Richard Fry wrote:
"The elements (bays) in these arrays all are one wavelength apart, and
driven with equal power and phase.

# Elements C-pol Gain (dBd)
1 -3.55
2 -0.21
4 3.09
8 6.34"

Bailey`s Table 10-I, which Richard Clark referred to as "naive", appears
on page 484 of "TV and Other Receiving Antennas".
The heading is Array Gain (approximate rule).

Nunber( of Half-Wave Rods (N) and
Numeric PowerRatio Gain (dB) 1
0
2 3
4 6
8 9

First difference from Richard Fry`s table is the loss of 3.55 dB as the
result of circular polarization (mostly) as half of the power which a
linearly polarized reference dipole would
use is cross-polarized.

The steps between doubling the number of elements in Richard Fry`s table
are all nearly 3 dB. Bailey says "approximate rule". He is vindicated.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote
First difference from Richard Fry`s table is the loss of 3.55 dB
as the result of circular polarization (mostly) as half of the power
which a linearly polarized reference dipole would use is
cross-polarized.

That, and the fact that the radiation pattern from each element is not the
pure cosine function assumed for a 1/2-wave dipole. It has slightly less
gain peak gain.

The steps between doubling the number of elements
in Richard Fry`s table are all nearly 3 dB.

"Nearly" is right, but the difference is not uniform for successive doubling
of apertures.

A small variation in the bay-bay spacing (departing from 1 wavelength) is
needed as a function of the number of bays, to maximize the peak gain from
this type of an array. The arrays in my table all have exactly 1-wavelength
element spacing, and the peak gain from arrays of them is lower than
expected for lower numbers of elements, and higher than expected for higher
numbers of elements -- which stretches/compresses that nominal 3 dB delta.

Fine points, to be sure.

RF

I don't think that's a valid excuse. The 3 dB rule applies to phased
arrays only when mutual coupling is ignored or in a few special cases.
Mutual coupling had to have been known at least at the time of the
invention of the Yagi-Uda antenna in 1926, and probably long before
that. It was being calculated for geometrically simple antennas at least
as early as 1943 (cf. R. King, Proc. IRE). Work proceeded rapidly
through the '40s, with papers describing increasingly accurate
techniques with antennas of increasing complexity.

We now have the means to calculate mutual coupling much more easily than
before, and for geometries which were impossible to deal with before we
had computers to do the work, but I don't think we've modified our
understanding of the phenomenon for many decades (some notable antenna
charlatans notwithstanding). Anyone measuring the gain of a short Yagi,
the gain of which routinely exceeds 3 dB per doubling of elements by a
considerable margin, must have become aware of the shortcoming of the 3
dB rule.

I suspect that if we were to read the cited quotations very carefully,
we'd see qualifications that explain neglecting mutual coupling.

Roy Lewallen, W7EL

Ian White GM3SEK wrote:
+3dB is a valid generalization, based on sound physics - but it is only
a generalization.

At the time those Grand Old Men were writing their textbooks, such
generalizations were the best that anybody could manage. But they had no
way of checking their accuracy - or more important, why and when they
start to become INaccurate.

50 years on, we do have a way, and we now know much more than they did.
That makes it very dangerous to quote those Grand Old Generalizations as
accurate and universal truths. Richard was quite correct to describe the
"+3dB rule" as "naive" - because, at today's level of knowledge, it is.

But we still need to know that the +3dB generalization exists; and
understand the fundamental reasons for it. That fundamental
understanding is what protects us against stupid mistakes.

Roy Lewallen, W7EL wrote:
"I don`t think that`s a valid excuse."

That old authors were satisfied with approximations may have less to do
with ignorance than with not having computers and programs to make
analysis fast and easy. The computer gurus have done well. A seconndary
effect of a paucity of computer power is a requirement for more
measurements. As the title of this thread is:"Accuracy of Antenna
Testing Ranges". measurement is still a concern.

As one who was doing plenty of tests and measurements 50 years ago, I`d
like to testify that if I could get 1-dB accuracy, I was satisfied.
Bailey may not have thought that was good enough accuracy, but I think
it was realistic for the period in the field. I`m sure the NBS did
better. But for ordinary purposes. 1 dB is probably good enough for
graphs and tables to be comparable in accuracy to the measurements you
can make. Of course, everyone wants complete accuracy.

Richard Fry`s and Arnold Bailey`s tables were within 1-dB. I think it`s
satisfactory.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
1 dB is probably good enough for
graphs and tables to be comparable in accuracy to the measurements you
can make. Of course, everyone wants complete accuracy.

