LF antennas]

Andre Kesteloot akestelo@bellatlantic.net
Fri, 27 Feb 1998 19:48:29 -0500

Peter Martinez wrote:

> >From Peter Martinez G3PLX Cumbria
> Regarding the horizontal versus vertical debate.
> My reading of the theory side, mostly gleaned from books such as
> The Services Handbook of Radio volume 5 (Transmission and
> Propagation) and Ionospheric Radio by K Davies, show that there is
> very little propagation of horizontal polarised waves along the ground
> on VLF, for two reasons.
> (1) The cancellation of horizontally polarised waves along the ground
> (at any frequency) means that the antenna will radiate zero signal at
> zero elevation over perfect ground.
> (2) Surface wave propagation can only be vertically polarised since,
> in order for the wave to hug the curvature of the earth, you need a
> current in the ground surface along the line of propagation, and you
> don't get that with a horizontally polarised wave.
> That doesn't mean that we can't get skywave on this band, but the
> skywave will be very different from that which we know on HF. The
> reflection height on LF is around the 70km mark during the daytime
> rising only to 90km at night. If we work out elevation angles for the
> sort of distances we think we might be able to work by skywave,
> they are quite low. Although we think in terms of firing straight up
> and down on 160m and 80m at night in order to bounce off the
> F-layer at a height of 500km or more when the E-layer has vanished,
> the situation on LF is that reflection is still from the D layer at night
> and it's only the absorption that gets less when the sun goes down
> that will give us any increase in signal and hence a modest increase
> in range. We will still want to get as much RF along the horizon for
> skywave at whatever distance, and that probably means vertical
> polarisation.
> Radiation theory says that we don't need to stick rigidly to the idea
> that only a current in a wire will radiate. If we place any conducting
> object at a height H above the ground and put an RF voltage V on it,
> it will create an E-field of magnitude V/H in it's vicinity. That will be
> the same as from a vertical wire of height 2H with V on it, and will
> thus radiate just as well. Some may say that the only way to get a
> voltage V on this conducting object will be to connect a wire to it
> and pump current along it, and THAT radiates. Both views are right
> of course, but the point is that a horizontal wire at a height H with a
> voltage V on it will radiate vertically polarised RF in all directions
> (including off the ends, as someone else noted) and we can work
> out how much by using H as the effective height, work out the
> equivalent radiation resistance for this height, and multiply by the
> square of the RF current. If we feed this current into one end rather
> than the centre of the top section, we just get a bit of extra
> horizontal radiation off the sides. Since this (like the vertical
> component) is "very small" compared to the other losses at LF, this
> lost horizontal RF just increases the effective losses by a "very
> small" amount: the fact that our longwire is radiating a bit of extra
> horizontal doesn't reduce the amount of vertical that it radiates.
> 73
> Peter