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

Alberto,

Okay, I have now interpreted your model (the Table of values for the
4-drop-wire 75 metre tower).

The tower is rather thick (2 metres diameter); and as well the drop
wires are not really wires (0.1 metres diameter (?)) --- these
conductors must be a cage of wires.

I can initially offer several comments:

1) For PEC (perfect electrical conducting) ground models, I recommend
you use MININEC. I have found very good agreement between NEC-4 and
MININEC for PEC ground.

2) You have too many segments for the thick conductor diameters. The
minimum segment length/conductor diameter should be not much smaller
than 5/1. This means that for a 75 m tower 2 m in diameter minimum
segment length should be not much less than 7/1. But this segment
length is too long for the skirt wires and the fed wire. I have tried
a segment length of 3 m (25 segments for the tower you used 100), and
this seem okay. If I change segment length above and below this
length, the reactance changes very little, and the resistance changes
by only an ohm or two.

3) You did not tell me the frequency for this antenna. At 1 MHz I
calculate (using EZNEC-4D) 94.6 + j 42.8 ohms. The field strength at
200 m (ground wave field strength) is 1.322471 V/m for 1 kW --- hence
the inverse distance field strength at 1 km (Eu) is 264.5 mV/m. This
seems to me to be a bit low --- and I have used a conductor loss for
copper. If we feed the 75 m tower directly, as a base driven quarter
wave antenna (frequency 1 MHz), Eu = 293.3 mV/m.

4) Let me comment on folded unipoles. Let Ro represent the base
resistance of the radiating system operating as a series fed antenna,
and let Io represent the total base current for an input power of W
watts. When excited as a folded unipole, the total antenna current
for the same power input will be Io as before, except that with one
conductor grounded and the other fed by the transmitter, the latter
carries only a portion M of the total current.

The value of M will differ with the relative radii of the two
conductors. If both are the same M = 0.5; if a large tower is fed by
a single small drop wire M will be very small. If one wishes to raise
M, a number of drop wires should be used; for a thick tower perhaps 8
drop wires (or more (?)) --- but I have not really studied this
issue.

Do you have measured impedances and field strength for comparison?

Jack

_____________________________________________
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>
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Received on Wed Apr 29 1998 - 09:46:53 EDT