"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

"Thomas Magma" wrote in message
...

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.

There is a correction that should be made here. Polyvinyl chloride has high
radio frequency losses, and the addition of plasticizers usually increases
these losses. But these dielectric losses are NOT due to conduction.
Rather, they are the result of hindered rotational movement in the chemical
dipoles within the polymer structure itself. In an insulator, when an AC
voltage is applied, most of the current through the capacitor formed by the
insulator leads the applied voltage. In a perfect capacitor, the current
leads the voltage by 90 degrees. But in a real capacitor, the insulator has
dielectric losses which means that the current leads the applied voltage by
less than 90 degrees; i.e. a portion of the current is now in phase with the
applied voltage. This current produces heating of the insulator. AT A
GIVEN FREQUENCY, the capacitor acts as if is a pure capacitance in series
with a resistance (or in parallel with a conductance). This model of a real
capacitor is only valid at that ONE frequency. At DC, for example, most
capacitors show extremely little conduction. Their insulation resistance
can be over 10^10 ohm-cm. At high RF frequencies, the dielectric loss
increases.

In the case of polyvinyl chloride, which is a hard, very brittle material,
additives known as plasticizers are compounded into the PVC to produce the
desired mechanical properties. A little plasticizer makes PVC tougher and
easier to process. A lot of plasticizer makes PVC soft and pliable. Clear
vinyl tubing can be as much as 40% plasticizer. Plasticizers are not
chemically attached to the PVC polymer. This means that over time, the
plasticizer can leach out or evaporate from the soft vinyl, leaving it hard
and brittle again. Everyone is probably familiar with vinyl automobile seat
covers. When your car is parked in the hot sun, a portion of the
plasticizer evaporates out. Eventually the vinyl cracks and tears, and you
wind up with a greasy, difficult to remove, oily film on the inside glass of
the car. The plasticizer has left the vinyl, causing the cracking, and
condensed on the glass making a greasy mess. The sticky, gooey mess seen on
old vinyl power cords is also due to the plasticizer leaving the PVC and
accumulating on the surface.

Did you ever wonder what was meant when coaxial cable was described as
having a non-contaminating vinyl jacket? This means that the plasticizer in
the cable jacket leaches out, but very slowly compared to the service life
of the cable. In older, and cheaper coax cable, conventional plasticizers
are used which leach out or evaporate fairly quickly. This makes the cable
stiffer and more prone to cracking. But long before this happens, the
plasticizer has migrated into the polyethylene insulation surrounding the
inner conductor, greatly increasing its RF losses. This can take just a few
years. In some of the newer cables, a foil or metalized polyester layer
surrounds the polyethylene under the shield. This effectively prevents the
migration of the plasticizer.

To go back to the antenna issue, polyurethane foams of low density (lots of
void space) have a low dielectric constant and small loss tangent (small
dissipation factor). "The Handbook of Antenna Design" By A. W. Rudge, K.
Milne, A. David Oliver, and P. Knight, has a discussion of high strength
polyurethane foams as radome materials. However these foams are different
from the "Great Stuff" foams in a can that you buy at the local hardware
store. These foams are moisture cured so their dielectric losses will be
somewhat higher. Do not confuse these with latex foams which have much
greater dielectric losses. Also remember that these uncured urethane foams
have 4,4-methylene bisphenyl isocyanate as one component. This is a nasty
material from a safety viewpoint (a skin and lung allergic sensitizer), so
follow the instructions carefully about gloves and eye protection.

To conclude, I would avoid the PVC material as a protective cover. Most
common fiberglass tubes are either fiberglass/polyester or fiberglass/epoxy
composites. Both materials have some dielectric loss but far less than PVC.
Urethane foam will be a fairly good material to hold the antenna rigid
within the tube. However the tube and the foam WILL detune the antenna
meaning you will need to do some experimentation before you can produce the
desired results.

73, Dr. Barry L. Ornitz WA4VZQ

"NoSPAM" wrote in message
...
"Thomas Magma" wrote in message
...

