On Nov 26, 9:32*am, "Thomas Magma"
wrote:
From modeling I did a long time ago: *there is a slight advantage to
using high velocity factor line, but it's very marginal. *Just as a
dipole doesn't need to be resonant to do a good job radiating (and
receiving), so the elements in the coaxial collinear don't need to be
resonant. *The phasing among the elements is dictated by the coax
between the feedpoints. *Each gap between two elements is a feedpoint;
across it is impressed the line voltage. *Since each line segment is a
half wave long and the conductors are reversed at each junction, the
voltage across each feedpoint is the same and in phase, less a small
amount for line loss. *The element currents depend on mutual coupling
among the elements, but my simulations for VF=0.66 to VF=1.00
indicated that the current phases were always very nearly the same.

Generally, you'll want all the elements to look the same from the
outside. *The top element should be the same length as the rest. *It's
common to short the coax an electrical quarter wave up from the
highest gap between elements; that reflects back an open circuit to
the bottom of the top element, so really you could just as well make
the top element a tube the same OD as the rest of the elements,
connected to the inner conductor of the next lower section. *The
feedpoint impedance at the bottom of the antenna is just the parallel
combination of all the feedpoints, which are generally each fairly
high (since each one is feeding a full-wave doublet, essentially), but
with ten or so sections, the net is modest, generally around 100 ohms.

Whatever the feedpoint impedance is, you need to match to it properly--
to whatever degree of matching is "proper" in your book. *I generally
use a simple "L" network: *a variable C across the feedpoint, and an
inductor to the feed line center conductor. *It matches the impedance
and can tune out some reactance. *Then you need to decouple the
antenna from the feedline, and from other metal in the area where it's
mounted. *I generally use *self-resonant coils in the small feedline,
one immediately below the antenna and one another quarter wave lower.
You could also try sleeves or radials...

Summary: *the coax provides proper feedpoint phasing (even if the
elements are shorter than 1/2 wave because of the VF of the line
used); a matching network lets you match to 50 ohms (or other
impedance if you want); decoupling keeps "antenna" current off the
feedline.

Cheers,
Tom

Thanks Tom for the detailed explanation. My current sketch of my antenna
design uses a quarter wave sleeve on the lower end of the antenna to stub
the current off the feedline ground. I am hoping to see the antenna having a
(somewhat) characteristic impedance of 50 ohms since the transmission
elements although made out of copper pipe and copper rod are designed to be
50 ohm and the antenna is single end fed (unlike a dipole). I will make some
provisions for a small tuning structure. If I do have to tune, the nice
thing is that it will be a receive only antenna and I can get away with
small RF components.

Tom, since you seem to know quite a bit about collinear coaxial antenna
design, do you know if I should be using an even or odd amount of half wave
elements? I planned on four, do you see a problem with this.

Thanks again,
Thomas

A couple comments:

First, about the feedpoint impedance. With four elements (a rather
short antenna at UHF), there are three gaps between elements, that
represent three feedpoints. Each feedpoint directly couples to the
two attached elements, and through mutual coupling to the other
elements. Consider driving just two such elements: it's just a
(nominally) full wave doublet, and the feedpoint impedance is pretty
high. Large diameter elements drop the feedpoint impedance, but even
with large (not "huge") elements, the impedance of such a doublet will
be several hundred ohms. To a rough approximation, in the coaxial
collinear, you are simply putting three of those in parallel, since
you're connecting them through 1/2 wave (electrical length) coax. If
the feedpoint is just a half wave (electrical length) of coax away
from the bottom gap between elements, you'll see the same impedance
echoed there. In ten element designs, I typically see around 100 ohms
at that bottom antenna feed spot. I fully expect to see quite a bit
higher than that with only four elements. You can simulate this
pretty easily with a program like EZNEC: just make a structure with
four elements, with equal voltage sources between each pair of
elements; figure the parallel combination of the reported impedances
at each feedpoint, and that will be a good estimate of the impedance
you'll see any even number of half-waves of feedline removed from the
antenna, assuming negligible loss in the feedline.

Second, about number of elements: it really doesn't matter a whole
lot. It will affect the impedance you see at the feedpoint, but the
antenna gain should go up monotonically with the number of elements,
if you get the phasing right and decouple the antenna from its
surroundings appropriately.

(Beware sleeve decoupling: it may not be as effective as you
think...)

I learned about these antennas when I got ****ed that I could never
seem to make one work right when I followed the instructions given in
the old ARRL pubs. I finally sat down with pen and paper and EZNEC
and worked out what was really going on. One thing I learned was that
the "magic" of the wax fill in the Stationmaster commercial antennas
wasn't really magic; neither the line velocity factor nor the
dielectric environment just outside the radiating elements is super-
critical. It IS important to get the element lengths right so the
phasing of the feedpoints is right.

Once I understood, building one that worked right became pretty easy.
Three steps: (1) use the coax VF and length to get the phasing
right. Accept whatever feedpoint impedance that gives you. (2) Use a
matching network (lumped or distributed, your choice) to match to that
impedance. (3) Decouple the antenna from its surroundings (including
but not limited to the feedline), so it can do its work properly. And
I suppose (4) mount it up high and in the clear for best coverage.
The rest is things like proper choices for good mechanical strength
and reliability.

Cheers,
Tom