Dear Richard:
I am obliged to you for your useful comments.
In the large TEM cells that I use to about 200 MHz (with care above
about 150 MHz) a very small probe is inserted through the roof of the cell.
That probe is used, with the sort of processing you mentioned, to provide
one verification of the E inside of the cell. (The other verification comes
from measuring the power going in and coming out of the cell, and the
internal dimensions.) The probe-scheme's success is helped by the fact that
E can go up to 200 v/m (or more) so a lot of signal is available.

Interesting things happen to some electronic equipment well before 200
v/m. A TEM cell with a spectrum analyzer can inform about what a device is
radiating.

Still, outdoors, a one meter rod over a large ground screen placed well
clear of other structures provides the means to measure (with reasonable
uncertainties) the low TOA vertical E field up to 20 or so MHz.

A scheme that I saw at NBS circa 1975 (now NIST) to measure an estimate
of total field uses orthogonal, very small doublets with diodes at their
center. They received a patent on the idea. To bring the DC to a measuring
device, lossy transmission lines were used so as to make the transmission
lines "invisible" to the field. In other words, the step up from
rod-and-screen is a big step.

In my youth, I measured VHF signals propagated by FM broadcast stations
in the "hills" of West Va. to enhance the VHF propagation models that then
existed. The purpose was to be able to predict interference to a radio
astronomy site. We used tuned dipoles elevated some standard distance that
I do not remember.

As many here have said: It is not easy to measure fields.

73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Richard Clark" wrote in message
...
On Sat, 17 Dec 2005 23:38:13 -0500, "J. Mc Laughlin"
wrote:

I have dealt with approximations of the subject device. In each
case,
an extremely high input impedance amplifying device is placed at the base
of
the antenna that has a known voltage amplification and a 50 ohm output
impedance. Knowing that a close approximation of the open circuit
voltage
is amplified by a known amount, a calibrated, tuned voltmeter (at 50
ohms)
is able to measure the size of vertically polarized E (with the usual
uncertainties). (and a bit of arithmetic)

Hi Mac,

I too, will jump in with alternatives to this short, thin rod feeding
an infinite Z. It makes for a simple specification, but when the
frequency begins to climb such is not very practical. Input Z's tend
to be dominated with strays and that "short" rod begins to become
enormous. Such artifacts of the MF era are quickly discarded.

The NIST methods (NIST technical note numbers 1309 and 1098) employ
resonant sized dipoles feeding a DC Hi R (and hence AC Hi Z load) at
the gap of the elements. By DC Hi R, the detector filter employs
50KOhm components in a balanced cascading filter that in turn feeds a
Hi R voltmeter through 250KOhm leads (carbon impregnated plastic
conductors to decouple both loading and induction).

Uncertainty, worst case, is 1dB.

Schelkunoff's algorithm is used to find the length of the dipole (no
real surprise here for halfwave length). The effective length is not
half, but rather closer to 62 - 63%.

73's
Richard Clark, KB7QHC