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 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

On Fri, 28 Nov 2008 17:08:06 -0800 (PST), K7ITM wrote:

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?

I don't. I wanted a number to show how accurate the cut would need to
be if he wanted the minimum VSWR point to be accurate to within 1MHz.
I picked 1MHz because the tolerance is easily scaled to other
bandwidth and accuracy numbers. My main point was that a fixture of
some sort was necessary to obtain that level of accuracy.

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.

Well, at 418MHz, one wavelength (electrical) is about 600 mm.
4 degrees is:
600 mm * 4/360 = 6.7 mm
Yeah, that's fairly loose and could be done with diagonal cutters and
a tape measure. Normally, I would punch the numbers into an NEC
model, but I couldn't figure out how to model a radiating coax cable
section as an antenna element (using 4NEC2). I sorta faked it with
wire segments, but got stumped on what to do with the dielectric and
it's velocity factor.

Here's one way to build a coax cable colinear (for 2.4GHz):
http://www.nodomainname.co.uk/Omnicolinear/2-4collinear.htm
Note the measurements in Fig 1 for where to measure the half
wavelength sections. I'm suspicious. Of course, at 418MHz, it's less
critical.

Using foam-
Teflon coax makes it easy to do: the insulation doesn't melt when you
solder things together.

I've only played with the RG6/u CATV flavor, where everything is
crimped. I never have tried to solder the stuff.

RG8/u with foam teflon dielectric:
http://www.westpenn-cdt.com/pdfs/coaxial_spec_pdfs/50%20ohm%20cables/25810.pdf
Looks nice.

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.

See photos of the jig at bottom of:
http://www.nodomainname.co.uk/Omnicolinear/2-4collinear.htm

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.

I hate to admit that I made a mistake, but as you and Roy Lewallen
point out, my explanation of how this antenna operates is almost
certainly wrong. I'll do a fast measurement tomorrow to satisfy my
curiousity, but from your explanation and Roy's, I've erred big time.
With a constant current distribution along the length of the antenna,
and a constant voltage at the various feed points, it's a fair
conclusion that the power radiatated around each of these feed points
are equal. I goofed(tm).

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

Gone sulking...

--
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:
. . .
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

Jeff Liebermann wrote:
. . .
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.

There's quite a handful of potential problems with this:

1. You might have been in the near field. The relationship between field
strength in the near field and the radiated far field is very complex.
You can't determine the field in one based on measurements in the other.
2. If you're in the near field, the field strength you measure at a
given point depends on the type of antenna used. In the far field, the
field impedance (E/H) is a constant value, but not so in the near field.
In various places in the near field, an antenna which responds more
strongly to the E field (a "high impedance" antenna) will show higher
readings where the field impedance is high, and lower where it's lower.
In any case, the relationship between radiated field and local near
field strength isn't simple.
3. The power applied to the antenna is radiated in all directions,
although of course unequally. As I explained in my last posting, the
total field is the vector sum of the fields from the individual parts of
the antenna. Sampling near the antenna gives you no idea of how the
fields sum at a distant point.
4. It's very difficult to make even roughly accurate measurements even
at HF, let alone UHF or higher. One of several problems is that it's
extremely difficult to decouple the feedline when an electrically small
probe is used, so you end up not measuring what you think you are.

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.

That's interesting, and I'd like to get some more information about it.
Perhaps blocking the bottom had a greater effect on the pattern, moving
the maximum away from the direction of the other end of the path?

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.

Here's a model of a coax collinear, but using coax with unity velocity
factor. This "Franklin" array model was created by Linley Gumm, K7HFD.
Coaxial cable is modeled as a combination of transmission line model, to
represent the inside of the coax, and a wire to represent the outside.
The technique is described in the EZNEC manual. See "Coaxial Cable,
Modeling" in the index. I've posted the EZNEC equivalent to
http://eznec.com/misc/rraa/ as COAXVERT.EZ. The accompanying Antenna
Notes file is also there as COAXVERT.txt.

