" wrote in message
news:u9_ae.18115$NU4.14900@attbi_s22...
Richard,
You state that you used a dipole to compare with, which was at the same
height !.
Which antenna was altered so that the elevation angle of maximum gain
was the same for both antennas.such that max gain measurements
were truly comparable?
Where was the height of the "curtain" measured or referred to
so that "same height" could be justified ?
( You also did say it was for SW use which is certainly different
to ground wave use)
Presumably, the comparison was for the same type of polarization
and ignored differences created by the side addition of other
types of polarization.
Without further information the "Facts" could be seen as
correct to plus or minus 100 percent measurement error!



And that sums up most antenna testing rather well!

--
Ed
WB6WSN
El Cajon, CA USA

Art Unwin wrote:
"Remember, Richard was replying to the initial post which was very
specific in nature regarding lab testing and degree of error."

Antenna test facilities involve far fields. Kraus says on page 831 of
his 3rd edition of "Antennas":
"---it is obvious that measurement usually takes place in the far
field."

This can be far indeed with highly directive antennas.

My initial response included:
"A lab may put its stamp of approval on your instrument, but your best
assurance may be measurement of known values. The termperature of
ice-water or the voltage of new dry cells, for example. You usually can
try several dry cells for confirmation or averaging.

In antennas, one strategy for successful gain determination is
comparison with an antenna of known gain."

My posting was imperfect. There`s nothing that can`t be improved, but
were I re-writing my posting, I can`t think how I might improve it.

I don`t think my example of checking gain of an array using skywaves was
amiss. We build shortwave antennas to use skywaves. We give antenna
gains in free-space because it makes sense.

I said we built a small-scale model first because we can measure the
model`s characteristics without a helicopter. The full-scale antenna
performed exactly like the model. Computer modeling has eliminated the
small-scale model step in new designs.

We checked only the first off of the new design, with the full
confidence that subsequent antennas of the same design would perform the
same.

Of the first antenna, we measured everything including the currents
along each element. We used an R-F ammeter in a loop suspended from the
element and towed along with a string. We read it using a telescope. The
antenna was a scientific success as well as a practical success. This
differs from some of the oil wells I was to drill later, though some of
those succeeded too.

I can only post what I know and it will never satisfy what everybody
wants to read. Sometimes my postings are more responsive than others.
That`s part of the fun.

Best regards, Richard Harrison, KB5WZI

On Sun, 24 Apr 2005 05:44:34 +0000 (UTC), "Reg Edwards"
wrote:

All electrical calibration and testing laboratories issue tables of
claimed accuracies of measurements. Measurement uncertainties stated
on calibration certificates are legally binding. All stated
measurement results must be traceable to International Standards or a
laboratory or testing station loses its status.

Consequently there is no incentive for a laboratory to overstate its
capabilities in its sales literature. Indeed, it is dangerous,
illegal even!

Naturally, laboratories can differ widely, one from another.

It would be interesting to compare laboratory uncertainties with
performance figures claimed by antenna manufacturers. Or anyone else.

Does anyone have typical examples of measurement uncertainties claimed
by antenna testing stations? Answers in decibels please.

A reply from a testing station, at HF or VHF, would be specially
appreciated.

As stated by Ian, there's no simple answer. The bane of antenna
testing is reflections reflections reflections.

It may come as a surprise to our correspondent who likes to disparage
"gurus" that "standard-gain" antennas are widely used as reference
standards. To head off the question of how the standard gain is
determined, that is done by testing three "identical" antennas in
pairs; each one against the other two, with one the source and the
other the receiver. A bit of algebra and you have the gain of each
one individually.

http://www.mi-technologies.com/literature/a00-044.pdf

The foregoing paper might help answer Reg's question about achievable
accuracy.

While not addressing hf and vhf measurements, some of the following
might be of interest.

Indoor measurements are usually conducted in anechoic chambers where
the shape is often tapered to control reflections and the walls are
covered in absorber material. A chamber will have a "quiet zone"
where the reflections are specified to be X db down. Very often the
antennas under test are being characterized for side lobe levels or in
the case of monopulse radar, the null depth of the difference
pattern(s). If you're trying to measure a 60 dB null, it doesn't pay
to have a quiet zone of -40 dB.

These measurements also require an amplitude and phase front that
mimics a source at infinite distance. This used to require huge
chambers, often hundreds of feet long. A new way to accomplish this
is to "fold" the range by using specially shaped reflectors to flatten
the amplitude/phase across the test aperature. This has the added
benefit of shorter cables between sources, DUT and measurement
receiver. At X and K band, cable loss can be a killer. Likewise
moving cables around and even temperature changes can affect the
measurments.

