On 6 Jan, 16:06, (Richard Harrison) wrote:
Wim, PA3DJS wrote:

"Determining the radiated power from near field measurements is
theoretically possible, but not easy in the real world for most
amateurs."

My experience in tuning commercial 2-way HF radios for maximum power
radiated. It could be obtained by placing my multimeter (using crystal
rectifiers for a.c. scale rectification) with its test leads attached on
the hood of the vehicle containing the radio and tuning for maximum. The
measured performance proved as good as any other tuning method.

Best regards, Richard Harrison, KB5WZI

But that doesn't change anything that Wim stated which is entirely
correct.
You did not "determine" radiated power.......period
Art

art wrote:
"You did not 'determine" radiated power.....period"

Exactly right. Power in the near field is largely reactive. To determine
radiated power you measure the in-phase volt-amperes with a wattmeter.
The multimeter on the hood maximizes output same as a Bird wattmeter for
practical results, but you hanen`t quantified watts out.

Best regards, Richard Harrison, KB5WZI

On 7 ene, 04:25, (Richard Harrison) wrote:
art wrote:

"You did not 'determine" radiated power.....period"

Exactly right. Power in the near field is largely reactive. To determine
radiated power you measure the in-phase volt-amperes with a wattmeter.
The multimeter on the hood maximizes output same as a Bird wattmeter for
practical results, but you hanen`t quantified watts out.

Best regards, Richard Harrison, KB5WZI

Hi Richard,

You can determine radiated power and radiation pattern by near field
measurements (theoretically). Please search for "poynting theorem"
and "Huygens principle", "Huygens Source" or "Fresnel diffraction
theory". Most texts require differential vector calculus.

Because determining radiated power based on E- and H-Field measurement
is generally not feasible for amateurs (and even for many
professionals), I suggested another approach based on E-field
measurements and EM simulation.

As absolute E-field measurement and EM simulation are within the reach
of amateur radio operators, one can both tune for maximum field and
determine absolute radiated power.

Of course you don't know how much power is dissipated in nearby
structures (buildings, etc).

Best regards,

Wim
PA3DJS
www.tetech.nl

Wim, PA3DJS wrote:
"Of course you don`t know how much power is dissipated in nearby
structures (buildings, etc)."

Yes. Standard broadcast field strengths are measured at distances along
radial paths begining at one mile from the station to ensure far field
determination. Conductivity of the earth affects received signal
strength. Over sea water, the millivolts per meter decline inversely
with distance. Over the earth, decline is more rapid depending on
conductivity. Some sites along the radials are unacceptable due to
proximity of conducting structures. The more measurements along a
radial, the better. 25 sites per radial is about the minimum. In the
USA, the FCC has published Groundwave Field vs Distance Charts, and made
them a part of its rules. One can try to fit his measured data to the
FCC curves. Groundwaves is what broadcasting is all about at these
frequencies.

Best regards, Richard Harrison, KB5WZI

To add to what PA3DJS has observed:

I have seen the use of near-field measurements to determine directivity and
gain at the (then) NBS in Bolder, Col. A huge x-y table was used to move a
probe over the face of an energized microwave antenna. Lasers (with cycle
counters and mirrors) were used to measure the actual coordinates of the
probe! A staggering amount of data was collected over a long period of time
and then digested over another long period of time. This was done inside of
a very large building where reflections were minimal.

To use this technique at, say, 10 MHz with a yagi would be improbable, but
not impossible.

73, Mac N8TT

--
J. McLaughlin; Michigan, USA
Home:
"Wimpie" wrote in message
...
On 7 ene, 04:25, (Richard Harrison) wrote:

snip
Hi Richard,

You can determine radiated power and radiation pattern by near field
measurements (theoretically). Please search for "poynting theorem"
and "Huygens principle", "Huygens Source" or "Fresnel diffraction
theory". Most texts require differential vector calculus.

Because determining radiated power based on E- and H-Field measurement
is generally not feasible for amateurs (and even for many
professionals), I suggested another approach based on E-field
measurements and EM simulation.

As absolute E-field measurement and EM simulation are within the reach
of amateur radio operators, one can both tune for maximum field and
determine absolute radiated power.

Of course you don't know how much power is dissipated in nearby
structures (buildings, etc).

Best regards,

Wim
PA3DJS
www.tetech.nl

J. Mc Laughlin wrote:

...
To use this technique at, say, 10 MHz with a yagi would be improbable, but
not impossible.

73, Mac N8TT


It would be interesting to hear ideas of those on "probes", methods,
etc., on how to do such, would be attempted--implemented-imployed ...
never say never.

I think it would be interesting to hear on endeavors along these lines,
brave hearted individuals may be willing to share their experiences,
findings, guesses, etc. ... the bark is often much more dangerous
sounding than the the "bite." :-)

Frankly, "re-inventing the wheel" is welcomed here!

Regards,
JS

John Smith wrote:

...
Frankly, "re-inventing the wheel" is welcomed here!

Regards,
JS

Yanno? That was a vague response, the above ...

