Dear Rik, LF group,
At 17:41 25/06/2002 +0200, you wrote:
Has anyone ever tried a short vertical dipole on LF (either built and
tested one or simulated one) ?
The free space radiation resistance of a short vertical dipole would be
only half of that of a vertical monopole of the same size (Ra =
197*H^2/L^2, where H = dipole height and L = wavelength), so that would be
a 3dB drawback. But close to (a perfect) ground the radiation resistance
should increase (double) again, and win the 3dB back.
The main disadvantage would probably the relative low antenna capacitance,
one would need a large loading coil (= large loss).
But on the other hand a dipole could be less dependent on
ground/environement conditions (not as much as a loop but better than a
vertical monopole).
A short vertical dipole in free space would have a symmetrical current
distribution, maximum in the middle and zero at the ends. Placing it close
to a ground plane would modify this because displacement current would flow
between the ground plane and the lower half of the dipole, increasing the
current towards the bottom end of the dipole making the current
distribution more even in the lower part of the dipole. The current
distribution in the case where the lower end of the dipole was very close
to the ground would be very similar to that of a short monopole with an
elevated feed point - if the lower end of the dipole actually was in
contact with the ground plane, it would be a monopole of course.
You could increase the capacitance by adding end loading to the dipole - if
the lower end of the dipole were close to the ground, in effect you would
have a top-loaded vertical with an elevated feed driven against a counterpoise.
A practical difficulty would be caused by the asymmetrical nature of the
dipole - it would be necessary to have some way of adjusting the voltages
applied to the upper and lower legs of the dipole to get equal current in
both legs, and zero net current on the feed line, in order to achieve the
proper dipole operation. Since the lower end of the dipole would be a high
voltage point, and close to the ground, there would be increased dielectric
losses in the ground under the antenna, which would tend to reduce any
advantage of this antenna configuration.
Cheers, Jim Moritz
73 de M0BMU
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