I remember asking my college prof back in the '50's:
How can we trust a graph where none of the measured
values actually fall *on* the graph line?
--
73, Cecil http://www.qsl.net/w5dxp

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"Frank" wrote in message
news:VPqbe.902$0X6.797@edtnps90...
Thanks again Ed. From everyone of your posts I learn something new.

The MIL-STD-461E requirement for absorbed is a 10 dB return loss at 250
MHz.

I have 24" tall pyramidal foam, and that meets the requirement. As
frequency decreases, the foam essentially disappears. By 10 MHz, it has
almost no effect.

I think we were using 12" pyramdal foam, even on the floor, with inverted
foam to provide a walking area.

The pyramidal foam is expensive, about $50 / sq ft. If you want more
return loss, you need taller pyramids; those mythical governmental labs
have had foam up to 72" tall (and the wall absorbers tend to droop a bit
g).

With a 3m chamber, anything greater than 12" is not really practical.

Well, that's kind of what I was trying to say. The right way to do the work
is to start with the size of the device you need to test, then consider the
test standard that you need to apply, and that will tell you how much
"working volume" you need inside the chamber. Then, you can decide on
anechoic treatments, and that then defines the size of the shielded chamber.
This "working outward" approach is the right way, but if you find that you
have now specified a 30' high by 50' wide by 100' long chamber, likely you
can't afford that much "goodness." g

Most people find themselves in a situation where they have a chamber of some
kind, and then they are challenged to do good work on a product inside that
volume. Sometimes you can do "good enough" work, with a lot of effort and
some known limitations. Sometimes what you do is pretty decent, and good
enough for "pre-compliance" requirements. If your product line is rather
consistent (size, peripherals, external cabling), you can often use data
from a fancy, fully capable lab and compare that with data generated in your
own limited facility. When you find the deviations, you can use those as
future "correction factors."


--
Ed
WB6WSN
El Cajon, CA USA

"Frank" wrote in message
news:VPqbe.902$0X6.797@edtnps90...
Thanks again Ed. From everyone of your posts I learn something new.

The MIL-STD-461E requirement for absorbed is a 10 dB return loss at 250
MHz.

Assume you would test the chamber return loss with a tuned dipole having
free space return loss 10dB.

Again, the 250 MHz verification of return loss is measured with a horn
antenna, typically a double-ridged model like the ARA 2020 or the EMCO 3106.

A newer technique is to use ferrite tiles, especially on the floor. They
are less than a half-inch thick, and perform much better at low
frequencies. And the cost is about $100 / sq ft. I like to think of my
walls and ceiling as covered with $5 bills, and the floor carpeted with
$10's.

Your anechoic chamber is never really perfect; however, it becomes "good
enough" when you run out of money.

I have heard of the ferrite floor tiles, and are probably a much better
solution than inverted pyamids fitted into the floor mounted pyramids.

Before I installed the ferrite floor tiles, I had considerable problems with
resonances, starting around 7 MHz and continuing through about 150 MHz,
associated with the chamber XYZ dimensions. After the ferrite installation,
the resonances have nearly disappeared.


I did a lot of analysis to figure out what was required, but never got to
finish it, on account of being laid-off! Nobody ever seems to want to
spend the money to get it right.

You can write that on your chamber wall (but management will be ****ed).

OK, just for trivia's sake. If the antenna base was cylindrical, painted
grey crinkle, had a 6-position range switch and a brown bakelite top
insulator, it was an Empire VA-105.

Describes it perfectly

But, if it was almost a cube, painted battleship grey, had a black front
panel and an 8-position range switch, it was a Stoddart 92138-1 (that
number is a hazy memory). Both were passive antennas. The Empire was used
with the NF-105 receiver,

That was the one I used, now you mention it I remember the model number as
the NF-105


So you're older than dirt too? g


--
Ed
WB6WSN
El Cajon, CA USA

"Reg Edwards" wrote in message
...
Reg propped up this tar baby and everyone's taken a punch at it.

Perhaps it is time to check in and see if you have your answer yet
Reg.

==========================================
Wes,
Not everybody has yet taken a punch at it. There are several regular
names who are missing.

All I want is a number, eg., of decibels, preferably from a standards
lab.

But it has only been been demonstrated "Measurements" is not a
"Science" - it is an "Art". Perhaps I can clarify my question.

Suppose a customer, perhaps an antenna manufacturer, walks into the
lab wheeling behind him a weird contraption (we've heard of them) and
asks for the forward and reverse gains to be determined and for a
calibration certificate to be issued.

