W2SH Said:
If, one had, say, a 120-foot vertical antenna, should one just feel great
about having a full-size quarter-wavelength antenna on Top Band and leave
well enough alone, or should one then add top-hat capacitance of such an
amount that the current maximum moves up the vertical from ground?
KN4LF Says:
I've been experimenting with a similar transmitting antenna arrangement for
4 years with great success.
I use what I call a linear loaded voltage fed T antenna with the entire
vertical section and linear load section made out of 450 ohm ladderline. The
vertical section is 80 feet high, with a 46 foot linear load horizontal
section one foot above ground on the far end, climbing to 5 feet above
ground at the tuning doghouse. The capacity hat is comprised of four 61 foot
wires using #12 stranded wire, spaced one foot apart and sloping down to 70
feet. The feedpoint is 5 feet above ground and terminates in the tuning
doghouse to a legal limit plus rated homebrew parallel matching network and
driven against nine 1/8 wave radials five feet above the ground surface and
one 1/4 wave radial on the ground, four 10 foot ground rods and a 150 foot
deep well casing.
Per the EZNEC 3.0 modeling program, my 80 foot voltage fed Tee has a near
perfect textbook oval radiation pattern, with 1.95 dbi gain at a takeoff
angle of 20 degrees, a 3 db beamwidth of 51.2 degrees, F/B of 0.30 db,
feedpoint impedance of 628.6-j19350, a 1 mile mV/m of 134.22 using 1000
watts, with the highest current point elevated at approximately 60 feet
above ground. However for all intents and purposes the highest current is
nearly equally distributed along the 80 foot vertical section, thanks to the
capacity top hat and 46 foot linear load horizontal section.
With the flip of a switch the linear loaded voltage fed T becomes an
inverted V doublet, an idea borrowed from KK4TR. This antenna has given me
160 meter DXCC with 117 worked and 108 confirmed in approximately 4 years.
25% of the DXCC contacts were via CW and 75% via phone. 103 DXCC contacts
were with 130 watts PEP, 114 with 700 watts PEP.
BTW the ET3PMW has been stronger here in central Florida by approximately 6
db when the antenna is in the inverted V doublet mode. KK4TR Joe in
Brooksville FL has worked the ET3 on his version of this antenna, in the
inverted V doublet mode. Must be the magnetic field power coupling thing
that fellow propagation expert Bob NM7M has discussed on this reflector
recently.
73,
T. F. Giella, KN4LF
Plant City, FL
KN4LF 160 Meter Amateur Radio Resources And More:
http://www.kn4lf.com
Florida Meteorological & Climate Institute:
http://www.kn4lf.com/sub/fmci.htm
Giella Website Design:
http://giellawoodsignandwebsitedesigns.com
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