"Frank" wrote in message
news:U_Uae.64373$VF5.13953@edtnps89...
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.

Everbody loves to argue about antennas; their calibration, application &
accuracy! In the EMC area (my side of the elephant), we are frequently
looking for emissions with a maximum limit so low (imposed by the standard)
that we have to be inside a shielded enclosure. Since the cost of a chamber
increases as the square (or maybe the cube) of its volume, only
extraordinarily well-funded (uhh, governmental) labs can afford really huge
chambers. Thus, most EMC testing happens in more modest volumes (my chamber
is 36' x 24' x 9').

Because the standard recognizes that a lot of the required test frequency
range practically puts the measurements in less than far-field conditions,
the standard gets very picky in defining the acceptable antennas and the
test setup and methodology. Here's what MIL-STD-461E says about conical
logarithmic spiral antennas:

"Previous versions of this standard specified conical log spiral antennas.
These antennas were convenient since they did not need to be rotated to
measure both polarizations of the radiated field. The double ridged horn is
considered to be better for standardization for several reasons.At some
frequencies, the antenna pattern of the conical log spiral is not centered
on the antenna axis. The double ridged horn does not have this problem. The
circular polarization of the conical log spiral creates confusion in its
proper application. Electric fields from EUTs would rarely be circularly
polarized. Therefore, questions are raised concerning the need for 3 dB
correction factors to account for linearly polarized signals. The same issue
is present when spiral conical antennas are used for radiated susceptibility
testing. If a second spiral conical is used to calibrate the field correctly
for a circularly polarized wave, the question arises whether a 3 dB higher
field should be used since the EUT will respond more readily to linearly
polarized fields of the same magnitude."


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.

The 41" (or really, 104 cm, gotta get with the program!) the monopole rod
goes way back, to the early 50's. It was originally intended to go down to
150 kHz, and the designs (Stoddart, Empire, Fairchild, Singer, AHS, EMCO)
were all variations of a 41" rod atop a box containing manually switched
transformers. Later designs incorporated remote switching, but these were
still passive antennas, with horrible efficiency and high antenna factors/

A big change happened in the early 70's, when active designs came out. The
41" rod was still there (some designs added a big capactive top-hat for
greater pick-up), but it now stood on a switchless box that had a very high
input impedance FET. (Don't touch that rod; ESD!) But this design allowed
antenna factors to approach 0 dB, and yielded a flat gain across 11 octaves!
(That nice for automated acquisition systems.)

OTOH, these may not really be antennas any more. They certainly can't be
driven with RF power to act as a radiator, so maybe we should be calling
them "field probes" instead of antennas.

Since you asked about the rod calibration procedure, here's some background
on it, again from MIL-STD-461E:

"There are two different mounting schemes for baluns of available 104
centimeter rod antennas with respect to the counterpoise. Some are designed
to be mounted underneath the counterpoise while others are designed for top
mounting. Either technique is acceptable provided the desired 0.5 meter
electrical length is achieved with the mounting scheme. The 10 pF capacitor
used with the rod antenna in 5.16.3.4.c(3) as part of the system check
simulates the capacitance of the rod element to the outside world. With the
rod antenna, the electric field present induces a voltage in the rod that is
applied to the balun circuitry. One of the functions of the balun is to
convert the high impedance input of the antenna element to the 50 ohm
impedance of the measurement receiver. The 10 pF capacitor ensures that the
correct source impedance is present during the check. Some antennas have a
10 pF capacitor built into the rod balun for calibration purposes and some
require that an external capacitor be used. For measurement system checks,
establishing the correct voltage at the input to the 10 pF capacitor can be
confusing dependent upon the design of the antenna and the associated
accessories. Since, the electrical length of the 104 cm rod is 0.5 meters,
the conversion factor for the induced voltage at the input to the 10 pF
capacitor is 6 dB/m. If the limit at the measurement system check frequency
is 34 dBuV/m, the required field level to use for measurement system check
is 6 dB less than this value or 28 dBuV/m. The voltage level that must be
injected is:
28 dBuV/m – 6 dB/m = 22 dBuV

Since the input impedance at the 10 pF capacitor is very high, a signal
source must be loaded with 50 ohms (termination load or measurement
receiver) to ensure that the correct voltage is applied. A “tee” connection
can be used with the signal source connected to the first leg, the 50 ohm
load connected to the second leg, and the center conductor of the third leg
connected to the 10 pF capacitor (barrel referenced to the balun case).
Sometimes a feed-through accessory that acts as a voltage divider is
supplied with a rod antenna for the purpose of determining antenna factors.
The accessory usually includes the required 10 pF capacitor inside the
accessory. If the accessory is used for injecting the measurement system
check signal, caution needs to be observed. Since the accessory is intended
for only determining antenna factors, the procedures provided with these
accessories may not address the actual voltage that appears at the 10 pF
capacitor. The design of the accessory needs to be reviewed to determine
that the correct voltage is obtained. For a common design, the voltage at
the capacitor is 14.6 dB less than the signal source level and 5.0 dB
greater than the indication on the measurement receiver."