What I meant to say is, YES! I worry that "in close" measurements with
my field strength meter are/is inaccurate--and, I am ignorant on how to
correct this (without going to far-field.) Indeed, it can easily be
proven this is the case, EZNEC gives much different results than those
plotted with a "close" field strength meter--has anyone found different.
If so, I am GREATLY interested, and what am I doing wrong?

Sometimes it is just good to ask ...

Warm regards,
JS

John Smith wrote:
J. Mc Laughlin wrote:

...
To use this technique at, say, 10 MHz with a yagi would be improbable,
but not impossible.

73, Mac N8TT


It would be interesting to hear ideas of those on "probes", methods,
etc., on how to do such, would be attempted--implemented-imployed ...
never say never.


The challenge is that one needs a probe that measures 3 axis E and H.
There are some clever designs out there based on a sphere with 6 half
loops sticking out. They've been used to make near field measurements
of broadcast stations, among other things. See, for instance, Gassman
and Furrer, 1993. Silva, et al., published an interesting fiber optic
probe scheme in 97. Driver and Kanda published a optically linked
sensor for making Poynting vector measurements in the near field in 1988
(IEEE Trans EMC). In the microwave area, the probe is usually an open
ended section of waveguide.

There are some clever techniques (see, for instance, the work of
Bolomey) where you put an array of small (non resonant) dipoles with
switches in the near field, and turn them on and off. You look at the
antenna's feedpoint impedance and from that, you can tell what part of
the field is affected by that dipole.

Once you've got your near field data, you need to post process. A
gentleman, A.C. Newell, literally wrote the book(s) on this technique at
NIST/NBS.






I think it would be interesting to hear on endeavors along these lines,
brave hearted individuals may be willing to share their experiences,
findings, guesses, etc. ... the bark is often much more dangerous
sounding than the the "bite." :-)

Frankly, "re-inventing the wheel" is welcomed here!

Regards,
JS

Dear Jim Lux W6RMK:

I was not able to examine the probes that I saw in use at NBS in what was
probably the mid-70s.

The probes that I did see and use comprised three, orthogonal, very short
doublets with attached means for rectifying. The resulting DC was conveyed
away through a resistive, plastic transmission-line crafted to be almost
transparent to RF. These probes were used to estimate the size of strong EM
signals in the vicinity of equipment so as to be able to put better numbers
on EMC capabilities.

While one can make a reasonable estimate of field strength inside of a TEM
cell (a big piece of transmission line) from external measurements, it is
desirable to be able to estimate FS at spots inside of the cell. It is
important to note that even with care, significant uncertainties remain in
the estimates. All measurement comprise a minimum of two numbers: an
estimate of the value and an estimate of the uncertainty of that estimate.
You, and most on the group, know this, but it needs to be repeated.

Safety of life was involved with the testing then done. With the profusion
of transmitters in close proximity of safety equipment today, it is a wonder
that more lives are not lost with inappropriate actuations just within
present cars. A reminder that the need for savvy RF engineers will not
diminish, and sending that work off-shore to a low-bidder is dangerous and
probably criminally negligent. The major US car makers, to pick an
industry, expend a large, expensive effort to see that their cars are safe
using resident engineers.

Warm regards, Mac N8TT
--
J. McLaughlin; Michigan, USA
Home:
"Jim Lux" wrote in message
...
:
J. Mc Laughlin wrote:

...
To use this technique at, say, 10 MHz with a yagi would be improbable,
but not impossible.

73, Mac N8TT






The challenge is that one needs a probe that measures 3 axis E and H.
There are some clever designs out there based on a sphere with 6 half
loops sticking out. They've been used to make near field measurements of
broadcast stations, among other things. See, for instance, Gassman and
Furrer, 1993. Silva, et al., published an interesting fiber optic probe
scheme in 97. Driver and Kanda published a optically linked sensor for
making Poynting vector measurements in the near field in 1988 (IEEE Trans
EMC). In the microwave area, the probe is usually an open ended section of
waveguide.

There are some clever techniques (see, for instance, the work of Bolomey)
where you put an array of small (non resonant) dipoles with switches in
the near field, and turn them on and off. You look at the antenna's
feedpoint impedance and from that, you can tell what part of the field is
affected by that dipole.

Once you've got your near field data, you need to post process. A
gentleman, A.C. Newell, literally wrote the book(s) on this technique at
NIST/NBS.

Wimpie wrote:
On 7 ene, 04:25, (Richard Harrison) wrote:

art wrote:

"You did not 'determine" radiated power.....period"

Exactly right. Power in the near field is largely reactive. To determine
radiated power you measure the in-phase volt-amperes with a wattmeter.
The multimeter on the hood maximizes output same as a Bird wattmeter for
practical results, but you hanen`t quantified watts out.

Best regards, Richard Harrison, KB5WZI


Hi Richard,

You can determine radiated power and radiation pattern by near field
measurements (theoretically). Please search for "poynting theorem"
and "Huygens principle", "Huygens Source" or "Fresnel diffraction
theory". Most texts require differential vector calculus.


This is what is done on a near-field antenna range. For certain
scanning geometries, the math is fairly straightforward (e.g. Fourier
transforms). For others, it gets complex, especially if the scanning
points are not evenly spaced.


A further complication is that you need to make 3 axis measurements of
both E and H.

Jim, W6RMK