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

Sylvio Damiani,

I have modelled folded unipole antennas in some detail, c.f my
comments in IEEE AP-S Magazine, August 1997 issue. This discussion
relates to Tino Trainotti's so-called asymmetric dipole --- which is a
130 metre grounded tower with four drooping radials, where the top
section is fed like a folded unipole (three drop wires).

At the time I did my initial analysis I was using NEC-2, but I have
redone my analysis using NEC-4D (double precision). I use Roy
Lewallen's program --- EZNEC pro.

For this study I had a number of concerns: in particular connecting
"thin" wires to a "thick" tower; and the fact that the tower was
grounded.

For the initial study, since I was using NEC-2 I simulated ground
connection by using four or six resonant radials just above the
ground. I also used three sources, each on a short 1-segment wire
connecting to the bottom end of each drop wire.

In the final analysis since I was using NEC-4D I grounded the tower,
by assuming a 135 m tower, where the base of the tower was 5 m below
ground level. This I though would best simulate the several (I
believe 12) ground rods, arranged in a circle around the base of the
tower. I also used a single source, since this is the way the tower
was fed --- i.e. I joined the ends of the drop wires by a parimeter
wire and fed this cage by a short 1-segment wire, centred source on
this wire --- see below.

I calculate a FS of 127.7 and 108.8 dB microvolts/m at 1 km and 10 km
(50 kW transmitter); compared with 128 and 108 dB microvolts/m
measured. The impedance I calculate is 148 + j 9, measured values are
said to be between 150-200 ohms.

I must say I am astonished at the apparent close agreement with real
world measurements, since the antenna is a cage of thin wires, and as
well there are four drooping radial wires; all connecting to a thick
tower.

I have not modelled this antenna system using MININEC --- but I can
tell you I have experienced difficulty modelling closely spaced wires
(transmission lines) using MININEC. NEC places the source on the
centre of a segment. MININEC places the source on either end of a
segment. To place a source on the centre of a short feed wire using
MININEC you have to use 2 or 3 segments on the wire. I discuss this
subject in my paper "Modeling HF Antennas with MININEC -- Guidelines
and Tips from a Code User's Notebook", The ARRL Antenna Compendium
Volume 3, 1992, pp. 156-164 --- but I have not pursued this subject.

John S. Belrose

PS

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.

--------------------------------
>Antenna modelers: how to model a vertical folded monopole, say with 6
>folded wires, for the AM Broadcasting band?
>
>I've tried once but the measured input impedance is much higher than
>the MININEC-Win prediction. Also the unattenuated field at one km for
>one kW power presents some discrepancy with real world
>results. Thanks.

_____________________________________________
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 - 13:05:57 EDT