"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