Date: Sat, 28 Sep 1996 16:30:07 PST
From: wb2vuo@juno.com (William K Hibbert)
To: qrp-l@Lehigh.EDU
Subject: [642] Tilted Long Wire Antenna
Message-ID: <19960928.163230.4335.3.wb2vuo@juno.com>

                 A Tilted Off-Center Fed Long-Wire

        In the December, 1995 "QST", there is an article on a tilted
(sloped) long-wire array called "The Super Sloper", by Roger Sparks,
W7WKB.  What Roger had done was to model a tilted long-wire with a
parasitic director with his computer.  What he found can be applied
to a simpler array utilizing just the long wire with an offset feedpoint.

        A basic long-wire will have a pattern that differs from the
classical 1/2-wave antenna.  The radiation will be more off the ends of
the wire instead of the sides, with the major lobes lining up closer to
the wire direction as the length in increased.  The major lobes form a
squashed cloverleaf pattern, with the center of the pattern partially
filled with the minor lobes. (By definition, a minor lobe is any lobe
that is -10dB when compared to the strongest major lobe...) These lobes
run from 45-degrees off-axis for a 1-wave long-wire, to 25-degrees off-
axis for an 8-wave long-wire.  Most amateurs don't have room for anything
greater than 8 wavelengths long, even on 10 Meters, so we won't discuss
them here.

        One thing that is overlooked is that these major lobes also
display a radiation angle, (vertical angle), equal to the offset angle.
By tilting the wire, the major lobes can be depressed to the horizon,
which will make the antenna more unidirectional.  A good compromise
angle is 30-degrees, although even a 15 - 20 degree angle will produce a
front-to-back ratio.  This angle can be achieved with a 40-foot high
support at one end, and a 10-foot high support at the other, with
overall lengths of 70 - 150 feet.

        The offset feed point is used to enhance the unidirection
characteristics and to feed the antler at a current node, which brings
the feedpoint impedance down to a more manageable level.  Basically, the
feedpoint is 1/4-wave from the high end of the wire, with the total
length
of the wire being equal to an integral multiple of 1/2-wave, (1-wave, 1
1/2-wave,2-wave, etc...)  For the example, I will model this antler for
20 Meters, with a 1-wave version, and a 1 1/2-wave version:

   1-Wave Tilted Wire              1 1/2-Wave Tilted Wire

Frequency    = 14.200 MHz         Frequency    = 14.200 MHz
Total Length =   67.5 feet        Total Length =  101.5 feet
Feedpoint    =   16.5 feet        Feedpoint    =   16.5 feet
Approx. Gain =    2.5 dB          Approx. Gain =    3.0 dB

        The length of the 1 1/2-wave Tilted Wire may look familiar, and
it should.  A G5RV is 1 1/2-waves at 20 Meters, but is fed in the center.
By moving the feed to the end, the feedpoint impedance will fall in the
range of 130 - 170 Ohms, which, when fed through a 4:1 balun, and 50 ohm
coax will yield an SWR of 1.6:1 or lower.  This is easily in the range
of even the most basic ATU's, and may be usable without a tuner at all.

        The forward pattern will be about 90 - 120 degrees wide, with a
F/B ratio of 6 - 20 dB, depending on tilt, height, etc...  The 1-wave
version, if fed directly with RG-8X or other 50-ohm coax, will have an
SWR of about 2.5:1, and will need a tuner in line for most operations.
The lightest weight feedline for the Tilted Wire would be 1/2-wave of
twinlead from the feedpoint to the balun, and then coax to the ATU.  The
1/2-wave section will bring the same impedance down to the balun, and
you won't have the weight of the balun at the feedpoint.

        The principle can be extended out as far as you have room.  Just
keep the total length at an integral multiple of 1/2-wave, and the
feedpoint at a 1/4-wave or 3/4-wave point from the end.  Keep this design
in mind for Field Day!

72/73, Keith, WB2VUO, QRP-L #582
Trustee, KB2YTW/B 10 Mtr Beacon (28.2860 MHz)
"In the Depths of the Great Bergen Swamp...FN13ac"



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