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.

There is a correction that should be made here. Polyvinyl chloride has
high radio frequency losses, and the addition of plasticizers usually
increases these losses. But these dielectric losses are NOT due to
conduction. Rather, they are the result of hindered rotational movement in
the chemical dipoles within the polymer structure itself. In an
insulator, when an AC voltage is applied, most of the current through the
capacitor formed by the insulator leads the applied voltage. In a perfect
capacitor, the current leads the voltage by 90 degrees. But in a real
capacitor, the insulator has dielectric losses which means that the
current leads the applied voltage by less than 90 degrees; i.e. a portion
of the current is now in phase with the applied voltage. This current
produces heating of the insulator. AT A GIVEN FREQUENCY, the capacitor
acts as if is a pure capacitance in series with a resistance (or in
parallel with a conductance). This model of a real capacitor is only
valid at that ONE frequency. At DC, for example, most capacitors show
extremely little conduction. Their insulation resistance can be over
10^10 ohm-cm. At high RF frequencies, the dielectric loss increases.

In the case of polyvinyl chloride, which is a hard, very brittle material,
additives known as plasticizers are compounded into the PVC to produce the
desired mechanical properties. A little plasticizer makes PVC tougher and
easier to process. A lot of plasticizer makes PVC soft and pliable.
Clear vinyl tubing can be as much as 40% plasticizer. Plasticizers are
not chemically attached to the PVC polymer. This means that over time,
the plasticizer can leach out or evaporate from the soft vinyl, leaving it
hard and brittle again. Everyone is probably familiar with vinyl
automobile seat covers. When your car is parked in the hot sun, a portion
of the plasticizer evaporates out. Eventually the vinyl cracks and tears,
and you wind up with a greasy, difficult to remove, oily film on the
inside glass of the car. The plasticizer has left the vinyl, causing the
cracking, and condensed on the glass making a greasy mess. The sticky,
gooey mess seen on old vinyl power cords is also due to the plasticizer
leaving the PVC and accumulating on the surface.

Did you ever wonder what was meant when coaxial cable was described as
having a non-contaminating vinyl jacket? This means that the plasticizer
in the cable jacket leaches out, but very slowly compared to the service
life of the cable. In older, and cheaper coax cable, conventional
plasticizers are used which leach out or evaporate fairly quickly. This
makes the cable stiffer and more prone to cracking. But long before this
happens, the plasticizer has migrated into the polyethylene insulation
surrounding the inner conductor, greatly increasing its RF losses. This
can take just a few years. In some of the newer cables, a foil or
metalized polyester layer surrounds the polyethylene under the shield.
This effectively prevents the migration of the plasticizer.

To go back to the antenna issue, polyurethane foams of low density (lots
of void space) have a low dielectric constant and small loss tangent
(small dissipation factor). "The Handbook of Antenna Design" By A. W.
Rudge, K. Milne, A. David Oliver, and P. Knight, has a discussion of high
strength polyurethane foams as radome materials. However these foams are
different from the "Great Stuff" foams in a can that you buy at the local
hardware store. These foams are moisture cured so their dielectric losses
will be somewhat higher. Do not confuse these with latex foams which have
much greater dielectric losses. Also remember that these uncured urethane
foams have 4,4-methylene bisphenyl isocyanate as one component. This is a
nasty material from a safety viewpoint (a skin and lung allergic
sensitizer), so follow the instructions carefully about gloves and eye
protection.

To conclude, I would avoid the PVC material as a protective cover. Most
common fiberglass tubes are either fiberglass/polyester or
fiberglass/epoxy composites. Both materials have some dielectric loss but
far less than PVC. Urethane foam will be a fairly good material to hold
the antenna rigid within the tube. However the tube and the foam WILL
detune the antenna meaning you will need to do some experimentation before
you can produce the desired results.

73, Dr. Barry L. Ornitz WA4VZQ


Thanks Barry for the in-depth enlightenment of radome material Do you
happen to know the answer to this question: When you tune the near field
effects of PVC out, what is the end result in loss? and how is this compared
to fiberglass?

