On Nov 26, 9:32*am, "Thomas Magma"
wrote:
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

A couple comments:

First, about the feedpoint impedance. With four elements (a rather
short antenna at UHF), there are three gaps between elements, that
represent three feedpoints. Each feedpoint directly couples to the
two attached elements, and through mutual coupling to the other
elements. Consider driving just two such elements: it's just a
(nominally) full wave doublet, and the feedpoint impedance is pretty
high. Large diameter elements drop the feedpoint impedance, but even
with large (not "huge") elements, the impedance of such a doublet will
be several hundred ohms. To a rough approximation, in the coaxial
collinear, you are simply putting three of those in parallel, since
you're connecting them through 1/2 wave (electrical length) coax. If
the feedpoint is just a half wave (electrical length) of coax away
from the bottom gap between elements, you'll see the same impedance
echoed there. In ten element designs, I typically see around 100 ohms
at that bottom antenna feed spot. I fully expect to see quite a bit
higher than that with only four elements. You can simulate this
pretty easily with a program like EZNEC: just make a structure with
four elements, with equal voltage sources between each pair of
elements; figure the parallel combination of the reported impedances
at each feedpoint, and that will be a good estimate of the impedance
you'll see any even number of half-waves of feedline removed from the
antenna, assuming negligible loss in the feedline.

Second, about number of elements: it really doesn't matter a whole
lot. It will affect the impedance you see at the feedpoint, but the
antenna gain should go up monotonically with the number of elements,
if you get the phasing right and decouple the antenna from its
surroundings appropriately.

(Beware sleeve decoupling: it may not be as effective as you
think...)

I learned about these antennas when I got ****ed that I could never
seem to make one work right when I followed the instructions given in
the old ARRL pubs. I finally sat down with pen and paper and EZNEC
and worked out what was really going on. One thing I learned was that
the "magic" of the wax fill in the Stationmaster commercial antennas
wasn't really magic; neither the line velocity factor nor the
dielectric environment just outside the radiating elements is super-
critical. It IS important to get the element lengths right so the
phasing of the feedpoints is right.

Once I understood, building one that worked right became pretty easy.
Three steps: (1) use the coax VF and length to get the phasing
right. Accept whatever feedpoint impedance that gives you. (2) Use a
matching network (lumped or distributed, your choice) to match to that
impedance. (3) Decouple the antenna from its surroundings (including
but not limited to the feedline), so it can do its work properly. And
I suppose (4) mount it up high and in the clear for best coverage.
The rest is things like proper choices for good mechanical strength
and reliability.

Cheers,
Tom

"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

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

"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

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

"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 Nov 27, 2:46*pm, Jeff Liebermann wrote:
....
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.

But why would you care to try to get it within 1MHz? With only four
radiating elements, the beam 3dB width will be roughly 8 degrees if
the bottom of the antenna is a wavelength above ground (30 degrees in
freespace...). There's not much point in putting a lot of effort into
get closer than perhaps 4 electrical degrees along the line, and I
don't believe even that is necessary to get good performance. That's
several mm, and should be easy with such short lengths. Using foam-
Teflon coax makes it easy to do: the insulation doesn't melt when you
solder things together. I cut the sections to matched lengths, use a
little jig to trim the layers to the same lengths on each, and then
put a wrapping of 30AWG or so silver plated wire (wire-wrap wire)
around each joint to hold it while soldering. That makes it easy to
adjust before soldering, and solid after.

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.


There's very little loss in a half wave of decent coax at 450MHz.
That means that the voltage across the lowest junction between
sections is echoed up the antenna at each other junction. In
freespace, by symmetry, the currents will be very nearly the same
going down from the top as going up from the bottom. My model over
typical ground (bottom a wavelength above the ground) shows current
symmetry within a percent or so, assuming equal voltages driving each
of the three junctions. If you wish, you can use the parameters of
the line you're actually using to figure the differences among the
feedpoint voltages, based on the loads at each junction. When I've
done that in the past, the differences are practically negligible.
You can iterate, feeding those voltages back into the model to find
new load impedances, etc., repeating till you're happy that the models
have converged. Recent versions of EZNEC even let you put the
transmission line into the model, along with its loss.