CM Coaxial Vertical Antenna
CM
CM ! Wire # 16 for I srcs, shorted/open TL, and/or loads.
CE
GW 1,1,0.,0.,6.76615,.02081892,0.,6.76615,.000127
GW 2,1,0.,0.,5.766841,.02081892,0.,5.725204,.000127
GW 3,1,0.,0.,4.684258,.02081892,0.,4.725896,.000127
GW 4,1,0.,0.,3.684949,.02081892,0.,3.643311,.000127
GW 5,1,0.,0.,2.602366,.02081892,0.,2.644002,.000127
GW 6,1,0.,0.,1.603057,.02081892,0.,1.561419,.000127
GW 7,1,0.,0.,.5204737,.02081892,0.,.5621104,.000127
GW 8,11,0.,0.,6.76615,0.,0.,5.766841,.00635
GW 9,11,.02081892,0.,5.725204,.02081892,0.,4.725896,. 00635
GW 10,11,0.,0.,4.684258,0.,0.,3.684949,.00635
GW 11,11,.02081892,0.,3.643311,.02081892,0.,2.644002, .00635
GW 12,11,0.,0.,2.602366,0.,0.,1.603057,.00635
GW 13,11,.02081892,0.,1.561419,.02081892,0.,.5621104, .00635
GW 14,6,0.,0.,.5204737,0.,0.,0.,.00635
GW 15,1,0.,0.,0.,.02081892,0.,.02081892,.000127
GW 16,1,208.1892,208.1892,208.1892,208.1913,208.1913, 208.1913,2.0819E-4
GE 1
FR 0,1,0,0,144.
GN 1
EX 0,16,1,0,0.,1.414214
NT 16,1,15,1,0.,0.,0.,1.,0.,0.
TL 1,1,2,1,50.,1.040946,0.,0.,0.,0.
TL 2,1,3,1,50.,1.040946,0.,0.,0.,0.
TL 3,1,4,1,50.,1.040946,0.,0.,0.,0.
TL 4,1,5,1,50.,1.040946,0.,0.,0.,0.
TL 5,1,6,1,50.,1.040946,0.,0.,0.,0.
TL 6,1,7,1,50.,1.040946,0.,0.,0.,0.
TL 7,1,15,1,-50.,1.040946,0.,0.,0.,0.
RP 0,181,1,1000,90.,0.,-1.,0.,0.
EN

I've seen models using coax with VF = 0.82 having a good pattern.

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.

This isn't the first time that's happened.

Roy Lewallen, W7EL

On Fri, 28 Nov 2008 20:55:31 -0800, Roy Lewallen
wrote:

1. You might have been in the near field. The relationship between field
strength in the near field and the radiated far field is very complex.
You can't determine the field in one based on measurements in the other.

That probably a good start. My testing was a 2.4GHz. My field
strength meter was just a shottky diode, balun, ferrite/choke
isolation, DC amp, and battery. Not very fancy and also not very
sensitive. I tried to calibrate it against a microwave oven leakage
meter, but go nowhere.

My guess is that I was about 20cm away from an 8dBi vertical in one
test. The antenna was a Tecom colinear. See omnis at:
http://11junk.com/jeffl/antennas/tecom/
I still have some of these antennas and plan to repeat my testing. At
2.4GHz, one wavelength is about 12.5 cm, so 20cm is well within the
near field. There was also a bunch of other antennas nearby, which
certainly contributed some reflections.

2. If you're in the near field, the field strength you measure at a
given point depends on the type of antenna used. In the far field, the
field impedance (E/H) is a constant value, but not so in the near field.
In various places in the near field, an antenna which responds more
strongly to the E field (a "high impedance" antenna) will show higher
readings where the field impedance is high, and lower where it's lower.
In any case, the relationship between radiated field and local near
field strength isn't simple.

Umm... you lost me, but I'm not at my best right now. I'm in the last
2 weeks of radiation oncology. No problems but I currently fade
fairly fast in the late evening. I'll decode it all tomorrow.