Whew! That's why I'm glad I only use, and not design or calibrate, those
things!


--
Ed
WB6WSN
El Cajon, CA USA

"Ed Price" wrote in message
news:H_Vae.2007$pk5.904@fed1read02...

Everbody loves to argue about antennas; their calibration, application &
accuracy! In the EMC area (my side of the elephant), we are frequently
looking for emissions with a maximum limit so low (imposed by the
standard) that we have to be inside a shielded enclosure. Since the cost
of a chamber increases as the square (or maybe the cube) of its volume,
only extraordinarily well-funded (uhh, governmental) labs can afford
really huge chambers. Thus, most EMC testing happens in more modest
volumes (my chamber is 36' x 24' x 9').

Last place I worked with EMC facilities they only had a 3 m cube chamber.
The dimensions you quoted are huge compared to my experience. (I think ETC,
in Airdrie Alberta, had a similar chamber to yours; also General Dynamics in
Calgary had two similar chambers. Also Nortel has some EMC capabiltiy.)
The insides were covered in microwave absorber, and there was some question
as to how effective the absorber was at 30 MHz. It must have done
something, since before the absorber was installed it was interesting to see
the effects on a transmitter keyed inside a shielded enclosure.

Because the standard recognizes that a lot of the required test frequency
range practically puts the measurements in less than far-field conditions,
the standard gets very picky in defining the acceptable antennas and the
test setup and methodology. Here's what MIL-STD-461E says about conical
logarithmic spiral antennas:

"Previous versions of this standard specified conical log spiral antennas.
These antennas were convenient since they did not need to be rotated to
measure both polarizations of the radiated field. The double ridged horn
is considered to be better for standardization for several reasons.At some
frequencies, the antenna pattern of the conical log spiral is not centered
on the antenna axis. The double ridged horn does not have this problem.
The circular polarization of the conical log spiral creates confusion in
its proper application. Electric fields from EUTs would rarely be
circularly polarized. Therefore, questions are raised concerning the need
for 3 dB correction factors to account for linearly polarized signals. The
same issue is present when spiral conical antennas are used for radiated
susceptibility testing. If a second spiral conical is used to calibrate
the field correctly for a circularly polarized wave, the question arises
whether a 3 dB higher field should be used since the EUT will respond more
readily to linearly polarized fields of the same magnitude."

Very interesting Ed, will forward your comments to my last company. Doubt
they will do anything tho, as they never want to spend any money. Assume
the recomended type of antenna is a linearly polarized log periodic.

The 41" (or really, 104 cm, gotta get with the program!) the monopole rod
goes way back, to the early 50's. It was originally intended to go down to
150 kHz, and the designs (Stoddart, Empire, Fairchild, Singer, AHS, EMCO)
were all variations of a 41" rod atop a box containing manually switched
transformers. Later designs incorporated remote switching, but these were
still passive antennas, with horrible efficiency and high antenna factors/

I remember the Singer (Was it Singer-Metrics), and using it to measure
radiated spurious in a cow pasture at 50 m from a 1kW TMC linear (Canadian
Marconi, Montreal). The test monopole had a cylindrical base with a rotary
switch.

A big change happened in the early 70's, when active designs came out. The
41" rod was still there (some designs added a big capactive top-hat for
greater pick-up), but it now stood on a switchless box that had a very
high input impedance FET. (Don't touch that rod; ESD!) But this design
allowed antenna factors to approach 0 dB, and yielded a flat gain across
11 octaves! (That nice for automated acquisition systems.)

OTOH, these may not really be antennas any more. They certainly can't be
driven with RF power to act as a radiator, so maybe we should be calling
them "field probes" instead of antennas.