Thomas

"Thomas Magma" wrote in message
...

Thanks Barry for the in-depth enlightenment of radome material Do you
happen to know the answer to this question: When you tune the near field
effects of PVC out, what is the end result in loss? and how is this
compared to fiberglass?

When you tune the antenna to compensate for its surroundings, usually by
shortening the elements since the real portion of the permittivity of the
close surroundings is capacitive, you just have to accept the added loss due
to the imaginary portion of the permittivity, the dielectric dissipation.

When designing a radome, look for a material with the lowest dissipation
factor (or loss tangent which is another way of describing the dissipation
losses). The added capacitive reactance can be tuned out, but it is still
best to use a material with a low dielectric constant. This does not say
that the antenna pattern will not be affected, however, as the antenna
lengths and spacings are changed.

A good analogy to illustrate this is the fact that an aircraft cannot be
perfectly stealthy. If the aircraft is made entirely from microwave
transparent material, the difference between that material's dielectric
constant and that of the surrounding air will still generate reflections
that will show up on radar. In fact, if you make the aircraft out of
completely absorbing material, the bow wave compression of the air will
create a slight dielectric discontinuity which still reflects microwaves.
Of course, the radar cross section will be far smaller. The idea is to make
the radar target appear so small that it is ignored.

While the dielectric constant of PVC is slightly lower than fiberglass
reinforced polyester or fiberglass reinforced epoxy, its lost tangent is
higher. Also, the mechanical strength of PVC is less than the fiberglass
reinforced plastics, so you can make the radome thinner with the FRP
materials. This benefits the detuning as well as the losses.

Before closing, I would like to comment on an additional related issue
brought up by my friend AE6KS...

"Jeff Liebermann" wrote in message
...

If you want to try a real disaster, try black drainage PVC pipe.
Carbon filled. For a good acid test, try putting a pipe section in a
microwave oven.

I would be willing to bet that the carbon black had little to do with the
losses. It only takes a tiny amount of carbon black to make the plastic
quite black looking and to provide good ultraviolet light protection - just
a few percent at most. I once needed a good microwave absorber for an
instrument I was building. Not wanting to wait on Emerson & Cuming to sell
me some of their ECCOSORB ® material, I decided to make my own. Just down
the hall was our polymer testing lab where they could blend polymer chips
with various additives and mold me some 4" x 4" blocks from the blend. I
decided to have them make a polyethylene blend with 30% carbon black added.
The finished material was a dull black, and it would even leave nice black
marks when rubbed across paper. I was confident when I placed a 1/2" thick
block of the material between two WR-90 10 GHz flanges that it would not let
much signal through. Boy was I surprised when it offered very little
attenuation! It took a while to think about what was happening. The
polyethylene was a low loss dielectric material at these frequencies, and
the carbon black particles were extremely small. The result was a plastic
that contained conductive particles that were insulated from each other.
The particles were so small that even at 10 GHz, they were far too small to
couple much microwave energy into them. What I really needed was long
carbon fibers that made electrical contact with each other in the plastic.
I didn't have time to study this as the Eccosorb arrived and I used it in my
instrument. But it taught me a valuable lesson about absorbers.

By the way, http://www.eccosorb.com has a number of good technical tutorials
on absorbers and dielectrics.

73, Barry WA4VZQ

On Wed, 26 Nov 2008 10:50:37 -0800, "Thomas Magma"
wrote:

Thanks for all the good points, but you haven't scared me away yet

Good. I don't mind spending the time if you're willing to build it.
None of my objections are particularly fatal. However, I would
suggest you at least investigate alternatives, which in my opinion,
work better (and are easier to build). For example, the 4 bay stacked
vertical folded dipoles, with coaxial power dividers, is far less
complexicated, and methinks works better. I was building these for
463/468MHz in about 1968(?) out of strips of 1/2" wide aluminum and
pop rivets. If you're interested, I'll see if I can find some photos
or scribblings. There are a few in this photo:
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn02.html
but I can't distinguish mine from the stock dB Products antennas.
Incidentally, that's a great example of how *NOT* to install antennas.
Those are all transmit antennas with no ferrite isolators. The
intermod generated was monumental.

