[Lf] [Fwd: LF: short vertical dipole]

[Andre Kesteloot]

James Moritz wrote:

> 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