Since you asked about the rod calibration procedure, here's some
background on it, again from MIL-STD-461E:

"There are two different mounting schemes for baluns of available 104
centimeter rod antennas with respect to the counterpoise. Some are
designed to be mounted underneath the counterpoise while others are
designed for top mounting. Either technique is acceptable provided the
desired 0.5 meter electrical length is achieved with the mounting scheme.
The 10 pF capacitor used with the rod antenna in 5.16.3.4.c(3) as part of
the system check simulates the capacitance of the rod element to the
outside world. With the rod antenna, the electric field present induces a
voltage in the rod that is applied to the balun circuitry. One of the
functions of the balun is to convert the high impedance input of the
antenna element to the 50 ohm impedance of the measurement receiver. The
10 pF capacitor ensures that the correct source impedance is present
during the check. Some antennas have a 10 pF capacitor built into the rod
balun for calibration purposes and some require that an external capacitor
be used. For measurement system checks, establishing the correct voltage
at the input to the 10 pF capacitor can be confusing dependent upon the
design of the antenna and the associated accessories. Since, the
electrical length of the 104 cm rod is 0.5 meters, the conversion factor
for the induced voltage at the input to the 10 pF capacitor is 6 dB/m. If
the limit at the measurement system check frequency is 34 dBuV/m, the
required field level to use for measurement system check is 6 dB less than
this value or 28 dBuV/m. The voltage level that must be injected is:
28 dBuV/m - 6 dB/m = 22 dBuV

Since the input impedance at the 10 pF capacitor is very high, a signal
source must be loaded with 50 ohms (termination load or measurement
receiver) to ensure that the correct voltage is applied. A "tee"
connection can be used with the signal source connected to the first leg,
the 50 ohm load connected to the second leg, and the center conductor of
the third leg connected to the 10 pF capacitor (barrel referenced to the
balun case). Sometimes a feed-through accessory that acts as a voltage
divider is supplied with a rod antenna for the purpose of determining
antenna factors. The accessory usually includes the required 10 pF
capacitor inside the accessory. If the accessory is used for injecting the
measurement system check signal, caution needs to be observed. Since the
accessory is intended for only determining antenna factors, the procedures
provided with these accessories may not address the actual voltage that
appears at the 10 pF capacitor. The design of the accessory needs to be
reviewed to determine that the correct voltage is obtained. For a common
design, the voltage at the capacitor is 14.6 dB less than the signal
source level and 5.0 dB greater than the indication on the measurement
receiver."

Whew! That's why I'm glad I only use, and not design or calibrate, those
things!

It does seem a bit confusing. I have never seen this procedure before, and
do not understand how a physical length of 1.04 m can have an electrical
length of 0.5m. I guess the 10pf capacitance of the rod is its capacitance
with a defined ground plane size. I don't think I would be 100% convinced
as to the procedures accuracy unless I could verify it with a known E field.
At least, in principal, I understand what is being done.
--
Ed
WB6WSN
El Cajon, CA USA

Frank
VE6CB

"Frank" wrote in message
news:1V8be.56318$yV3.14588@clgrps12...

"Ed Price" wrote in message
news:H_Vae.2007$pk5.904@fed1read02...

Everbody loves to argue about antennas; their calibration, application &
accuracy! In the EMC area (my side of the elephant), we are frequently
looking for emissions with a maximum limit so low (imposed by the
standard) that we have to be inside a shielded enclosure. Since the cost
of a chamber increases as the square (or maybe the cube) of its volume,
only extraordinarily well-funded (uhh, governmental) labs can afford
really huge chambers. Thus, most EMC testing happens in more modest
volumes (my chamber is 36' x 24' x 9').

Last place I worked with EMC facilities they only had a 3 m cube chamber.
The dimensions you quoted are huge compared to my experience. (I think
ETC, in Airdrie Alberta, had a similar chamber to yours; also General
Dynamics in Calgary had two similar chambers. Also Nortel has some EMC
capabiltiy.) The insides were covered in microwave absorber, and there was
some question as to how effective the absorber was at 30 MHz. It must
have done something, since before the absorber was installed it was
interesting to see the effects on a transmitter keyed inside a shielded
enclosure.

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. 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).

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.

With the dark blue pyramids and black tiles, a chamber looks like a bat
cave. One vendor decided that the new millenia needed white paint on the
foam; another vendor touts pyramids that have a 90-degree axial rotation
part way up the taper, and yet another truncates the pointy tips, telling us
that works better. It's just like the antenna game.

Here's what MIL-STD-461E says about conical logarithmic spiral antennas:

"Previous versions of this standard specified conical log spiral
antennas. These antennas were convenient since they did not need to be
rotated to measure both polarizations of the radiated field. The double
ridged horn is considered to be better for standardization for several
reasons.

Very interesting Ed, will forward your comments to my last company. Doubt
they will do anything tho, as they never want to spend any money. Assume
the recomended type of antenna is a linearly polarized log periodic.

No, 461 doesn't like log periodics either, saying:

"Other linearly polarized antennas such as log periodic antennas are not to
be used. It is recognized that these types of antennas have sometimes been
used in the past; however, they will not necessarily produce the same
results as the double ridged horn because of field variations across the
antenna apertures and far field/near field issues. Uniform use of the double
ridge horn is required for standardization purposes to obtain consistent
results among different test facilities."

