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