Accuracy of Antenna Testing Ranges
 

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.
----
Reg, G4FGQ

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.

There is no simple reply, Reg, but you're very welcome to come down and
read three box-files full of references on this subject.

It all depends what you're trying to measu simple forward gain or the
complete directional radiation pattern; absolute or relative gain; and
whether the antenna is a beam or something less directional.

The kind of measurement that is subject to the least errors is a
comparison of forward gain between two or more directional antennas that
are very similar. The more similar the antennas under test (AUTs) are,
the better the errors in each individual measurement will match and
cancel out. The more directional the AUT is, the less its gain
measurement will be affected by unwanted reflections.

The largest source of error in this case is probably in the uniformity
of field strength and phase across the test space where you will
position the AUT. There is no single answer in dB for this: you would
have to estimate the error-bars by modeling on a case-by-case basis.

Amateur measurements, such as those made by VHF Groups in the USA,
typically use a ground reflection range technique that creates a test
volume at a height of about 6-10ft above ground, to make it easily
accessible by standing on a picnic table and waving the antenna about by
hand, but these practical needs will also increase the errors compared
with a professional range with remote-controlled positioning and more
time to do it properly.

However, within their limitations, careful amateur measurements can make
valid better/worse comparisons between very similar antennas.
Reproducibility of gain measurements on the same yagi is within a few
tenths of a dB... and the more similar your AUTs are, the closer you can
approach this limit when comparing different antennas.

Absolute gain measurement is an additional can of worms. The most common
amateur mistake is to attempt to measure gain in dBd by comparing a long
yagi against a reference dipole. BIG MISTAKE! A dipole is so
non-directional, it makes the so-called "reference" measurement very
vulnerable to stray reflections that a sharper beam just doesn't see, so
any so-called "standard dipole" is in fact totally worthless.

Or even worse than worthless, the "results" can be anything you want,
wish or dream of. Amateur antenna literature is full of such examples,
all fueled by over-active imagination.

The solution is to use a reference antenna that is as directional as the
AUTs, and to measure or compute its gain by some other means. For
example, there is an IEEE standard gain reference antenna that has been
designed to be both directional and reproducible (in the sense that its
gain is quite tolerant of construction errors) and the gain of that
antenna has been very carefully measured under the best possible lab
conditions. For microwaves, the usual reference is a standard horn
antenna whose gain can be both measured and computed.

What amateur groups like Scott's tend to do is to keep a "gold standard"
reference yagi that is used for all their own measurement meets - and
above all, to put much more faith in the *relative* gain comparisons
than in the claimed absolute gains.

For HF antennas, the required physical size of the test range scales up
with the wavelength, and all the problems about range reflections and
non-directional of AUTs become impossible for professionals and amateurs
alike. That means even professionals are thrown back to computer
modeling... which amateurs can do equally well.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Reg Edwards" wrote in message
...
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.
----
Reg, G4FGQ

Interesting topic Reg. I have always been concerned with uncertainties
involved in antenna measurements. ATR antennas do not always provide the
source for calibration, but assume it could be ANSI/IEEE Std 149-1979.
ETS-Lingren quotes, for their conical log spiral, model 3102 antenna factor
uncertainty as SAE, ARP 958 1M (With which I am not familiar). The antenna
factor uncertainty is specified as +/- 0.8dB from 1 - 10 GHz. More data,
including other conical log spirals, biconicals etc. is available on
ETS-Lingren's web site at www.ets-lingren.com if anybody is interested.

One company I worked for was making measurements in a 3 meter shielded ATR.
Their distance measurements from the source were measured from the support
pole of a conical log spiral, when it should have been measured from the
tip. With the 3102 antenna this introduced a 3 dB error. Radiated spurious
limits quoted in V/m were also assumed to be peak, but not specified. Some
research indicated that these limits are in fact RMS -- another 3dB error!
When used for linear field measurements the conical log spiral gain is 3 dB
below that for circular polarization. Conical log spirals are calibrated
with a circularly polarized signals.

Agilents 11940A has an antenna factor uncertainty of +/- 2dB from 30 MHz to
1 GHz, and is calibrated in the far field, for near field measurements.
ETS-Lingren's antennas are all calibrated at 1m irrespective of frequency.

When questioned about accuracies of measurements people usually say "This is
the way we have always done it". I have very little experience with HF
outdoor measurements, but have heard of sites using high wooden towers to
minimize ground reflection effects. A helicopter is required to plot the
field strength. Very few EMC antenna manufacturers seem concerned with low
frequency far-field measurements. Siemens was about the only company I
could find for such measurements.

Regards,

Frank (VE6CB)

Very interesting post Reg
I see gain figures in Db alone as misleading
since one isn't aware if lobe thickness ( and elevation angle) is taken into
consideration.
As Ian points out, comparison to a dipole is fatal since comparison of max
gain inevitably
involves different angles of elevation ( I assume that labs take this into
consideration
but I have no proof of it.) As I have stated in the past, different antenna
designs provide
different lobe thicknesses, such that comparisons with each other can
provide higher antenna
gains to the lab's whim if the elevation angle is not taken into
consideration.
Taken to the extreme, all antennas with the yagi design can be declared
equal in gain
when measured at the elevation angle where the leading lobes intersect
Regards
Art


"Reg Edwards" wrote in message
...
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.
----
Reg, G4FGQ

Reg, G4FGQ wrote:
"Naturally, laboratories can differ one from another."

