Jon,
Two things explain your problem - and they're both "to do" because of
"wavelength".
First your 160m OCF is very close to ground in terms of wavelenth.
1.8 MHz is 166.67 meters
2.0 MHz is 150 meters.
Difference: 16.67 meters or about .
3.5 MHz is 85.71 meters
4.0 MHz is 75 meters or about 18 feet difference.
Difference: 10.71 meters or about 11 feet difference.
Proximity to ground and other structures will detune your antenna - making a
calculated length actually resonant at a higher frequency. We usually add
5% to the calculated length of a diple (already factored into the 468/f(MHz)
formula. Closeness to ground and the proximity effect is proportional to
the wavelenth of the frequency under consideration. More effect at greater
wavelengths. Thus 160 m suffers most of all from this as 50 feet is but 15%
or so of a wavelength at 1.8 MHz. (I've only used 39 inches equals one
meter in the above calculations, so my figures aren't exact. I also used
300/f(MHz) to calculate wavelength.)
In all cases it is "better" to have an antenna slightly longer than slightly
shorter. You can take away but it is structuraly weaker to "add on".
You might consider the Inverted L antenna for 160 meters if you have the
room - it gives superior performance on both close in and distant
propagation. If you're just interested in DX and don't mind if your high
angle radiation is surpressed, go for an Tee antenna. You'll need twice the
top section however for the Tee antenna. (These are wonderful antennas.)
W1BB - Stew Perry did many experiments with top band (160 meter) antennas
and he really thought the Inverted L with a 1/4 wave resonant top section
(around 135 feet) on top and "whatever vertical section was available" was
the "best" antenna for many people. This antenna worked well for local and
for DX but it had to be matched with a capacitor as it would be much longer
than the standard 1/4 wave vertical (or Inverted L).
One trick that I've used to benefit when I couldn't put up such an antenna
(extended 1/4 wave inverted L) and could only fit a 1/4 wave length of wire,
was to use "twin lead" or TV line. You feed one wire at the beginning, then
short both wires at the far end, and the remaining wire at the beginning
gets connected to ground. This arrangement doubles the radiation resistance
and consequently halves the ground losses of the antenna.
In this design, you take a 135 foot section of twin lead (75 ohm or 300 ohm)
and short the "far end" by stripping the insulation, and tying the wires
together and soldering them. Using twin lead feedline has a problem.
Supporting 135 feet of this stuff by tying an insulator on the end of the
"shorted and soldered" end (by making the stripped ends of sufficient length
to allow an insulator to be placed there) is a problem because the wire
frequently (also known as "always") breaks at the end. To eliminate this,
you can take two pieces of plexiglass of a size about 6 by 4 inches, score
the area so that the twin lead will "fit" into the groove, and then with
small brass or stainless bolts and nuts, secure the sandwitch. I seal the
end of the twin lead with "Liquid Tape" after wrapping it with a good grade
of plastic electricians tape. DO NOT OMIT this part, and do it to ALL
outside antenna connections. It is a valuable "trade secret!
To do this easily, get some round (not lockwashers) and large pancake
washers - you'll need about four or five depending on the thickness of the
twin lead per bolt. Use these inside to allow the twin lead to be held
comfortably.
I've also scored the plexiglass at right angles to the twin lead - it will
behave like the serated jaws of a Vice-Grip (tm) pliers and hold the twin
line sufficently.
What the twin lead does is to double the impedance of the antenna. If the
measured antenna has a typical value of 36 ohms (which is standard for a 1/4
wave vertical wire), then having the two wires will double it to around 70
ohms. Also the ground losses as they are proportional to their fraction of
the radiational resistance of the antenna will go down by a factor of two.
Some people have used this 160 meter 1/4 wave antenna and put a series trap
(resonant at 3.7 MHz) a distance of about 55 feet from the feed point on 80
meters, the antenna appears to have a short at 55 feet, and thus appears to
be a 1/4 wave antenna on that band. However, using this arrangement you
cannot use the 55 foot long antenna on 30 meters. With a wire to the coil
of the relay this antenna will then work on 160/80 and the 80 meter antenna
will perform on the 3rd harmonic or about 10.5 MHz. (Just like a 7 MHz
dipole works on 21 MHz).
I hope this information is helpful to you.
73
David N1EA
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