On 4/25/2017 10:12 PM, Jeff Liebermann wrote:
On Tue, 25 Apr 2017 17:24:37 -0700, (Dave
Platt) wrote:
In article ,
Jeff Liebermann wrote:
On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote:
I think the real problem is this antenna for 2 meter operation is 20
feet long!
Yep. The problem with the alternating coax cable antenna design is
that only every other 1/2 wave section radiates. The result of half
the radiation is half the gain. Or, as you've noticed, the antenna is
twice as long as it might be with phasing elements between 1/2 wave
sections.
Do you have a sim or model of that situation, showing that there aren't
significant RF currents on every other half-wave section? Somehow I
can't make sense of how that would happen.
No, I don't. I'll see what I can find but I don't recall ever seeing
such a model.
My mental model of the alternating-sections design has been that all
of the sections do radiate... the alternating hookup forces them to
radiate in phase with one another, rather than out of phase (and thus
tending to squint the pattern badly upwards and downwards).
Now, I have heard that the alternating-coax collinear doesn't have as
much gain as an array of separate dipoles hooked up with a phasing
harness... but I've always read that as being explained by the fact
that the upper sections are carrying smaller RF currents than the
lower because some power has been radiated away before the signal
reaches the upper part of the antenna.
On every other 1/2 wave element, the wire that carries the signal is
inside a shielded and grounded conductor. I don't think it's going to
radiate.
Huh, what? Doesn't the shield carry the signal as well? Every "other"
section is exactly the same as the non-other sections. Which ones
radiate and which other-ones don't? Maybe I've got the wrong image in
my mind. I thought each section was coax but they connect wire to
shield both ways at each junction.
http://www.rason.org/Projects/collant/collant.htm
So, I'd appreciate enlightenment here!
Well, I'll see what I can find and do. If necessary, I'll throw
together a model. Modeling coax cables with NEC2 might be difficult
or impossible, but I'll see if I can fake it:
"The Dipole and the Coax"
https://www.antennex.com/w4rnl/col0606/amod100.html
"Neither software core (NEC and MININEC) is capable of physically
modeling conventional coaxial cables. The transmission line
function within NEC creates lossless non-radiating mathematical
models of lines and hence cannot capture common mode radiation.
Therefore, the method used to show common mode radiation is to
place a third leg into the dipole."
and so on... This is probably more than I want to attempt without
some study time.
"Collinear antenna structure"
https://www.google.com/patents/US6771227
"Collinear antenna of the alternating coaxial type"
https://www.google.com/patents/US20040125038
--
Rick C