I have used such a range to measure antennas from L to Ka band.

Outdoor ranges often "feature" the ground reflection, since it is
difficult to eliminate it physically. This is particularly true at
hf/vhf. I have used a technique that utilized the time-domain
capability of a modern network analyzer (HP-8510) to identify the
reflection and then place absorber material to attenuate it.
Similarly, a frequency-domain measurement, that includes ground
reflection, can be transformed to the time domain where the reflection
is gated out and then transformed back to the frequency domain for
"reflection free" analysis.

See also:

http://www.lehman-inc.com/pdf/mag2.pdf

Wes,
What you have posted is very interesting and is not spewing out alot of
stuff
regarding isentropic gain etc that is really not relevent to an actual
testing range.
Rather than deflect away from Reg's needs may I go back to the "compared to
a dipole"
statement which Richard keeps brushing off. If the gains are different then
the
angle for max radiation is different and if you do not take this into
account by searching
for the individual point of maximum gain position then the the measurements
are in total error.
To put antennas at the same height and then measuring at the same stationary
point for receive,
switching back and forth
is not a true comparison because of the different elevation angles. If one
was to compare a long yagi
to a dipole ando make it a true comparison measurement one must surely take
into account the two degree
or so difference when positioning the listening posts and not relying on a
single
listening position which to me appears to be a NO No .
Richards response to the "error" question totally ignored TOA saying they
are usually
the same . He also ignored what he considered as an "equal" height for the
curtain,
i.e the top,bottom or the center line of the curtain array which alone would
introduce
error with respect to comparible measurement. If Richard was pointing out
that his was a typical
professional method of measurement then I would view his statement in
complete disbelief.
Your posting, thankyou, confirms my thinking in that the use of a dipole
only confirms the reliability
of the set up used and that is the end of it with respect to measurement of
a competing antenna
where I suspect a pro lab would identify the particular resulting elevation
measurement.
If the last sentence is in error I would apreciate a correction
Regards
Art


"Wes Stewart" wrote in message
...
On Sun, 24 Apr 2005 05:44:34 +0000 (UTC), "Reg Edwards"
wrote:

All electrical calibration and testing laboratories issue tables of
claimed accuracies of measurements. Measurement uncertainties stated
on calibration certificates are legally binding. All stated
measurement results must be traceable to International Standards or a
laboratory or testing station loses its status.

Consequently there is no incentive for a laboratory to overstate its
capabilities in its sales literature. Indeed, it is dangerous,
illegal even!

Naturally, laboratories can differ widely, one from another.

It would be interesting to compare laboratory uncertainties with
performance figures claimed by antenna manufacturers. Or anyone else.

Does anyone have typical examples of measurement uncertainties claimed
by antenna testing stations? Answers in decibels please.

A reply from a testing station, at HF or VHF, would be specially
appreciated.

As stated by Ian, there's no simple answer. The bane of antenna
testing is reflections reflections reflections.

It may come as a surprise to our correspondent who likes to disparage
"gurus" that "standard-gain" antennas are widely used as reference
standards. To head off the question of how the standard gain is
determined, that is done by testing three "identical" antennas in
pairs; each one against the other two, with one the source and the
other the receiver. A bit of algebra and you have the gain of each
one individually.

http://www.mi-technologies.com/literature/a00-044.pdf

The foregoing paper might help answer Reg's question about achievable
accuracy.

While not addressing hf and vhf measurements, some of the following
might be of interest.

Indoor measurements are usually conducted in anechoic chambers where
the shape is often tapered to control reflections and the walls are
covered in absorber material. A chamber will have a "quiet zone"
where the reflections are specified to be X db down. Very often the
antennas under test are being characterized for side lobe levels or in
the case of monopulse radar, the null depth of the difference
pattern(s). If you're trying to measure a 60 dB null, it doesn't pay
to have a quiet zone of -40 dB.

These measurements also require an amplitude and phase front that
mimics a source at infinite distance. This used to require huge
chambers, often hundreds of feet long. A new way to accomplish this
is to "fold" the range by using specially shaped reflectors to flatten
the amplitude/phase across the test aperature. This has the added
benefit of shorter cables between sources, DUT and measurement
receiver. At X and K band, cable loss can be a killer. Likewise
moving cables around and even temperature changes can affect the
measurments.

I have used such a range to measure antennas from L to Ka band.

