Re: Modeling N4KG-fed vertical

Pete,

Your memo is interesting for me since I intend to present a paper in a
special session at the ACES '96 Meeting in Monterey in March on elevated
radials. This session is entitled "Hams at ACES".

For an insulated tower (a tower standing on a base insulator at a lower
height then then the height of the radials) the shield of the coaxial cable
(or antenna tuning system ground) would be connected to the radials, which
come up to the tower but are insulated from the tower. The centre
conductor of the coax (or the antenna feed) would connect to the tower.

For a grounded tower the feed arrangement which you referred to as an
N4KG-fed HF vertical, apparently suggests reversing the feed arrangement.
The shield of the coax is attached to the tower at radial height, and the
centre conductor to the radials. Whatever I would think you would need
lots of ferrite beads on the coax to avoid problems with equipment ground
and current on the outter surface of the shield of the coax.

I have not tried modelling a grounded tower with elevated radials as you
suggest -- all my modelling has been concerned with the use of elevated
radials to eliminate the need for grounding a tower in the conventional
manner -- that is using many buried radial wires. I do not really see the
need for the arrangement you suggest, since grounded towers can be shunt
fed -- but I guess N4KG's tower is perhaps not very well grounded.

This antenna will be tricky to model, and for what purpose, but let me tell
you how I think you can model such an antenna arrangement:

1) NEC-2 does not permit connecting a thin wire to a thick tower. NEC-4
and MININEC permit this. But I do not have NEC-4 on my PC, and MININEC is
not very suitable since the ground beneath the tower is assumed to be
perfectly conducting. So I am stuck with using NEC-2. So to get a feeling
for the problem let us use wire antenna elements for radiator and radials.

2) First we have to simulate a grounded tower. Let us model a quarter
wavelength vertical wire antenna (# 10 wire), say 20 meters long for 3.75
MHz. Connect 4-resonant on the ground radials to the bottom end of this
vertical wire antenna. For a radial height of 5 mm the length of the
radials should be 11.1 m long for average ground (3 mS/m, 13).

3) Now install your 4 elevated radials, again using resonant radials, and
your feed arrangement. But this will be tricky. We could perhaps use
segment taper, tapering to 0.001 wavelengths on all radial wires (ground
mounted and elevated) and on the lower end of the vertical wire above
radial height. The peripherial wires joining the radials would be (say)
0.003 wavelength long (length joining each radial) with 3 segments on each
wire, and the wire connecting to the vertical wire should be at least 0.003
wavelength long again with 3 segments on the wire -- play around with the
geometry so you can realize this. Place the feed in the center of the wire
connecting the radial system to the tower.

This all assumes that you are using a current balun at the feed point. We
cannot model coax connected the way you have described with an uncerntain
equipment ground somewhere else.

4) Now we still have a piece of wire to deal with, viz. the wire
connecting the elevated radials with the on the ground radials. Depending
on the length of this wire you could use segments of length 0.001
wavelengths, or taper to this segment length on both ends of this wire.

So now we have an antenna system, but for what purpose? We have simulated
a fairly effective "connection" to ground, so what do we need the elevated
radials for?? Good luck.

Please give me a reference to N4KG vertical.

73, Jack, VE2CV

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>Another newbie question: How would I set up the NEC model of what is
>referred to in ham circles as an N4KG-fed HF vertical? Physically, this
>consists of a grounded tower with or without top-loading from antennas. One
>connects the coax feedline's center conductor to 4 x 1/4 wave elevated
>horizontal radials, which meet at but are insulated from the tower. The
>shield of the coax is attached to the tower at that height, which is
>determined such that the part of the tower above the connection is an
>electrical quarter-wave at the operating frequency.
>
>I don't have any trouble describing the physical layout of conductors, but
>how do I feed them?
>
>Thanks!
>
>73,
>
>Pete N4ZR (n4zr_at_ix.netcom.com)

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 Thu Jan 18 1996 - 17:19:00 EST