NEC-LIST: Standard Reference Antennas used for MF Broadcasting

In recent conference papers published by Kabbary et.al [1997,1999] it
is reported (and re-reported) that the CFA consistently out-performs a
conventional quarter wavelength monopole antenna by 3-10 dB.

This is deemed to be reasonable in accord with these authors' point
of view.

They say "it is well known that the radiated power from a dipole or
monopole antenna has low efficiency." I continue with paraphrasing
rather than direct quote: The radiated power in the far field occurs
where E and H are in space quadrature and in time-phase, hence the
Poynting vector P = E x H produces real power radiation.

However, in the near field E and H fields are 90-degrees out of time
phase, resulting in reactive or non-radiating power. So far so good
(more or less).

But then they claim that the E and H field components in the far field
are much weaker than the reactive components in the near field. Which
explains why conventional antennas posses large inductive fields and
are not efficient radiators.

The CFA is (said to be) fundamentally an antenna which is designed to
move radiated power production from the conventional far field to the
near field, minimizing reactive power or inefficiency problems
associated with standard antenna designs.

Thus, it is not a surprise for them to find (so they say but we have
yet to see a graph showing this) that the field strength for a CFA is
astonishing great, which no one can believe, until they see for
themselves.

It is further claimed (correspondence Stuart-Belrose) that NEC cannot
predict the performance of a CFA, largely because NEC does not
correctly predict the H field associated with the capacitor element of
the CFA. That is a separate issue which is not the purpose of this
note.

Here I want to address the efficiency of a standard broadcast antenna.
The FCC standard reference antennas are based on an assumed sinusoidal
current distribution, for antennas of height h/wavelength. But real
antennas are electrically longer then their physical height, and
impedance and gain are characterized by their electrical height. For
h/wavelength (physical height) equal to 0.311 and 0.5 FCC field
strengths at 1 km for a radiated power of 1 kilowatt are 321.8 mV/m
and 380.5 mV/m.

In earlier correspondence I have been making reference to an antenna
75 metres in height, frequency 1161 kHz (h/wavelength = 0.29).
According to NEC-4D the field strength for this antenna at 1 km. for a
transmitter power of 1 kw, with 80 radials buried 0.2 m, average
ground, is 338 mV/m.

This is in accord with my reasoning above, and this also shows that a
MF monopole over an extensive ground screen is a very efficient
radiator. The FCC standard gain curves are for a no-loss antenna. My
calculations above are for an antenna over real ground. These sort of
field strengths are in accord with with those achieved, as can be seen
in hundreds of proof of performance reports for MF broadcast antennas.

So where is the loss that the CFA antenna circumvents, that can make
it so efficient? The only explanation (if we need one) is that the
CFA must produce little or no sky-wave. But NEC-4 does not predict
this, but neither does NEC-4 predict that the CFA is an antenna with
much gain.

So the saga continues, but I wish to state that in accord with my view
the conventional MF monopole can be a very efficient radiator.

Regards, Jack

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Thu May 06 1999 - 18:47:43 EDT