[Fwd: LF: Vertical versus Inverted-L]
Thu, 24 Dec 1998 09:49:20 -0500
> Mike Dennison wrote:
> > > Modelling I have done also strongly suggests that the type of top
> > > loading, and whether or not it is "lop sided", does not make much
> > > difference to the pattern, which is onmidirectional (the overhead null
> > > or minimum is not of much significance for DX working).
> > >
> > > I too have observed what seem to be moderately directional properties,
> > > but I believe this could be more to do with the surrounding environment
> > > rather than the antenna wire as such. Nearby buildings and trees
> > > distort the electric flux pattern (short verticals are fairly high
> > > impedance affairs) and the near field distortion takes its toll on what
> > > the computer model (usually with no nearby objects included) says is
> > > omnidirectional.
> > >
> > > Also at moderate distances the ground conductivity likely varies with
> > > path, and that too can lead to wrong deductions being made about
> > > directional antennas, when it is actually differential path loss.
> > >
> > > However, an intersting topic ....
> > >
> > > Cheers (and beers),
> > >
> > > Bob ZL2CA
> > >
> > Yes, quite true, but in the case under discussion the comparison
> > was between two antennas erected at different times in the same
> > environment - actually about 2m apart. The site itself should favour
> > the south-east as the ground slopes that way to a river 1km away,
> > and the entire earth system is in that direction.
> Downward slopes are "good" for launching/receiving RF on most bands. I
> am not sure what comment to make about earth wires on one side only, but
> offhand I do not see why that has much impact on radiation in other
> directions. Earthing is mainly to do with "site efficiency" and not
> "site directivity" at wavelengths that are much longer than the
> (electrically) small antenna. Having earthing all around the base of
> the antenna should improve radiation in all directions, as it allows
> more current to flow in the vertical than was previously the case
> (ground loss dominates at LF, the more earthing the better).
> My QTH has fairly high hills about 1 kilometre south of me, but clear in
> other directions. Relative to another Wellington LF station (ZL2BBJ),
> on I get moderately better reports to/from the north, and moderately
> worse reports to/from the south. These hills are in the near field for
> LF (wavelength about 1600 metres at 180 kHz).
> > I am pleased to have generated some discussion on the subject.
> > On a similar subject, I have often seen theoretical comparisons
> > between an inverted-L and a T. The theory says that the L will have
> > some high angle radiation from the single top wire. But the
> > radiation from the two top wires on the T "cancel each other out",
> > so it is more efficient. But is this true?
> > Where does this cancelling take place? Is the RF radiated, but out
> > of phase, and therefore gets cancelled in the "ether"? Or is it
> > genuinely not radiated, and therefore concentrates the RF in the
> > vertical section? I suspect it is the former, in which case an L is no
> > less efficient for low angle working, and is more efficient for high
> > angle.
> > Mike, G3XDV (IO91VT)
> In the top loading, the current is maximum where it connects to the "up
> wire". The current tapers off to zero at outer tips of the top
> loading. In a T, the current divides going to each side of the top
> loading (+ in one side, - in the other, relatively speaking), which has
> the main impact of reducing the horizontally polarised component of
> radiation. When "viewed" from some distance away, the net result of +
> and - currents is to reduce the horizontal component compared to one
> current alone. The horizontal component is probably not "cancelled" but
> it is much reduced. The horizontal component is not much use at LF, and
> mostly "warms up the ground" rather than helps with a QSO. In an L
> antenna, the all current in the top loading flows to one side, so has a
> much greater horizontal component than for a T.
> However, practicalities of making the most of a given QTH over-ride
> theories, and an L antenna is far better than no antenna!
> Bob ZL2CA
> All the best for the festive season ....