"Jeff Liebermann" wrote in message
...
On Tue, 25 Nov 2008 14:55:52 -0800, "Thomas Magma"
wrote:

I am about to attempt to build a UHF collinear coaxial antenna and am
trying
to finalize a design.

What design? Drawing? Description? NEC model? Numbers?
UHF is about 300 to 1000MHz. Any particular frequency?

Incidentally, it's not a "coaxial antenna". It's an end fed vertical
colinear using coaxial cable elements.

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?

Yep, lots wrong. End fed colinear antennas are convenient but far
from ideal. They're also deceptively simple where the problems only
show up after the antenna is built.

1. Most of the RF comes out the bottom of the antenna. Very roughly,
the first dipole belches 1/2 the RF, the next dipole belches 1/4 the
RF, then 1/8th, and so on. This is NOT exact, but close enough to
illustrate the problem. You can make it as long as you want, but if
somehow manage to cover up the lower part of the antenna (a common
problem on a rooftop or side mounted on a tower), most of the signal
is history.

2. The alternating 1/2 wave coax cable type antenna is twice as long
as necessary. Every other 1/2 wave coax section is essentially a
non-radiationg phasing section. That's convenient for construction,
but not very compact. A similar antenna, using a simple 1/2 wave
hairping stub, with be half the length, with the same gain.

3. Coax is lossy. Coax phasing sections add un-necessary loss that
is not present in an antenna that uses (for example) a hairpin stub or
coil instead. Your copper pipe and air dielectric idea eliminates
this problem, but I thought I would throw this in for those building
them from coax cable scraps.

4. End fed antennas tend to have pattern uptilt. That may or may not
be a problem depending on your unspecified application. The uptilt
doesn't show up on free space models, but is certainly there if you
include the effects of a rooftop, ground, or mast arm. If this is
going on a mountain top, you might consider mounting it upside down.

You can reduce the uptilt problem somewhat by cutting the antenna in
half and feeding it in the middle (forming a dipole), rather than end
feeding it. Several commercial antennas work this way. That also
eliminates the need for ground plane radials at the base.

5. The effects of the radome can be critical. I built such a UHF
antenna for 463MHz long ago. It worked well enough with exposed
sections. However, when I potted it with urathane fence post foam in
a PVC pipe enclosure, the center frequency drifted downward
sufficiently to render the antenna useless.

6. Cutting the coax sections accurately is difficult. If you're not
using a fixture for cutting, forget it.

7. Making it out of copper pipe is rather expensive but certainly
possible. Making the insulators will be somewhat of a challenge.
There's no velocity factor involved (Air=1) so the measurements will
be simple. However, since there's an overlap between sections, I'm
wondering from where to where you should measure. If you cut the
outer copper tubing to exactly 1/2 wave, then you need a very thin
insulator between sections to prevent shorts. Methinks there will
need to be some cut-n-try along with some careful measurments (swept
VSWR) along the way.

Good luck.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Hi Jeff,

Thanks for all the good points, but you haven't scared me away yet My
target frequency is around lets say 418MHz (that's not really it, I like to
remain anonymous). It was interesting what you said about the radome and how
it detuned the antenna. Do you think it was mainly the PVC or the urethane
foam that caused the issue. I plan to use a fibreglass tubing and spacers so
hopefully I don't see as much near field effects as you did. I have learned
that some PVC pipes have certain conductive additives and are not so good
for antenna use, plus it might be tough trying to sell a 'poop pipe' antenna
commercially if it ever became a product of ours.

Do you happen to know if I should be using a odd or even number of half wave
elements in my design? I'm beginning to think it doesn't really matter.

Thomas