For present purposes actual forward and reverse figures don't matter.

But for the two figures to be of value the uncertainties in the
determination should be stated on the certificate (a legal document).

What are TYPICAL uncertainties, in dB, which appear above the Head of
the Laboratory's signature.



Reg:

I'm primarily doing the US MIL-STD-461E protocol, on widely varying test
specimens (hand-held, man-worn, 2-meter tall racks, 2-meter diameter
parabolic tracking antennas), so I am getting my antennas calibrated for 1 &
3 meter separation distances.

The lab I use has an outdoor range, and I get a Certificate of Conformance
for each antenna which includes the following statements:

"Test and Measurement Equipment used for performance verification is
calibrated with traceability to the U.S. National Institute of Standards and
Technology. Inspection records, test data, and other evidence of conformance
are on file at the sellers facility are are available for inspection on
request."

They further state a "Calibration Uncertainty of (2 sigma) (+/- 1 dB)" and
reference "SAE ARP-958". I then get a listing of the specific instruments
used in the generation of my antenna data, along with their calibration
certificate numbers and the date of each calibration period expiration.

As for the data, I get a tabulation and a plot. For a typical antenna (an
EMCO 3115 double-ridged horn covering 1 GHz to 18 GHz), I get a tabular
array of antenna correction factor, dB return loss, SWR, numeric gain and
dBi, in 250 MHz step increments, across the range. I also get a continuous
swept plot of return loss, and from this, I infer that the lab uses my
unknown antenna as the transmitting element on their range.

This data and the C of C is enough to keep my internal Metrology guy happy,
and it gives me a sufficiently warm feeling. I have been using the same
antenna lab for almost 10 years, and I informally track the calibration data
from year to year on each of my antennas. So far, the data never has looked
"copied", fudged, or otherwise egregious. However, that may just mean I'm
easily fooled.


--
Ed
WB6WSN
El Cajon, CA USA

Reg:

I'm primarily doing the US MIL-STD-461E protocol, on widely varying
test
specimens (hand-held, man-worn, 2-meter tall racks, 2-meter diameter
parabolic tracking antennas), so I am getting my antennas calibrated
for 1 &
3 meter separation distances.

The lab I use has an outdoor range, and I get a Certificate of
Conformance
for each antenna which includes the following statements:

"Test and Measurement Equipment used for performance verification is
calibrated with traceability to the U.S. National Institute of
Standards and
Technology. Inspection records, test data, and other evidence of
conformance
are on file at the sellers facility are are available for inspection
on
request."

They further state a "Calibration Uncertainty of (2 sigma) (+/- 1
dB)" and
reference "SAE ARP-958". I then get a listing of the specific
instruments
used in the generation of my antenna data, along with their
calibration
certificate numbers and the date of each calibration period
expiration.

As for the data, I get a tabulation and a plot. For a typical
antenna (an
EMCO 3115 double-ridged horn covering 1 GHz to 18 GHz), I get a
tabular
array of antenna correction factor, dB return loss, SWR, numeric
gain and
dBi, in 250 MHz step increments, across the range. I also get a
continuous
swept plot of return loss, and from this, I infer that the lab uses
my
unknown antenna as the transmitting element on their range.

This data and the C of C is enough to keep my internal Metrology guy
happy,
and it gives me a sufficiently warm feeling. I have been using the
same
antenna lab for almost 10 years, and I informally track the
calibration data
from year to year on each of my antennas. So far, the data never has
looked
"copied", fudged, or otherwise egregious. However, that may just
mean I'm
easily fooled.
--
Ed
WB6WSN
El Cajon, CA USA

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

Much obliged to you Ed for interesting infomation from a reliable
source.

Including 2-Sigma uncertainty limits of +/- 1 dB.

Thanks.
Reg, G4FGQ

On Fri, 29 Apr 2005 20:27:51 -0700, "Ed Price"
wrote:

I had considerable problems with
resonances, starting around 7 MHz and continuing through about 150 MHz,
associated with the chamber XYZ dimensions.

Hi Ed,

When I was in the Navy, my buddy had, in his former life, been a pipe
organ technician (the old fashion type, not the Hammond home organ
type). He had worked in the really large Pizza 'n' Pipes types of
concessions up and down the Pacific coast, and in the classic theaters
of the 30s vintage (Paramounts, Pantages, Orpheums, etc.). He related
how those venues made sure that when constructed, no two walls met at
90° nor were parallel so as to break up resonances.

73's
Richard Clark, KB7QHC