Re: NEC-LIST: Early History in using NEC to study Reradiation Problem

Duncan,

Thank you for your cheery note concerning matters of re-radiation. This is
a subject that has interested me for about two decades or so. I have
folders full of computations, and correspondence, papers yet to be written.
Several topics studied are:

1) HF dipoles are typically fed by a coaxial cable, with and without a
balun. Does this matter? When should a balun be used? Even with a balun,
currents can be induced to flow on the outer surface of the coaxial cable,
if for example the arms of the dipole are of different lengths (the
off-centre-fed dipole is a popular antenna with amateurs in radio); if the
arms of the dipole droop, and one arm droops more than the other; or more
typically if the coaxial cable comes off at an angle that is not
symmetrical wrt to the arms of the dipole. That is why I like the drooping
dipole, with a centre support mast. The coax generally comes off
symmetrical wrt the arms of the dipole. My article on this was intended to
be a follow on article, to follow my article on baluns (reference QST June
1991, pp. 30-33). But that was 6-years ago!!

2) The so called "back stay" antenna is typically used as an HF antenna on
sailboats. But the aluminum mast and the rigging are a part of the antenna
system. So why both with the back stay antenna? Why not excite the mast
or the rigging? I have a folder full of computational data, and letters
received from radio amateur sailors --- I do not have a sail boat.

3) Often there is more than one HF antenna on boats. Our Department of
Fisheries and Oceans have two HF antennas on their patrol boats --- a
standard marine whip, and an inverted-L slung from the mast. Frequently
when transmitting on one antenna, the antenna tuner connected to the other
antenna is fried. Also, when using automatic antenna system tuning units
(ASTUs), it is found that when "tuning" one antenna, the ASTU connected to
the other antenna comes to life, and tries to do its job at tuning as well.
I have a folder full of data on this.

I have not tackled antenna farms --- unless requested to do so, there is
probably little point, since results would be very site selective.

With the risk of boring people to death on this subject, I will post my
reply. If I happen to receive correspondence, this could help me/or
encourage me in writing one or more papers on these subject studies.

Regards,

Jack, VE2CV

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Hi there Jack.

I think your note is most timely and contains many useful references. This
is an aspect which has long intrested me because of the 'hidden' problems
at large antenna farms (HF). The radiation patterns of all of the
antennas are computd/assumed for for level ground (often perfect), and
perfect conductors. For some time now I have unsuccessfully been trying
to get a contract for a small study related to this area.

You will also have seen an earlier file generated after the theft of the
copper antenna wires at Grahamstown's ionospheric station. One of the
options was zinc galvanized or stainless steel wires. I ruled out the
galvanizing on the basis of skin depth I seem to recall (must do the
calculation again), and then I got some very surprising inputs for the
relative magnetic permeability of stainles. This also told me to be very
cautious with the type of stainless one used. Anyway, that is all by the
by, but the stainless was also raised in terms of refurbishing a large
antenna farm near the coast. This is where I am angling for a contract as
per above to study the reradiation problem.

There have been some articles on the impact of re-radiation from things
like coaxial cables on the modern SSL HFDF system - already well-known for
Adcock type systems.

Well, that's it for now. My editor seems to have quit working on deleting
lines so I will also do the same.

By the way, funding here is so doggone tight, I doubt I'll make ACES next
year unless I happen to be across in the US on some IEEE funded business.

Keep up the good work. Best regards, Duncan.

*********************************************************************
PROF. DUNCAN C. BAKER, DEPT. OF ELECTRICAL AND ELECTRONIC ENGINEERING,
UNIVERSITY OF PRETORIA, PRETORIA, 0002, SOUTH AFRICA.

EDITOR-IN-CHIEF, ACES JOURNAL.

PHONE +27 12 420 2775 (OFFICE), +27 12 47 7480 (HOME)
FAX +27 12 43 3254 (OFFICE), +27 12 348 5314 (HOME)
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Received on Fri May 16 1997 - 10:47:19 EDT