"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