A lab may put its stamp of approval on your instrument, but your best
assurance may be measurement of known values. The temperature 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.

To determine the gain of a SW BC curtain antenna, we hung a 3-wire (to
match 600-ohms) folded dipole alongside and at the same height as the
curtain. We swiched transmission back and forth every 5 minutes between
the dipole and the curtain. We continuously measured and recorded the
signal strength for several days in the target area. We averaged
strengths of each signal and compared them for periods of the
recordings.

The HF dBd of the curtain agreed very well with that measured on the
model at 400 MHz in the lab before the curtain was built at full scale.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Reg, G4FGQ wrote:
"Naturally, laboratories can differ one from another."

A lab may put its stamp of approval on your instrument, but your best
assurance may be measurement of known values. The temperature 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.

Whow, thats a good idea, write it up for QST. They are looking for pearls of
wisdom
that can be useful for ham radio operators so that we may maintain our
perceived
leadership of the art of antennas......'Compare with a antenna of known
gain'...... Revolutionary!
Now why hasn't any Guru on this group thought of this before today?
Now we have to decide what we use to measure the gain and more important
not to compare or to compare at a single recieving point especially if the
receiving depends
on skip or propagation. Is it possible that Guru's are unaware that
elevation angles
can be different when comparing antennas? Another gem for the ARRL and
provided
solely by the leading gurus of AMATEUR radio operators no less. Ofcourse we
need
a telephone link with the country that we wish to hear the transmission,
some thing on the simple lines of
....."can you hear me now"
question as we switch antennas
between a dipole and a drape / curtain array every 5 minutes

Art

To determine the gain of a SW BC curtain antenna, we hung a 3-wire (to
match 600-ohms) folded dipole alongside and at the same height as the
curtain. We swiched transmission back and forth every 5 minutes between
the dipole and the curtain. We continuously measured and recorded the
signal strength for several days in the target area. We averaged
strengths of each signal and compared them for periods of the
recordings.

The HF dBd of the curtain agreed very well with that measured on the
model at 400 MHz in the lab before the curtain was built at full scale.

Best regards, Richard Harrison, KB5WZI

"Frank" wrote in message
news:oNOae.62351$VF5.45329@edtnps89...
"Reg Edwards" wrote in message
...
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.
----
Reg, G4FGQ

Interesting topic Reg. I have always been concerned with uncertainties
involved in antenna measurements. ATR antennas do not always provide the
source for calibration, but assume it could be ANSI/IEEE Std 149-1979.
ETS-Lingren quotes, for their conical log spiral, model 3102 antenna
factor uncertainty as SAE, ARP 958 1M (With which I am not familiar). The
antenna factor uncertainty is specified as +/- 0.8dB from 1 - 10 GHz.
More data, including other conical log spirals, biconicals etc. is
available on ETS-Lingren's web site at www.ets-lingren.com if anybody is
interested.

One company I worked for was making measurements in a 3 meter shielded
ATR. Their distance measurements from the source were measured from the
support pole of a conical log spiral, when it should have been measured
from the tip. With the 3102 antenna this introduced a 3 dB error.
Radiated spurious limits quoted in V/m were also assumed to be peak, but
not specified. Some research indicated that these limits are in fact
RMS -- another 3dB error! When used for linear field measurements the
conical log spiral gain is 3 dB below that for circular polarization.
Conical log spirals are calibrated with a circularly polarized signals.

A conical log spiral antenna's radiating plane moves along it's axis with
frequency. Various models place the support pole at the rear or at the
center of the radiating axis. In any case, use this class of antennas was
strongly discouraged after 1996 by MIL-STD-461D.


Agilents 11940A has an antenna factor uncertainty of +/- 2dB from 30 MHz
to 1 GHz, and is calibrated in the far field, for near field measurements.
ETS-Lingren's antennas are all calibrated at 1m irrespective of frequency.

Your should always calibrate your measurement antenna in accordance with the
applicable testing standard. For MIL-STD-461E, this means a 1-meter
distance. For commercial emission testing, that means separate calibration
tables for 3-meter, 10-meter & 30-meter ranges. And for some conditions,
like FCC Part 18 or broadcast station field-strength "footprints", you
should obtain a true far-field calibration. Calibration at any distance
other than the actual use distance is just not enough.

When questioned about accuracies of measurements people usually say "This
is the way we have always done it". I have very little experience with HF
outdoor measurements, but have heard of sites using high wooden towers to
minimize ground reflection effects. A helicopter is required to plot the
field strength. Very few EMC antenna manufacturers seem concerned with
low frequency far-field measurements. Siemens was about the only company
I could find for such measurements.

