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