL      | The mountains are high and
6730 S. Old Spanish Trail | the Emperor is far away.
Tucson, AZ 85747-9498     |
                          | You can't work 'em
INTERNET:   | if you can't hear 'em.

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