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Double-L Antenna For 80/160
Don Toman, K2KQ
toman@ibm.net
A popular misconception about vertical antennas for the low bands is
that they must have elaborate ground systems. Here’s a vertical antenna
for 80 and 160, fed with a single feed line that is simple, effective, and
requires no ground system. You won’t beat the 4-squares, but you will
hold your own against a grounded quarter wave with ridiculous amounts of
copper in the ground.
Rather than get into the theory of why this antenna works, I will
simply describe it here and let results speak for themselves. If
there’s a demand, I’ll do a follow-up article on the relevant
theory.
The antenna is a center-fed half-wave vertical with about 70 feet of vertical
length with the remainder of the top and bottom of the antenna bent
horizontal and parallel to each other. The antenna looks like a squared-off
letter "C" fed in the middle of the vertical part.
Thus, the 160-meter antenna is a 270-foot dipole fed in the center
with the bottom antenna wire bent parallel to the ground about 10 feet off the ground
and the top at 80 feet
off the ground. The horizontal parts are 100 feet long and
parallel to each other.
The 80-meter antenna is a 130-foot dipole, fed in the center with 70 feet vertical and 30 feet horizontal 10 feet off the ground
and 80 feet
off the ground.
Think of it as an inverted L fed against an L. The two Ls are
balanced with respect to each other and because the currents in the
horizontal sections are out of phase, the antenna has a minimum of horizontal
radiation.
The 80 meter
and 160 meter
antennas are separate, fed from a common coaxial feed line.
In my case, the 80-meter and 160-meter horizontal sections are about
30 degrees apart. The 160 horizontal wires run east while the 80 wires run
east-northeast.
On both bands, the bulk of the current flows symmetrically in the
center of the antenna, with the current peak about 45 feet off the ground at
the feed point. On 80, the current loop peak is about 0.16 wavelengths above
ground and on 160, the current loop is about 0.08 wavelength above ground.
The accompanying figure illustrates the antenna.
You can adjust the resonance of the antenna by adjusting the lengths
of lower horizontal sections. The small asymmetry doesn’t bother
anything. The center impedance of mine at resonance is very close to 50 ohms
on 160 and close to 70 ohms on 80. The 160 antenna presents high impedance at
80 meters
and the 80-meter antenna looks like a parallel capacitor across the 50-ohm
160 antenna. The 160 antenna covers 1800-1860 with under 2:1 SWR. I needed to take about 2 feet off the horizontal section to get mine
resonant at 1830. If 80-meter current flows in the 160 antenna, it tends to
flatten the current loop in the vertical section.
The 80 meter
antenna is resonant at 3750 with the 130-foot length shown. The VSWR is under
2:1 over the DX part of the ‘phone band. It needs to have some length
added to cover the CW portion. I haven’t tried to bring it to resonance
in the CW band, but have chosen to feed it through a tuner.
I originally had this antenna hung from trees. This year I put up an
80-foot Rohn-25G with three sets of guys. I hung the wires from ropes
attached to the tower so they are separated from it by about 3 feet. The coaxial feed
line comes off perpendicular to the antenna and is then taped to the tower. The
center conductor goes to the top and the shield goes to the lower part. Before
the coax turns on to the tower, I’ve wrapped some 30 feet of it into a
coil. I expected to see a lot of interaction, but the tower and guys seem
well off resonance at the operating frequencies and I didn’t see any to
worry about.
The first QSO on 80 was VK6LK, long path on SSB at sunset on
September 12. I’ve worked a few ZSs, HF0POL, LU, and the usual
Europeans. On 160, the first QSO was KP4SN on September 15. In the couple of weeks
since hanging it from the tower, I’ve worked ZS6UT, TU2MA, TL5A, VK6VZ,
VK6LK, VK3ZL and NL7Z and the usual horde of Europeans on 160 with no fuss. I
run about 800 watts out from a Ten-Tec Centurion.
Last January, I had about 200,000 points with 750 QSOs in the CQ WW
CW 160 contest using the predecessor hung from trees.
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Doble L para 80/16
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