Some more examples of commercial versions:
http://www.radiowrench.com/sonic/so02202.html
http://www.radiowrench.com/sonic/ (look for dB Products PDF's)

My
target frequency is around lets say 418MHz (that's not really it, I like to
remain anonymous).

Y'er no fun.

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.

Both. I suspect you have a sweeper and some means of measuing
reflection coefficient or VSWR in real time (on a scope). If not, the
reflection coefficient bridge is easy to build. Take any antenna you
look at the VSWR curve on the scope. Then, shove the pipe over the
antenna and watch what happens. If the tubing were fiberglass, some
thin plastics, or glass, nothing will change on the scope. PVC and
ABC will detune the antenna. So will common fence post compound
(urathane foam) but to a lesser degree. Packing the empty space with
styrofoam or styroam peanuts seems to work well enough and result in a
repairable antenna. Real fiberglass tubing (masts or marine hardware)
is easy enough to obtain, that I wouldn't bother with PVC. Besides,
fiberglass is nice and stiff, while PVC flops around in the wind.

I plan to use a fibreglass tubing and spacers so
hopefully I don't see as much near field effects as you did.

Yep. There's always hope.

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.

If you want to try a real disaster, try black drainage PVC pipe.
Carbon filled. For a good acid test, try putting a pipe section in a
microwave oven. If it stays cold, you win. If it gets hot, thing
again. If it melts and catches fire, forget it. It's also fun to
take an ordinary 440 yackie talkie or scanner, shove a piece of PVC
pipe over the antenna, and listen to the signal change. I like to do
this demo at radio club meetings. The best of the bunch is
fiberglass. A close 2nd is white ABS (acrylo-nitrile butadene
styrene) which is a bit difficult to find. It's commonly used in
vacuum forming and commonly found on GPS antennas and such.

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.

It matters quite a bit. However, I can't offer an answer. Some
designes use the bottom section as a matching transformer or
counterpoise. That mangles the count. I would have to see what
you're doing to make the determination.

Also, I could probably figure it out, but it's midnight and I'm beat.
I spent 5 days last week fighting a kidney stone and am still kinda
wiped from that. If you can't figure it it, bug me and I'll do the
dirty work. A good clue is that the center conductor of the input
coax connector must connect to the center wire which goes to the 1/4
vertical whip section at the top.

Also, there are patents worth reading:
http://www.google.com/patents?id=JMweAAAAEBAJ&dq=6947006
http://www.google.com/patents?id=qDYWAAAAEBAJ&dq=6947006
http://www.google.com/patents?id=XpgfAAAAEBAJ&dq=6947006
etc. The accompanying explanations are usually sufficient to figure
out how it works. You might notice that one of the construction
methods is applicable to your copper tubing idea.


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

"Jeff Liebermann" wrote in message
...
On Wed, 26 Nov 2008 10:50:37 -0800, "Thomas Magma"
wrote:

Thanks for all the good points, but you haven't scared me away yet

Good. I don't mind spending the time if you're willing to build it.
None of my objections are particularly fatal. However, I would
suggest you at least investigate alternatives, which in my opinion,
work better (and are easier to build). For example, the 4 bay stacked
vertical folded dipoles, with coaxial power dividers, is far less
complexicated, and methinks works better. I was building these for
463/468MHz in about 1968(?) out of strips of 1/2" wide aluminum and
pop rivets. If you're interested, I'll see if I can find some photos
or scribblings. There are a few in this photo:
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn02.html
but I can't distinguish mine from the stock dB Products antennas.
Incidentally, that's a great example of how *NOT* to install antennas.
Those are all transmit antennas with no ferrite isolators. The
intermod generated was monumental.

