At 08:27 PM 2007-11-06, Jim Miller WB5OXQ in Waco wrote:
>I wonder how messed up an antenna for 160 would be if the center was
>supported at 55' and the legs had to be bent like a letter W in
>order to get the required length for 1/2 wave at 1.9 mhz. The ends
>would wind up being only 20' from the ground. Would it be better to
>feed it with 450 ohm line and use the tuner?
>If my neighbor would just let me run 1 leg over her property I would
>not have to bend the legs at all. then 55 at center 30 at ends.
This will work, but it does represent a compromise. The gain will
probably be about 4 dB below the conventional inverted-V with the
same center height. The bandwidth will be a lot less (less than half
of a conventional inverted-V). The feedpoint impedance will be much
lower (also less than half that of a conventional inverted-V).
I modeled a configuration with the center at 55 feet and 61.5 foot
long bare #12 copper wires running down to 20 feet high on adjacent
corners of a 71.6 x 71.6 foot square lot. These connect to wires
running along opposite sides of the lot that are 71.6 foot long and
20 feet high. The gain is mostly straight up, with about 0.3 dBi
gain. The feed impedance is about 16 ohms and the 2:1 SWR bandwidth
is about 25 kHz at 1.9 MHz. Average gain (in three dimensions) is
about -6.8 dB, which is mostly (-6.1 dB) due to ground
losses. (Anything you can do to raise the average height of the side
wires will give you more signal and more bandwidth. To a lesser
degree, the same goes for making the W wider.)
A straight-line Inverted-V with 55 foot center and 30 foot high ends
has about 4.6 dBi gain (straight up). Feed Z is 45 ohms and the
bandwidth is just over 60 kHz. Average gain is -3.3 dB, again due to
ground losses.
(Note that I used a NEC-2 based program, which probably
underestimates ground losses for antennas low to the ground.)
Since the bandwidth of the Inverted-W antenna is quite narrow, open
wire feed with a balanced tuner might be a good choice. You probably
would want to avoid feedline electrical lengths near odd multiples of
1/4 wavelength, due to the transformation to very high (> 10 kohm)
impedances at the tuner which would result in very high RF voltages
at QRO power levels. TLW says 100 feet of 450 ohm ladder line
feeding a 16 ohm load will have 0.45 dB loss at 1.9 MHz.
Any horizontally polarized antenna at 55 feet (~0.1 wavelength) on
160m will be a cloud burner and best for short haul
communications. For much better DX performance, you could create a
55 foot vertical with one or more (more is better) sloping top
loading wires and even a modest ground system (either on-ground or
tuned-elevated radials, but not both).
73, Terry N6RY
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