Re: A few questions about collinear coaxial antennas
- From: "Thomas Magma" <somewhere@xxxxxxxxxxxxxxxxxx>
- Date: Tue, 25 Nov 2008 17:04:23 -0800
"Jerry" <jerrymartes@xxxxxxxxxxx> wrote in message
news:YG0Xk.1082$QX3.1048@xxxxxxxxxxxxxxxxxxxxxxx
"Thomas Magma" <somewhere@xxxxxxxxxxxxxxxxxx> wrote in message
news:_n%Wk.13$zQ3.3@xxxxxxxxxxxxxxx
Hello,
I am about to attempt to build a UHF collinear coaxial antenna and am
trying to finalize a design. I have done a lot of reading and am a little
confused on a few things. First off I have read contradicting statements
whether to use odd or even number of 1/2 wave elements. 1, 3, 5... or
1,2,4... Also I don't understand what the 1/4 wave whip is doing on the
top without a ground plane (found in most designs), is this necessary for
a receive antenna?.
Instead of using coaxial cable, I will be building the 1/2 wave and 1/4
wave transmission lines out of ridged copper pipe with air as it's
dielectric in order to maximize the velocity of propagation and therefore
making true 1/2 wave elements. Does anyone see anything wrong with this
approach?
Thomas
Hi Thomas
I think you can design and develop a very good colinear coaxial array at
UHF using copper pipe. Do you have any requirement for VSWR? Do you
have need for operating the antenna at other than one frequency? It isnt
easy to develop a good UHF colinear without good test equipment. How
will you measure input impedance? Do you care about the angle of the
radiation pattern maximum? End fed colinears will have beam squint with
frequency change.
Jerry KD6JDJ
Hi Jerry,
My application is at only one frequency so I intend to centre it on that
frequency and the VSWR I get is the VSWR I get. I would hope to be > 25 dB
return loss anyways. I do have a HP8714C network analyzer in the lab I will
be using so that is no problem. Due to the centre frequency (lower 400 MHz)
I figure I can only realistically have about 4 of the half wave elements
because of height, weight and wind loading. Oh wait was that 3 or 4 or 5
elements. I still haven't solved that fundamental issue yet. I don't suppose
the radiation pattern is too much of a concern at this point, as long as it
is omnidirectional.
Thomas
.
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