"Thomas Magma" wrote in message
news

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)



Hi Jerry,

It is a predetermined frequency that I am building the antenna for. It is
not determined if it will become a commercial product yet but I am trying
to design it as such. I can see that it might be a little time consuming
working with copper pipe, but once I get the formula right I should be
good to go. I'll start buy calculating the half wave elements based on
theory knowing my dielectric constant will be dry air or Argon.

The design I have sketch is pretty neat and clean (on paper anyways). It
has all the elements stacked directly on top of each other, unlike the
traditional staggered approach you see in other designs. Also the
dielectric chamber of the transmission elements are sealed and can be
filled with a noble gas such as Argon to prevent corrosion and detuning
from humidity.

My background is in receiver and transmitter design, so I'm quite familiar
with impedance matching and I understand how a Smith chart works on a
network analyzer.

I'm looking forward to working with copper pipe instead of 0201 capacitors
and a microscope!

Thomas

Hi Thomas

Your plan for this colinear antenna appears to be identical to the one I
designed for ACI in Van Nuys Calif.. It was stack of lingths of copper
tubes with no stagger. I dont remember what I finally did nor how it was
assembled. I do remember that it worked and that my supervisor was
impressed. Also, I remember that alot of impedance measurements were
performed.
I am sure you will get your antenna to work. I suspect you will get more
familiar with that Smith Chart in the process.

Jerry KD6JDJ