"Thomas Magma" wrote in message
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"Jerry" wrote in message
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"Thomas Magma" wrote in message
...
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

Hi Thomas

If you can use whatever frequency the antenna works best at, it may be
practical to build one then use the frequency of best performance with that
antenna. But, if you have some predetermined frequency that the antenna
must perform well at, there is a problem building prototypes. It can get
rather time consuming to build prototypes when using copper pipe.
Aparently you are confident that you can evaluate the antenna's input
impedance. I had figured that would be a fairly difficult task.

I'll be very interested in this project. Please keep the group informed
of your progress.

Jerry KD6JDJ (who has designed similar antennas for
commercial use)