Come on Carl! You're just being provocative. Someone may take that
seriously. You know very well that only the physical length matters
for narrowing the lobe.
Bob VE7BS
Hi Bob,
You have the right idea from the standpoint that 1500 feet of wire in a
100-foot linear spatial distance is still a 100-foot antenna.
The amount of wire in the antenna does not matter, or actually "drive" the
difference. The slower phase velocity along the wire is actually what makes
the difference. They are called "slow wave" antennas.
It can be slowed two ways, although some means of achieving slowing are not
practical for:
1.) Dielectric sleeve or other means of increasing shunting capacitance
2.) Increased series inductance
In the 70's, I tried grinding up ferrite and painting it on a wire. That was
before I understood, and while I was learning, what the mechanism at work
was. The think that set me off on that path was a lecture by John "Jack"
Kuechen (recently a SK, and I am not sure of the name spelling). He also
authored a book that was a collection of his lectures and work called
"Antennas and Transmission Lines". When it went out of print by Howard Sams
I convinced Martin Jue to pick up the rights for publication, so it is
probably still in publication. It is a good source of information on large
wire arrays, based on Kuecken's work with General Dynamics (real
measurements).
It isn't at all like a Beverage. The specific phasing mechanism at work is
that sections of the wire closer together are more out-of-phase. This
creates side nulls when things are correct.
For example, imagine a 1/2 wavelength wire. It has maximum sensitivity off
the broadside. Now if we slow the wave so much that the antenna has two
sections 1/8th wave in from the ends that are out-of-phase, we now force a
side null.
Where people get confused is they think this is some sort of Beverage mode,
and the same slowing with longer lengths will make the antenna "act longer".
What actually happens is we do have some narrowing of the main lobe with
modest wave slowing in long antennas, but we also create multiple nulls and
lose F/B ratio.
If people have some time, they can experiment with series inductors placed
at fractional wavelength intervals along the antenna in a model. What they
will find is this is no different than phasing the antenna with delay lines,
or any other method that slows the wave. The difficult part as the antenna
is made longer is bandwidth and rear null. The antenna, with a long antenna
and slow wave, can even tend to fire backwards. The termination impedance is
also modified, because the surge impedance is modified.
Once we a wavelength of space, or near a wavelength, we might as well use
two "Beverages" in stagger or echelon. It will have much higher F/B, much
more bandwidth, and be easier to build.
73 Tom
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