Re: NEC-LIST: Question on "vertical folded monopoles"

Gregory,

I will wait until I have my copy of the Communications Quarterly issue
on/or just off press (Spring 1998), in which Part 2 of my article on
elevation radials should appear. I will then send copies of Parts 1
and 2. And also a copy of my article in QEX.

But there is no problem to model elevated radials providing you have
NEC-2 or NEC-4. Forget about trying to model elevated radials using
MININEC.

1) First find the resonant length for the frequency, ground parameters
and height of interest. To do this put 2-radials end-to-end (this is
really a dipole at low height), and find the length for resonance.

2) Model your monopole or folded dipole with this radial length and
the number of radials you wish to model.

3) If you use EZNEC pro I can tell you a few tricks to make modelling
easy, and computation fast (at least fast as possible --- it still
takes several hours to a day to model 120 radials buried).

A word of caution: Expect a maximum attenuation (minimum radiation
efficiency), for GP antennas over buried radials, and a maximum
difference between GP antennas with elevated radials and GP antennas
with very many buried radials, when the frequency and earth parameters
are such that the loss tangent is one --- e.g. for average ground,
sigma 3 mS/m, dielectric constant 13, the complex permittivity for a
frequency of 4.1 MHz is 13 - j 13.2. The 80M band is my favorate
band, and I have modelled antennas over my so-called average ground,
in which the prediced performance is worse than for ground of poor
conductivity and ground of good conductivity!!

So we can calculate impedance and field strength for elevated radials.
The argument is how does this compared with field strength for GP
antennas with buried radials?? I know of no such measurements. So we
have to compare measured with predicted first for buried radials and
then for elevated radials. We need more measurements.

The classical paper by Brown et. al. [1937], see Proc. IRE, 25, Number
6, June 1937, pp. 753-787, is the only paper that I know that measured
antenna impedance, and field strength at one mile, inferred from the
field strength at 0.3 miles, as the number and length of radials, and
height of the antenna was changed. The field strength at 0.3 miles is
not an uinattenuated field strength. Unfortunately we do not know how
accurate the measurements were, certainly instrumentation was very
crude compared with today. And the field strength for 1 kW was
extrapolated since the power fed into each antenna was only 0.2 watt.

For a ground conductivity of 10 mS/m, dielectric constant 30 (their
measurements were made in an agriculture field), frequency 3 MHz, I
calculate the field strength at one mile for a radiator height
75-degrees, inferred from the field strength at 0.3 miles, equal to
158, 169.3, 178.4, and 183.7 mV/m, for 15, 30, 60 and 113 radials
0.274w long. Measured values were 153, 162, 176 and 179 mV/m
respectively.

For the same radiator (height 75-degrees) with four resonant radials,
height 2.5 m (2.5 m to be above head hight), length 24.09 m, I
calculate for comparison a field strength of 167.6 mV/m (-0.8 dB
compared with 113 radials buried). This is the typical difference I
find, which is generally less than a dB. Field strength for elevated
and buried radial systems are more or less comparable for MF broadcast
band frequencies, certainly the predicted difference is less than can
be measured, unless the conductivity is very poor.

Jack, VE2CV

------------------------------------

>John:
>
>Would you please send me reprints of your articles on raised resonant
>radials? I've had some problems modeling them.
>
>Thanks,
>
>N9GL
>Gregory D. Lapin, Ph.D., P.E.
>ComBioMed Laboratories
>1206 Somerset Avenue
>Deerfield, IL 60015 USA
>g.lapin_at_ieee.org
>
>
>> For readers interested in elevated resonant radials I have two recent
>> papers on the subject, see Communications Quarterly, Winter and Spring
>> 1998 issues. The latter paper discusses comparison between measured
>> and simulated, a real world broadcast station.

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
3701 Carling Avenue
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Tue Apr 21 1998 - 18:21:00 EDT