Re: NEC-LIST: Modeling loops

On Friday 6 December, in response to my posting yesterday, LB Cebik wrote:
>

>I'll look forward to your papers in QEX. Your experiences with modeling
>loops, deltas, half loops, etc. parallel my own experiences. Like you, I
>have had to dismiss the ARRL tradition of 1005/f. On 10 meters, I ran
>closer to 1040, depending on wire size--and wire diameter does make a
>difference, per Haviland. Split feed at corners or midway with even
>numbers of segments coincide nicely with single feed models. Have a
>couple of pieces, one on small quads (2/3 to 3/4 full size) and on
>multiband quads in the long wait at Communications Quarterly.

>On the verticals, I assume you have tried (but mention this just in case)
>feeding a small single segment (or very small number of segments) at the
>base and then fanning from the end junction of this wire. I have not
>tried this for monopoles, but it works well for dipoles, where the common
>wire is at the center. Also have not tried it except for thin wires--fat
>wires (tubing) may encounter the overlapping radii limits unless there is
>a source wire, then perhaps short horizontal wires to each fanned monopole
>end, then the fanned monople at its proper angle.
>

Hello LB,

Nice to hear from you.

It looks like we agree on the resonant dimensions for loops. Concerning
vertical fan monopoles, yes the apex of the fan does junction with a short
vertical wire at the base. The source is on the grounded end of this wire,
well removed from the multi-wire junction.

Actually I am trying to numerically model Marconi's December 1901 fan
monopole at Poldhu Cornwall. I do not know whether you know of my interest
in the history of radio. This antenna was a 54 wire fan, wires spaced 1
meter apart along a triatic cable at a height of 45.72 m. The apex of the
fan connects to a wire 3 m long.

Our experimental model exhibits resonances at 935 kHz, and 3.8 MHz;
anti-resonance at 2.4 MHz, 4.8 MHz and approaching an anti-resonance
between 7 and 8 MHz --- but no resonances above 3.8 MHz.

I have little confidence in out MININEC model for a 54 wire fan, but it
does give about the right fundemental resonant frequency; and we have
modelled a 12-wire fan, in a number of ways:

1) Equal length segments on all wires (length about 0.5 m); and

2) Tapering to 0.001-wavelengths at the junction of all wires.

The result is about the same. The fundemental resonant frequency is about
right, but I find NO higher order resonance. There are dips and wiggles in
the resistance and reactance with increase in frequency above fo, but the
reactance is alway inductive. The model exhibits a capacitive reactance
for frequencies above about 5 MHz. The "wiggles" are more evident, perhaps
I should say only(?) evident, for the case where each wire of the fan is
insulated from the triatic wire.

The only photograph of the Poldhu antenna, which has been published and
republished, clearly shows 12-wires, and there is a blob at the end of each
wire. But the view of GEC Marconi historians is that the photograph has
been retouched -- their view is that there were 54 wires, and the wires
were attached to the triatic cable, with insulators only at the ends of the
triatic.

We are currently constructing a 12-wire fan. We should have done this
earlier. We use a scale fact of of 75, thus the resonant frequency for our
model is about 70 MHz, and the radius of our elevated ground plane is about
3.5 wavelengths at this frequency. A finite ground plane does result in
impedance wiggles, due to reflections from the ends, but the impedance does
vary quite regularly with frequency (two or three points quite off the
curve).

This matter is of academic interest only, since the antenna's reactance at
frequencies above that which the antenna "jigger" (Marconi's words) tunes
it, inductance variously estimated to be about 42.3 microhenries,
dominates. The secondary of the "jigger" transformer is in effect a base
loading coil, which brings the resonant frequency of the antenna to 483 kHz
(according to out experimental model).

I will post this on the NEC-list. Perhaps someone can help.

73, Jack, VE2CV

John S. (Jack) Belrose, VE2CV
Director, Radio Sciences
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
Received on Fri Dec 06 1996 - 14:55:00 EST