The supporting tube certainly will affect the feedpoint impedance, but
in my experience, it does not materially affect the pattern. I deal
with the impedance through a matching network; it's no trouble to
adjust for a low enough reflection that I don't worry about it.
Decoupling is the more interesting problem, to me.

Cheers,
Tom

Jeff Liebermann wrote:
. . .
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.

I'm intrigued by this, and would like to know what "theory" it's based on.

The field radiated from a conductor is proportional to the current on
it. You'll see from either modeling or measurement that the currents on
all sections of a collinear array, or a long wire antenna for that
matter, are nearly the same. So in those directions in which the fields
reinforce, each section is contributing about the same amount to the
total field as any other.

Although the logic is sound for this particular situation, it can't be
used in general to assign particular amounts of radiated power to
particular parts of an antenna. The fields from two parts of the antenna
might partially or fully cancel in some directions, even though both are
producing large fields. Any part of the antenna which is carrying
current is involved in the radiation process, and the total field is the
vector, not algebraic, sum of those fields.

So if you have a valid method of determining how much of the total
radiated power comes from each part of an antenna, I'd be very
interested in learning more about it. References would be welcome.

Roy Lewallen, W7EL

On Fri, 28 Nov 2008 18:55:26 -0800, Roy Lewallen
wrote:

Jeff Liebermann wrote:
. . .
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.

I'm intrigued by this, and would like to know what "theory" it's based on.

I just knew this would create a problem. I'm open to corrections and
explanations. I'm still learning and tend to make some rather
disgusting fundamental errors.

It's an observation based upon my measurements with a field strength
meter on similar UHF colinear antennas (using 1/2 wave stubs for
phasing). Also on a center fed 2.4GHz Franklin sector antenna of
similar construction. Most of the voltage peaks were at the base of
the antenna, tapering off as the field strength meter was dragged to
the top of the antenna. Since the current through the antenna is
constant, I assumed that the bulk of the power came from the lower
elements of the antenna. My explanation was a geometric decrease in
radiatated power starting at the feed point.

I've also seen a similar effect with relatively high gain (10dbi)
2.4GHz omni antennas in WISP applications. Any blockage of the lower
sections of the antenna, had a much bigger effect on the range and
measure signal strength than covering roughly an equal amount near the
top of the antenna.

The field radiated from a conductor is proportional to the current on
it. You'll see from either modeling or measurement that the currents on
all sections of a collinear array, or a long wire antenna for that
matter, are nearly the same. So in those directions in which the fields
reinforce, each section is contributing about the same amount to the
total field as any other.

I can see that on some models. I never could successfully model an
antenna using coax cable sections as elements. Using a wire model,
the current distribution is constant along the length as you describe.
However, my field strength measurements show more RF towards the feed
point. It's difficult for me to tell exactly how much more RF because
my home made meter is not calibrated. I don't recall the exact
numbers but I can dig out the FSM and make some measurements on some
of the antennas I have hanging around on the roof this weekend.

Although the logic is sound for this particular situation, it can't be
used in general to assign particular amounts of radiated power to
particular parts of an antenna. The fields from two parts of the antenna
might partially or fully cancel in some directions, even though both are
producing large fields. Any part of the antenna which is carrying
current is involved in the radiation process, and the total field is the
vector, not algebraic, sum of those fields.

The models all show the total pattern produced by all the elements
combined. I haven't found a way to show the contributions by
individual elements, thus making it difficult to model my observation.

So if you have a valid method of determining how much of the total
radiated power comes from each part of an antenna, I'd be very
interested in learning more about it. References would be welcome.

Nope. I'll give in easily on this one as it's highly likely I'm
wrong. However, I will double check my measurements on the roof
tomorrow and see if they're reproducible. I may have simply goofed
and/or drawn the wrong conclusion.

Incidentally, I've been offering this observation for several years
and you are the first to question it.

Roy Lewallen, W7EL

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