Outdoor ranges often "feature" the ground reflection, since it is
difficult to eliminate it physically. This is particularly true at
hf/vhf. I have used a technique that utilized the time-domain
capability of a modern network analyzer (HP-8510) to identify the
reflection and then place absorber material to attenuate it.
Similarly, a frequency-domain measurement, that includes ground
reflection, can be transformed to the time domain where the reflection
is gated out and then transformed back to the frequency domain for
"reflection free" analysis.

See also:

http://www.lehman-inc.com/pdf/mag2.pdf

Art Unwin wrote:
"Richard`s response to the "error" question totally ignored TOA saying
they are usually the same."

Propagation dictates the take off angle that the signal actually follows
regardless of what your antennas do. We made meadurements on different
days so that propagation may have been different on different days. We
were checking over nearly the actual paths under what might be typical
conditions. Did the curtain produce louder signals? You bet!

Even though the curtain antenna had sharper vertical directivity as well
as sharper horizontal directivity than the lone dipole, these were the
goals of the design. Produce more signal on target to try to overcome
the myriad of jammers that were trying to drown us out.

During our tests, the paths between transmitter and the receivers were
the same in most cases. The width of a curtain was only about one
wavelength and the dipole was immediately adjacent to the curtain. The
curtain was two dipoles high, two dipoles wide and two dipoles deep as I
recall. Those dipoles in front were all driven in phase. Those behind
were tuned parasitic reflectors. It wasn`t unique at all. I`ve seen many
since then which look very much like our curtains. They were well
behaved and brought in lots of fan mail. They obviously radiated ok. The
reflectors seemed to shield the villiage behind them from being drowned
in radio frequency energy.

Whatever differences there may have been between the conditions imposed
on the dipole and curtain, they were tuned and loaded for the same
transmitted power. Received signal differences were likely due to gain
in the curtain versus gain in the dipole. Averiging a large number of
samples likely straightened out inevitable minor differences. I would
wager our results were good enough.
My employer was satisfied and all the contractors got paid.

Best regards, Richard Harrison, KB5WZI

Richard, it is now quite clear that you were not undertaking a test
referenced to a dipole. All you were doing is confirming a target area
under average conditions to ensure the language used was compatable
to the target area.....Period
More important to me is your statement that :

" Propagation dictates the take off angle that the signal actually
follows regardless of what your antennas do"

This statement seems to echo a conclusion arrived at by a regular poster
( I should call him a guru) on this group tho leaving me unconvinced.
Would you kindly point out to me what book you are extracting this
statement from so I may examine the boundaries under which that
statement is deemed correct?
Thanking you in advance
Art


"Richard Harrison" wrote in message
...
Art Unwin wrote:
"Richard`s response to the "error" question totally ignored TOA saying
they are usually the same."

Propagation dictates the take off angle that the signal actually follows
regardless of what your antennas do. We made meadurements on different
days so that propagation may have been different on different days. We
were checking over nearly the actual paths under what might be typical
conditions. Did the curtain produce louder signals? You bet!

Even though the curtain antenna had sharper vertical directivity as well
as sharper horizontal directivity than the lone dipole, these were the
goals of the design. Produce more signal on target to try to overcome
the myriad of jammers that were trying to drown us out.

During our tests, the paths between transmitter and the receivers were
the same in most cases. The width of a curtain was only about one
wavelength and the dipole was immediately adjacent to the curtain. The
curtain was two dipoles high, two dipoles wide and two dipoles deep as I
recall. Those dipoles in front were all driven in phase. Those behind
were tuned parasitic reflectors. It wasn`t unique at all. I`ve seen many
since then which look very much like our curtains. They were well
behaved and brought in lots of fan mail. They obviously radiated ok. The
reflectors seemed to shield the villiage behind them from being drowned
in radio frequency energy.

Whatever differences there may have been between the conditions imposed
on the dipole and curtain, they were tuned and loaded for the same
transmitted power. Received signal differences were likely due to gain
in the curtain versus gain in the dipole. Averiging a large number of
samples likely straightened out inevitable minor differences. I would
wager our results were good enough.
My employer was satisfied and all the contractors got paid.

Best regards, Richard Harrison, KB5WZI

" wrote about Richard Harrison's post:
Richard, it is now quite clear that you were not undertaking a test
referenced to a dipole. All you were doing is confirming a target area
under average conditions to ensure the language used was compatable
to the target area.....Period
More important to me is your statement that :

" Propagation dictates the take off angle that the signal actually
follows regardless of what your antennas do"
_________

Your arguments arise from trying to compare two different test goals, e.g.,
accurately measuring the free space az/el radiation patterns of an antenna
itself, versus how those radiation patterns may perform in a particular
application (height above ground, ground characteristics, ionospheric
propagation characteristics, reflection sources, target coverage zone, etc).