Regards,

Frank (VE6CB)


Perhaps the lack of interest in "low frequency far-field" measurements is
driven by an absence of any "low-frequency, far-field" compliance
requirements? OTOH, MIL-STD-461E is quite concerned with radiated E-field
emissions right down to 10 kHz, but at a 1-meter separation distance, this
is decidedly near-field! BTW, calibration of this standard's defined 10 kHz
to 30 MHz test antenna (an electrically short 41" monopole standing above a
small ground plane) is not done on an antenna range! The calibration
technique is all conducted, with a known signal being applied by coax,
through a shielded 10 pF capacitor, to the antenna input point of the
matching network (a box at the base of the 41" rod). The accuracy of the
calibration is dependent only on the test lab's ability to read the RF input
& output voltages.


--
Ed
WB6WSN
El Cajon, CA USA

To determine the gain of a SW BC curtain antenna, we hung a 3-wire
(to
match 600-ohms) folded dipole alongside and at the same height as
the
curtain. We swiched transmission back and forth every 5 minutes
between
the dipole and the curtain. We continuously measured and recorded
the
signal strength for several days in the target area. We averaged
strengths of each signal and compared them for periods of the
recordings.

The HF dBd of the curtain agreed very well with that measured on the
model at 400 MHz in the lab before the curtain was built at full
scale.

Best regards, Richard Harrison, KB5WZI

==================================

Richard, fine, so that was the measurement procedure. Thanks for the
description.

Now all we want to know is what was the uncertainty in the
measurement.

Was it within plus or minus x percent? Or perhaps plus or minus y
decibels?

Can you remember the uncertainty approximately?

Or perhaps it didn't matter what the uncertainty was. In which case
it was a waste of time making the measurement.
----
Reg, G4FGQ

Reg, G4FGQ wrote:
"Or perhaps it didn`t matter what the uncertainty was."

Examination of the comparative feild strength data left no doubt that
the antenna was working as expected. This was the first of several
similar antennas to be constructed. Before proceeding we needed
verification of the design and construction.. It worked and we built
more.

Best regards, Richard Harrison, KB5WZI

Ed, thanks very much for your most interesting comments.

A conical log spiral antenna's radiating plane moves along it's axis with
frequency. Various models place the support pole at the rear or at the
center of the radiating axis. In any case, use this class of antennas was
strongly discouraged after 1996 by MIL-STD-461D.

You raise an interesting point. The fact is, it never occured to me, yet is
is obvious when you think about it. This implies that at certain
frequencies a radiated spurious emission of a certain polarization could be
missed. As with conventional log periodics, at any given freqency, a
section of the antenna will be active, so I guess you would not get complete
rejection. The ETS-Lingren model 3102, has its support pole at the rear,
and the 3101 is about 1/3 from the rear. I was not aware of the
discouragement in the use of these class of antennas by MIL-STD-461D. Seems
pretty sad, when you consider the company I was working for advertised its
ATR capability, with no mention made of the MIL standard.

Your should always calibrate your measurement antenna in accordance with
the applicable testing standard. For MIL-STD-461E, this means a 1-meter
distance. For commercial emission testing, that means separate calibration
tables for 3-meter, 10-meter & 30-meter ranges.

I have seen cal data for 1, 3, 10, and 30m but all are concerned with
radiated EMC, and not antenna field strengths, which always was much more
interesting. Still the 30 m calibration would be acceptable for most HF
work -- at least above 5 MHz.

And for some conditions, like FCC Part 18 or broadcast station
field-strength "footprints", you should obtain a true far-field
calibration. Calibration at any distance other than the actual use
distance is just not enough.

Makes sense.

Perhaps the lack of interest in "low frequency far-field" measurements is
driven by an absence of any "low-frequency, far-field" compliance
requirements? OTOH, MIL-STD-461E is quite concerned with radiated E-field
emissions right down to 10 kHz, but at a 1-meter separation distance, this
is decidedly near-field!

At 10 kHz it is probably mostly capacative coupling at 1 m.

BTW, calibration of this standard's defined 10 kHz to 30 MHz test antenna
(an electrically short 41" monopole standing above a small ground plane)
is not done on an antenna range! The calibration technique is all
conducted, with a known signal being applied by coax, through a shielded
10 pF capacitor, to the antenna input point of the matching network (a box
at the base of the 41" rod). The accuracy of the calibration is dependent
only on the test lab's ability to read the RF input & output voltages.

Sounds like you are talking about a monopole made by EMCO, which had
switched frequency ranges. ETS-Lingren (I think they bought out EMCO) now
sell model 3301B that has a calibrated antenna factor down to 20 Hz. Must
have a very high gain amp, as the antenna factor is only about 25 dB at
20Hz. I have no idea how a cal procedure, using a 10 pF capacitor, can
relate the output level to an incident E-field on a 41" monopole. The
losses in the matching networks must be very high at the lower frequencies
also. Without attempting to analyze such a monopole, the radiation
resistance must be in the milli-ohm, to micro-ohm range.

--
Ed
WB6WSN
El Cajon, CA USA

Frank
VE6CB