Some more examples of commercial versions:
http://www.radiowrench.com/sonic/so02202.html
http://www.radiowrench.com/sonic/ (look for dB Products PDF's)

My
target frequency is around lets say 418MHz (that's not really it, I like
to
remain anonymous).

Y'er no fun.

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.

Both. I suspect you have a sweeper and some means of measuing
reflection coefficient or VSWR in real time (on a scope). If not, the
reflection coefficient bridge is easy to build. Take any antenna you
look at the VSWR curve on the scope. Then, shove the pipe over the
antenna and watch what happens. If the tubing were fiberglass, some
thin plastics, or glass, nothing will change on the scope. PVC and
ABC will detune the antenna. So will common fence post compound
(urathane foam) but to a lesser degree. Packing the empty space with
styrofoam or styroam peanuts seems to work well enough and result in a
repairable antenna. Real fiberglass tubing (masts or marine hardware)
is easy enough to obtain, that I wouldn't bother with PVC. Besides,
fiberglass is nice and stiff, while PVC flops around in the wind.

I plan to use a fibreglass tubing and spacers so
hopefully I don't see as much near field effects as you did.

Yep. There's always hope.

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.

If you want to try a real disaster, try black drainage PVC pipe.
Carbon filled. For a good acid test, try putting a pipe section in a
microwave oven. If it stays cold, you win. If it gets hot, thing
again. If it melts and catches fire, forget it. It's also fun to
take an ordinary 440 yackie talkie or scanner, shove a piece of PVC
pipe over the antenna, and listen to the signal change. I like to do
this demo at radio club meetings. The best of the bunch is
fiberglass. A close 2nd is white ABS (acrylo-nitrile butadene
styrene) which is a bit difficult to find. It's commonly used in
vacuum forming and commonly found on GPS antennas and such.

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.

It matters quite a bit. However, I can't offer an answer. Some
designes use the bottom section as a matching transformer or
counterpoise. That mangles the count. I would have to see what
you're doing to make the determination.

Also, I could probably figure it out, but it's midnight and I'm beat.
I spent 5 days last week fighting a kidney stone and am still kinda
wiped from that. If you can't figure it it, bug me and I'll do the
dirty work. A good clue is that the center conductor of the input
coax connector must connect to the center wire which goes to the 1/4
vertical whip section at the top.

Also, there are patents worth reading:
http://www.google.com/patents?id=JMweAAAAEBAJ&dq=6947006
http://www.google.com/patents?id=qDYWAAAAEBAJ&dq=6947006
http://www.google.com/patents?id=XpgfAAAAEBAJ&dq=6947006
etc. The accompanying explanations are usually sufficient to figure
out how it works. You might notice that one of the construction
methods is applicable to your copper tubing idea.


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

I'm anxious to get started so I've put my copper pipe design on hold well I
wait for parts and decided to start with a coax approach. So I hit the
hardware store and got some PVC pipe and mounting bits. I understand that
the PVC is not as good as fiberglass because of it's near field effects, BTW
if you can tune those effects out, what is the end result in loss? I plan
on using LMR-200 because of it's slight rigidity and it's high velocity
factor (83%). I bought 1-1/2 inch rubber washers with a 3/16 hole in the
center that will slide over the coax and then be pulled into the 1-1/4 inch
PVC this will center and support the coax up the length of the pipe. I will
try using some clamp-on ferrites that we have laying around to stub the
currents on the feed line and slide them around and see if I can tune the
antenna using the network analyzer. I still don't understand what that
quarter wave whip is suppose to do that sits on top of the array and I think
I will try to omit that in my first design (unless someone convinces me
otherwise).

Anyways, time to get my hands dirty and build me an antenna!

Thomas Magma

On Thu, 27 Nov 2008 10:10:25 -0800, "Thomas Magma"
wrote:

I'm anxious to get started so I've put my copper pipe design on hold well I
wait for parts and decided to start with a coax approach.

Sigh.

So I hit the
hardware store and got some PVC pipe and mounting bits.