Classic antenna test ranges are designed to measure the az/el radiation
patterns of antennas themselves, independent of their environment. What
that radiation will provide in terms of a desired "coverage" result is
another matter, and is the responsibility of the RF system designer -- not
the antenna test range.

RF

Visit http://rfry.org for FM transmission system papers.

wrote:
Rather than deflect away from Reg's needs may I go back to the
"compared to a dipole" statement which Richard keeps brushing off. If
the gains are different then the angle for max radiation is different
and if you do not take this into account by searching for the
individual point of maximum gain position then the the measurements are
in total error. To put antennas at the same height and then measuring
at the same stationary
point for receive,
switching back and forth
is not a true comparison because of the different elevation angles.

I don't think Richard is attempting to deny that.

His tests were not intended to measure the gain of the antenna. They
were intended to answer a much more practical question: "How much
stronger is the signal from the curtain array, as delivered into the BC
target area, compared with using a dipole?" That's what the station
owners wanted to know, and they specifically wanted that answer to
include all the variables of antenna patterns and ionospheric
propagation.

As you have correctly pointed out, in any environment except free space,
that number is not the same as the antenna gain in dBd. Anybody who has
thought about it is aware of the problem, and that clearly includes
Richard.

Everybody agrees with you, so you can stop banging on that open door.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White GM3SEK" wrote in message
...
wrote:
Rather than deflect away from Reg's needs may I go back to the "compared
to a dipole" statement which Richard keeps brushing off. If the gains are
different then the angle for max radiation is different and if you do not
take this into account by searching for the individual point of maximum
gain position then the the measurements are in total error. To put
antennas at the same height and then measuring at the same stationary
point for receive,
switching back and forth
is not a true comparison because of the different elevation angles.

I don't think Richard is attempting to deny that.

The question is about lab techniques and error measurement and
Richards post was in answer to that.
Another person insinuated that a person who
tests against a dipole and measures after a skip has taken place is in Lu
Lu land because he assumed he was literally describing a normal
lab test of comparing to a dipole!

Remember, Richard was replying to the initial post which was
very specific in nature regarding lab testing and degree of error !
Assumption has no part in a real laboratory.

His tests were not intended to measure the gain of the antenna. They were
intended to answer a much more practical question: "How much stronger is
the signal from the curtain array, as delivered into the BC target area,
compared with using a dipole?" That's what the station owners wanted to
know, and they specifically wanted that answer to include all the
variables of antenna patterns and ionospheric propagation.

I could not agree more and stated so in my last post


As you have correctly pointed out, in any environment except free space,
that number is not the same as the antenna gain in dBd. Anybody who has
thought about it is aware of the problem, and that clearly includes
Richard.

Then why is he introducing dbi into the subject using Kraus as a backup?
Why does he state that TOA are "usually" the same when the opposite is true
especially when comparing a curtain to a dipole ? I don't believe that to be
correct
In the absence of denial by a guru must I assume he is correct or he meant
something else
and everybody knows what he said is true?

So you agree with the poster who stated that if a person thought that a
dipole comparison test consistes of comparing after skip took place,
is in Lu Lu land?

Everybody agrees with you, so you can stop banging on that open door.
They do ....????

And the question regarding propagation and antenna function can we assume
he is correct on that also ? I don't like to "assume" that he meant
something different
and let the newbies as well as I to be lead astray.
Must I assume he is correct in that last sentence he made
where I am asking for a corroborating technical written statement ?
Richards last statment was /is an echo of a similar posting made a few
months ago and the Gurus said nothing to confirm or deny it's voracity?.
What are we meant to assume , that if a guru doesn't question it
it must be correct? I personally would rather see corrobaration
in a accepted technical writing than set up the beginnings of an old wives
tale
The old saying is still true , don't rely on one gurus answer, ask another
and then another and ensure that context is correct.
I await Richards response with interest so that I may read an authoritive
statement for myself without the need for "assumption" as to what he
"really"
meant to say but didn't. If you know what he "meant" to say on that last
remaining subject why not supply a helping hand ?

Art

--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Tue, 26 Apr 2005 08:16:56 +0100, Ian White GM3SEK
wrote:



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3s


So you've finally emigrated Ian!

Good luck in the move..

Peter, G3PHO