The MIL-STD defines a 104 cm rod from 10 kHz to 30 MHz, then a biconical
from 30 MHz to 200 MHz, and finally, horns above there. Since pyramidal
horns are only good for about an octave, a smart Navy guy added
exponentially flared ridges to the horns, and came up with multi-octave
horns. A typical horn for 200 MHz to 1 GHz has an aperture of about 1 meter,
then another horn tries to go from 1 GHz to 18 GHz. That's a bit too far for
me, as the antenna factor really climbs above about 14 GHz, so I switch to a
common, non-ridged horn for 12 GHz to 18 GHz. For 18 GHz to 26 GHz and 26
GHz to 40 GHz, I use standard-gain flared horns. With a pre-selected
spectrum analyzer, really good coax, and a couple of low-noise pre-amps,
that lets me get comfortably below the most stringent RE102 limits.


I remember the Singer (Was it Singer-Metrics), and using it to measure
radiated spurious in a cow pasture at 50 m from a 1kW TMC linear
(Canadian Marconi, Montreal). The test monopole had a cylindrical base
with a rotary switch.

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. 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, while the Stoddart
antenna was associated with the NM-22A (that's why the range switches were
different, to match the ranges on their associated receivers).


--
Ed
WB6WSN
El Cajon, CA USA

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. i.e. some physically realizable antenna with
a return loss of 40 dB at the test frequency. I suppose, with an inductivly
loaded dipole, you could test the return loss of a 3 m chamber down to 30
MHz. There were some questions raised about possible reflections in the 3 m
chamber due to imperfections in the installation of the pyramidal foam. I
tried sweeping from 1 to 10 GHz with the log spiral antenna, coupling to a
non-standard antenna, and performing an inverse FFT on the network analyzer
data to generate a time domain plot. I had very little success in actually
seeing reflections. For best resolution the ideal would have been to sweep
from 30 MHz to 20 GHz with two wide band antennas, but the company did not
want to spend the money for any new antennas. What I am thinking is that
careful return loss measurements may have shown if any reflections were
present.

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.

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.

With the dark blue pyramids and black tiles, a chamber looks like a bat
cave. One vendor decided that the new millenia needed white paint on the
foam; another vendor touts pyramids that have a 90-degree axial rotation
part way up the taper, and yet another truncates the pointy tips, telling
us that works better. It's just like the antenna game.

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

No, 461 doesn't like log periodics either, saying:

"Other linearly polarized antennas such as log periodic antennas are not
to be used. It is recognized that these types of antennas have sometimes
been used in the past; however, they will not necessarily produce the same
results as the double ridged horn because of field variations across the
antenna apertures and far field/near field issues. Uniform use of the
double ridge horn is required for standardization purposes to obtain
consistent results among different test facilities."

The MIL-STD defines a 104 cm rod from 10 kHz to 30 MHz, then a biconical
from 30 MHz to 200 MHz, and finally, horns above there. Since pyramidal
horns are only good for about an octave, a smart Navy guy added
exponentially flared ridges to the horns, and came up with multi-octave
horns. A typical horn for 200 MHz to 1 GHz has an aperture of about 1
meter, then another horn tries to go from 1 GHz to 18 GHz. That's a bit
too far for me, as the antenna factor really climbs above about 14 GHz, so
I switch to a common, non-ridged horn for 12 GHz to 18 GHz. For 18 GHz to
26 GHz and 26 GHz to 40 GHz, I use standard-gain flared horns. With a
pre-selected spectrum analyzer, really good coax, and a couple of
low-noise pre-amps, that lets me get comfortably below the most stringent
RE102 limits.

I think they were considering horns and low noise amps to get above 10 GHz.
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.

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

while the Stoddart antenna was associated with the NM-22A (that's why the
range switches were different, to match the ranges on their associated
receivers).


--
Ed
WB6WSN
El Cajon, CA USA

73,

Frank

"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

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

"Richard Clark" wrote in message
...
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


Umm, yes. Unfortunately, the suppliers of modular shielded enclosures are
rigorously orthogonal guys! g

--
Ed
WB6WSN
El Cajon, CA USA

--
J. Mc Laughlin; Michigan U.S.A.
Home:



.. 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


I too used a NF-105 with plug-in modules and a gas generator to run
propagation surveys in the hills of West Va. a very long time ago. I
received a first class education about real propagation and comparisons with
predictions. Sure wish I had had a computer.
Since we had a government station wagon (would drop off the generator,
drive ahead to get away from the noise, and take measurements) we were
concerned that the indigenous population might see us as "revenuers." We
never went up a road that had an advertisement for honey.
73 Mac - not quite as old as dirt N8TT