The prototype of a vertical radiating antenna is the VERTICAL DIPOLE.
This antenna is full size, and has to be mounted at least 0.25 wavelength
above ground in order for it to perform as expected (as in free space,
with maximum fieldstrength at the horison). One of the most difficult arrangements
is to feed it, without the RF-energy floating along the feeder which upsets
the the clean radiation pattern. Some attempts have been done using a "bazoka",
which formes a tube around the feeder. This bazoka is the negative part
of the dipole and has high RF-voltage at its end, which makes it hard to
have it in close range from the feeder. This bazoka is most often formed
as a cone with the top upwards.
On LF-bands it is not practical to make the vertical with the bazoka,
due to its height. We just have to reduce one parts length in the vertical
plane. As the vertical, as all antennas, has its mirror at the equivalent
RF-GROUND, it is most whise to shorten the lower part.
How can we make this lower part as efficient as possible ?
It depends upon what parameter we want to optimize. In the broadcast
industry they have access to the MW-band, where they must rely upon the
same kind of antenna as we do; the vertical. We have though a difference
in opinion what makes a good antenna. As numerous papers state regarding
this matter, it is the radiation efficiency one shall concentrate
on.
Radio amateurs has NOT ordered any scientific work in order to optimize
verticals, only the BC-industry has. Due to this fact it has not been scientifically
confirmed how to construct a vertical antenna system that has an optimized
efficiency at the really low take off angles.
Radials acts like WAVE-antennas; they have a certain wave velocity along
them selves. What figure this has is depending upon how the radials are
put out; slowest if burried and fastest if elevated above ground. The overall
ground conductivity is also a factor to count with, which in turn also
is a figure of what deviation the RF-ground has in comparison with the
real ground. The only ground the antenna relates to is the RF. The better
the conductivity is, the closer to the real ground comes the rf-ground.
In certain small-rocky and sandy soil the RF-ground can be SEVERAL meters
below the surface, also depending upon the frequency of interest.
In an overall point of view the velocity along a real radial is close
to 90 %. This means that after a wavelength long radial we have a buit
up phaseshift at its end of some 36°. After 2 wavelengths we will have
some 72°. When the shift reaches 90° and beyond, it starts to inhibit itself
in a radial point of view. When the phaseshift is 180° we have NO EXISTING
RADIAL ANYMORE, only a piece of wire acting like a mirror image.
For the invested amount of wire shall be as efficient as possible,
we have to stop at the point when the phaseshift is some 90° at the radials
end. (We can make the radial even longer, by means of controlling the radial
current, just as in the current controlled wire antenna. This is done by
connecting transmitting capacitors in series with the wire at certain intervals,
making the overall velocity factor closer to 100%). This point is at
2.5 wavelengths. We can be sure of that it is efficient, every meter
of it and worth the trouble putting it out.
In order to evaluate long radials behaviour, I made some tests some
few years ago;
I compared the eastcoast and the westcoast of USA (via short path).
With the radials in that aziumuth direction (290-330°) beginning with 1/4-wave,
the ratio between the W1-2-3 & 4 contra the W6's was memorized. After
making the wires some 1 wave long (the same night!) the comparison revealed
that the W6's were an honest 6 dB better than before in comparison with
the eastcoasters. This relationship has been there ever since when using
these long radials,
operating more than 10.000 hours...( ! ) to confirm it...
Beeing located quite far north (57.7°) and two of the most active regions
also are located in the northern hemisphere, means that the signals have
to travel through an area close to the north pole and most often affected
by the AURORA. This Aurora can be considered as a blotting paper for
signals between some 10 and 2 MHz. The higher take off angle You have,
on signals entering the area, the more attenuation of it You will get.
If You are fortunate to have good efficiency at the really low take
off angles (<10°), then Your signals have the chance to sneak under
it UN-ATTENUATED !
A band opener
and a band closer-capability.
Why is the low takeoff angles so important ? Why bother ?
This is why:
Most of the serious HAM operators knows by nature how their favorite
band behaves; when it opens up and closes down to a certain area. They
also know that certain neighbour stations can contact for example USA earlier
than them selves, or earliest about 20 minutes later when opening up and
latest at 20 minutes before themselves. This is mainly due to different
takeoff efficiency at low angles. Ofcourse a hilltop location enhances
the behaviour but is not always necessary. For a low frequency vertical
antenna long radials (at the greatcircle direction of interest)
does make the difference.
On LF it is known that the GRAYLINE offers some few minutes with better
signals at a far away location. Close to the equator this period is some
5-10 minutes, extending closer to the poles to some 1.5 hour, highly dependant
upon at what latitude You are located. At 57° N the normal grayline is
some 30 minutes (in the middle of June some 1 hour). With
long radials this grayline effect can be extended to 2.5 hours !
The behaviour is reciprocal; You can have QSOs with for ex. USA 2.5 hours
BEFORE THEIR SUNSET and also 2.5 hours AFTER YOUR SUNRISE. On a single
day (night) this means that You can have some of more QSO:ing towards USA than the average operator
next door.
(C) Björn Waller, SM6EHY 1998-99