3. The power applied to the antenna is radiated in all directions,
although of course unequally. As I explained in my last posting, the
total field is the vector sum of the fields from the individual parts of
the antenna. Sampling near the antenna gives you no idea of how the
fields sum at a distant point.

Agreed, but I was trying to sample what was being radiated from a
single element (or antenna section). I could see some peaks and nulls
as I moved along the length of the antenna, so I assumed that I was
seeing the contributions of each section (at the peaks).

4. It's very difficult to make even roughly accurate measurements even
at HF, let alone UHF or higher. One of several problems is that it's
extremely difficult to decouple the feedline when an electrically small
probe is used, so you end up not measuring what you think you are.

I know. My meter is battery operated and made to be viewed with
binoculars. I've used it to measure the total pattern on several
antennas by hoisting it up and down a fiberglass pole (or wood barn)
without any connecting wires. The problems are that it takes 2 people
to operate (the 2nd to watch the meter in the binoculars). The
contraption is also slightly directional, adding some additional
errors. However, the big problem is that its sensitivity absolutely
sucks. I need something better. I've tried to modify a Wi-Fi finder
to act as a signal strength meter. That's more sensitive and works
better but has a miserable 30dB(?) dynamic range. This is on the
things to do list (after 100 other unfinished projects).

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.

That's interesting, and I'd like to get some more information about it.
Perhaps blocking the bottom had a greater effect on the pattern, moving
the maximum away from the direction of the other end of the path?

Ummm... I wasn't really able to move the tower on which the antenna
was mounted. The problem was that I was stuck on the lower part of a
rooftop tower. On the roof was also a parapet and HVAC box that
blocked the downward view. The antenna was an overkill 12dBi
something (forgot model numbers) omni. The antenna was about 3 meters
from the parapet. We have a customer that was in the shadow area.
From his window, we could see the top half of the antenna, but not the
bottom. We installed an indoor dish antenna, but the office
aesthetics committee vetoed the installation. So, I raise the base of
the antenna, so that more of the bottom of the antenna was visible.
The problem with this was that the top part of the antenna was in the
middle of a latticework tower section used as a horizontal antenna
mounting arm. The upper 25 cm of the antenna was fairly well covered.
Yet, the improvement at the customers was both dramatic and adequate.
I left it that way for about 2 months. When the weather improved, I
replaced the antenna with a lower gain 8dBi omni, which improved the
signal even more. A month later, I installed two 120 degree Superpass
sector antennas (forgot exact model number), with some downtilt, and
the single increased yet again. My guess(tm) was that the effects of
covering the lower part of the original antenna was greater than
covering approximately the same amount at the top of the same antenna.
Maybe not.

Here's a model of a coax collinear, but using coax with unity velocity
factor. This "Franklin" array model was created by Linley Gumm, K7HFD.
Coaxial cable is modeled as a combination of transmission line model, to
represent the inside of the coax, and a wire to represent the outside.
The technique is described in the EZNEC manual. See "Coaxial Cable,
Modeling" in the index. I've posted the EZNEC equivalent to
http://eznec.com/misc/rraa/ as COAXVERT.EZ. The accompanying Antenna
Notes file is also there as COAXVERT.txt.

Nice and thanks. Forgive my use of a different modeling program but
it's one I know well, while EZNEC 5.1 is still somewhat of a mystery
to me. I converted the EZ file to NEC and ran the model without
modification. See:
http://11junk.com/jeffl/antennas/CoaxVert/
The geometry JPG shows the current distribution, which is as you
indicated, uniform. So much for my geometric decrease theory.

I'll play with it some more later. I don't really understand the TL
card, but will do some RTFM to see what I missed. 4NEC2 complained
about wire radius ratios, but I'll fix that tomorrow. I also want to
add a frequency sweep and move the design to UHF.

I've seen models using coax with VF = 0.82 having a good pattern.

Well, if the OP builds it with copper tubing, PTFE insulators, and air
dielectric, he can use a velocity factor = 1.0.



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