[Fwd: LF: Horizontal polarisation on LF?]

Frank Gentges gentges@itd.nrl.navy.mil
Sat, 17 Jul 1999 01:59:55 +0000 (GMT)


This discussion has some real merit.  At HF, and presumably at LF, a 
vertical wave only reflects off of the ground due to the "real" component 
of the ground impedence.  The result is that a lossy ground absorbs the 
vertical component of the wave as a fraction of the R value.

On the other hand, horizontal waves are reflected as well by "imaginary" 
component of the ground impedence as well as the "real" component.  Water 
has a very high dielectric constant in this case and thus can reflect the 
wave well.

At LF the antenna is very very close to the ground so the reflection may 
be a function of some components that are neglected for HF so there may 
be a problem with horizontal waves.

There is no problem, at least in concept, with erecting a dipole with some
capacity hats on the ends and then using a pair of tuning coils similar to
the single tuning coil used with the vertical Marconi. 

This concept may have real merit for sky wave LF DXing.

> > Hi all,
> >
> > Taking up the thread of the ERP discussion, and consideration of
> > verticals or a dipole:
> >
> > At the rather long wavelengths of interest, amateur antennas at LF are
> > usually very short in electrical length terms.  The presence of "earth"
> > can not be separated from the antenna environment.  The "earth" is a
> > combination of conductance and dielectric.  The launching of a radiated
> > wave is greatly facilitated by using the ground conductivity, and is why
> > VERTICAL POLARISATION is the main mode of antenna operation, and ensuing
> > GROUND WAVES are relied on for reliable coverage (at least that is the
> > reasoning for most of the broadcast and military LF systems).  Usual
> > practice is to have a "big antenna with lots of top loading" and "lots
> > of ground radials".  LF amateurs are mostly building small scale
> > versions of what the "big boys" do i.e. a Marconi T or variant.
> >
> > However, the best of amateur LF DX is usually a result of SKY WAVE
> > propagation, even if the amateur antennas are vertically polarised (the
> > polarisation reflected by the ionosphere could be random, but only the
> > vertical component is efficiently received).  If sky wave mode is what
> > is actually being sought after, then it may well be worth experimenting
> > with a horizontal "dipole"?  The ground conductivity will try to "short
> > out" the launching of a ground wave, but who cares if sky waves are the
> > desired main objective?  A horizontal wire electrically close to ground
> > provides a "near vertical incidence" radiation pattern, with most power
> > "going upwards", but with sufficient angular spread to provide
> > reasonable DX from a one hop path via an ionospheric reflection.

We may gain enough at reasonable sky wave angles to make up for much 
energy going straight up.

> >
> > One New Zealand amateur, Bruce ZL1WB, uses a very long wire strung out
> > over a gully, and with only 30 watts of RF power applied, is the most
> > often copied ZL LF amateur station in Australia (spanning the Tasman
> > Sea, a definite sky wave path, signals are zero much of the time during
> > daylight hours).  So there is a suggestion that the very long wire that
> > is mostly horizontal in nature does provide good LF DX.

Worthwhile data point to keep in mind.

> >
> > So, casting aside the preconditioning about vertical polarisation and
> > LF, I suggest that it would be well worth investigating from the point
> > of view of sky wave DX potential.

AMRAD may be able to get enough stations on the air to compare vertical 
vs. horizontal side in real time.

> >
> > Does anyone have a back yard that is a half wave across on LF :)  I do
> > not :(

I don't think this is required.  I would like to see dimensionaly similar 
horizonatal vs. vertical.  Otherwise we may find ourselves only proving 
that biger is better.

> > > > Regards,
> >
> > Bob ZL2CA

Good thinking for someone who hangs upside down allll the time. :-)
Maybe I should try it.

> >
> > PS I would ask Finbar if he could erect a second big mast for the other
> > end of an LF dipole?

Frank K0BRA