Topband: Beverages 2 basic questions

[Tom W8JI]

>  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