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