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

"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

"Jerry" wrote in message
...

"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

On Nov 25, 2:55*pm, "Thomas Magma"
wrote:
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

From modeling I did a long time ago: there is a slight advantage to
using high velocity factor line, but it's very marginal. Just as a
dipole doesn't need to be resonant to do a good job radiating (and
receiving), so the elements in the coaxial collinear don't need to be
resonant. The phasing among the elements is dictated by the coax
between the feedpoints. Each gap between two elements is a feedpoint;
across it is impressed the line voltage. Since each line segment is a
half wave long and the conductors are reversed at each junction, the
voltage across each feedpoint is the same and in phase, less a small
amount for line loss. The element currents depend on mutual coupling
among the elements, but my simulations for VF=0.66 to VF=1.00
indicated that the current phases were always very nearly the same.

Generally, you'll want all the elements to look the same from the
outside. The top element should be the same length as the rest. It's
common to short the coax an electrical quarter wave up from the
highest gap between elements; that reflects back an open circuit to
the bottom of the top element, so really you could just as well make
the top element a tube the same OD as the rest of the elements,
connected to the inner conductor of the next lower section. The
feedpoint impedance at the bottom of the antenna is just the parallel
combination of all the feedpoints, which are generally each fairly
high (since each one is feeding a full-wave doublet, essentially), but
with ten or so sections, the net is modest, generally around 100 ohms.

Whatever the feedpoint impedance is, you need to match to it properly--
to whatever degree of matching is "proper" in your book. I generally
use a simple "L" network: a variable C across the feedpoint, and an
inductor to the feed line center conductor. It matches the impedance
and can tune out some reactance. Then you need to decouple the
antenna from the feedline, and from other metal in the area where it's
mounted. I generally use self-resonant coils in the small feedline,
one immediately below the antenna and one another quarter wave lower.
You could also try sleeves or radials...

Summary: the coax provides proper feedpoint phasing (even if the
elements are shorter than 1/2 wave because of the VF of the line
used); a matching network lets you match to 50 ohms (or other
impedance if you want); decoupling keeps "antenna" current off the
feedline.

Cheers,
Tom

"Thomas Magma" wrote in message
...

"Jerry" wrote in message
...

"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)

From modeling I did a long time ago: there is a slight advantage to
using high velocity factor line, but it's very marginal. Just as a
dipole doesn't need to be resonant to do a good job radiating (and
receiving), so the elements in the coaxial collinear don't need to be
resonant. The phasing among the elements is dictated by the coax
between the feedpoints. Each gap between two elements is a feedpoint;
across it is impressed the line voltage. Since each line segment is a
half wave long and the conductors are reversed at each junction, the
voltage across each feedpoint is the same and in phase, less a small
amount for line loss. The element currents depend on mutual coupling
among the elements, but my simulations for VF=0.66 to VF=1.00
indicated that the current phases were always very nearly the same.

Generally, you'll want all the elements to look the same from the
outside. The top element should be the same length as the rest. It's
common to short the coax an electrical quarter wave up from the
highest gap between elements; that reflects back an open circuit to
the bottom of the top element, so really you could just as well make
the top element a tube the same OD as the rest of the elements,
connected to the inner conductor of the next lower section. The
feedpoint impedance at the bottom of the antenna is just the parallel
combination of all the feedpoints, which are generally each fairly
high (since each one is feeding a full-wave doublet, essentially), but
with ten or so sections, the net is modest, generally around 100 ohms.

Whatever the feedpoint impedance is, you need to match to it properly--
to whatever degree of matching is "proper" in your book. I generally
use a simple "L" network: a variable C across the feedpoint, and an
inductor to the feed line center conductor. It matches the impedance
and can tune out some reactance. Then you need to decouple the
antenna from the feedline, and from other metal in the area where it's
mounted. I generally use self-resonant coils in the small feedline,
one immediately below the antenna and one another quarter wave lower.
You could also try sleeves or radials...

Summary: the coax provides proper feedpoint phasing (even if the
elements are shorter than 1/2 wave because of the VF of the line
used); a matching network lets you match to 50 ohms (or other
impedance if you want); decoupling keeps "antenna" current off the
feedline.

Cheers,
Tom

Thanks Tom for the detailed explanation. My current sketch of my antenna
design uses a quarter wave sleeve on the lower end of the antenna to stub
the current off the feedline ground. I am hoping to see the antenna having a
(somewhat) characteristic impedance of 50 ohms since the transmission
elements although made out of copper pipe and copper rod are designed to be
50 ohm and the antenna is single end fed (unlike a dipole). I will make some
provisions for a small tuning structure. If I do have to tune, the nice
thing is that it will be a receive only antenna and I can get away with
small RF components.

Tom, since you seem to know quite a bit about collinear coaxial antenna
design, do you know if I should be using an even or odd amount of half wave
elements? I planned on four, do you see a problem with this.

Thanks again,
Thomas

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 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

"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

"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