Schedule 40, schedule 80, water, or electrical? They're all
different. Did you at least do the microwave oven test on a small
piece to see if you're headed for a problem? I've had a few surprises
with different vendors and styles.

I understand that
the PVC is not as good as fiberglass because of it's near field effects, BTW
if you can tune those effects out, what is the end result in loss?

No. You can't make the radome (pipe) big enough to get out of the
near field. Minimum is a few wavelengths.

Try a chunk of PVC over your 440 HT or scanner whip antenna and see if
you want to continue blundering along this path.

I plan
on using LMR-200 because of it's slight rigidity and it's high velocity
factor (83%).

The added rigidity doesn't buy you much if you're going to shove it
down a pipe.

I bought 1-1/2 inch rubber washers with a 3/16 hole in the
center that will slide over the coax and then be pulled into the 1-1/4 inch
PVC this will center and support the coax up the length of the pipe.

Why such a large diameter pipe? There's no difference in loss.

I will
try using some clamp-on ferrites that we have laying around to stub the
currents on the feed line and slide them around and see if I can tune the
antenna using the network analyzer.

Got a ferrite that works at 418MHz? Even if the ferrite does work,
the RF its blocking is converted to heat. Wouldn't it be better if
you built a proper matching contrivance to that RF is radiated instead
of absorbed? I suggest you lose the ferrites and band-aids as they
tend to hide design errors and inefficiencies.

I still don't understand what that
quarter wave whip is suppose to do that sits on top of the array

I hate easy questions. If you look at the construction of the
alternating coax sections, the top section will be one with the hot RF
lead eventually connected to the outside of the top coax section. In
other words, the outside of the coax is the radiating element.
http://www.rason.org/Projects/collant/collant.htm
Why bother using another coax section when it would be easier to just
use a piece of wire? Look at the Fig 3 drawing and just follow the RF
path from the coax entry at the left to the 1/4 wave element on the
right. That might also answer your question about odd/even sections.

and I think
I will try to omit that in my first design (unless someone convinces me
otherwise).

Not recommended, but you have the test equipment to determine if it's
a good or bad idea. Ummm... you were planning on testing this thing?

Anyways, time to get my hands dirty and build me an antenna!

Good luck, but first a little math. What manner of tolerance do you
thing you need to cut your coax pieces? Let's pretend you wanted to
get the center frequency accurate to 1Mhz. At 418MHz, one wavelength
is:
wavelength(mm) = 300,000 / freq(mhz) * VF
wavelength = 3*10^5 / 418 * 0.83 = 596 mm
That works out to:
596 / 418 = 1.4 mm/MHz
So, if you want the center frequency accurate to within +/- 1MHz, you
gotta cut it to within +/- 1.4 mm. Good luck. Like I previously
ranted, you'll need a cutting fixture. A steady hand, good eye,
quality coax, and plenty of patience are also helpful.

Incidentally, since the top 1/4 wave element represents something
close to perhaps 50 ohms, it would be interesting to measure the
amount of RF that isn't radiated and actually gets to the top section
of the antenna. If my analysis of the antenna is correct, the first
section (near the coax connector) radiates 1/2 the power. The next
section 1/4th. After that 1/8th, etc. By the time it gets to the top
of the antenna, there won't be much left. However, that's theory,
which often fails to resemble reality. It would interesting if you
stuck a coax connector on the top, and measured what comes out.

Happy Day of the Turkeys.


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

On Nov 25, 5:55*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

I just built one of these using thin wall 1 1/4" aluminum tubing and
bare 20ga copper wire for inside. It was a tri band
for 50, 144 & 440 mhz, section = 1/4 and 3 x 1/2 wave then top 1/4
section cut for 146mhz. SWR is higher than I
like but it receives and signal reports are so much better than my
factory 5/8 antenna I will now try to lower the swr.
I used short pieces of PVC to couple the tubing, its freestanding and
15' tall, very light weight, I used old swmming pool cleaner
telescopic pole for the elements.

N4aeq

unaxesc