RadioBanter - View Single Post - Coax Collinear Element Materials and Velocity Factor

Owen Duffy · April 6th 09, 12:50 AM posted to rec.radio.amateur.antenna

wrote in news:a706ece1-d519-47d0-8ffe-8e76cf03ff66
@z16g2000prd.googlegroups.com:

I always assumed having more 1/2 elements in a collinear was best,
because that obviously raises gain and lowers angle. So from that
point of view, I was thinking that it would actually be better to use
a low VF coax, since that would give you shorter length elements, thus
being able to fit more elements in a shorter space. But I've also read
that having the element lengths closer to actual 1/2 length (longer)
is actually more efficient than having more elements at a shorter (low
VF) length. Just wondering what people's opinion is on this... So lets
say you have a choice between using the insulated center conductor of
a VF 78 coax through brass tube outer elements and having room for a
few extra elements in a given length, verses using just an insulated
wire that has a 99 VF through brass tube outer elements, thereby
allowing slightly less elements because they're longer, but the
elements you do have are closer to actual 1/2 length. Which would be
best? And this is basically various UHF bands we're talking about.
Dave,

The popular explanation for these things constructed with reversing coax
sections is that the currents all along the vertical are (exactly) in
phase.

That explanation doesn't seem consistent with the nominal half wave
elements being in fact a halfwave at VF=0.67, ie about two thirds the
length. Nor does it deal with the fact that the element ends are
connected to each other, ie no charge difference permitted.

I have been playing with an NEC model of eight half wave
elements using RG213. The currents are certainly not exactly in phase,
not nearly, and the gain in freespace at 6dBi is less than I see claimed
for this antenna. People seem to justify a claim of dB gain as 3dB for 2
elements, 6dB for 4 elements, 9dB for 8 elements... in which case they
must mean gain to be wrt a dipole, and therefore gain of 8 half waves
would be 11.2dBi... very sus, too simplistic.

You asked specifically about efficiency. The efficiency in my NEC model
(which includes internal loss in the RG213) is very good, 99%.

Confusing part is that some of the designs have a half wave sticking out
the top, and others a quarter wave, similarly different treatments at the
bottom, and they aren't consistent about whether the coax breaks at or
near voltage maxima or minima.

I am still working on this, I suspect the antenna doesn't quite work as
often explained, and not nearly as good as claimed.

Owen