Folded Monopoles

Barry,

This is in reply to your correspondence dated 7 February.

Folded monopoles (dipoles) can be arranged as you have described as a
"coaxial" type or a two conductor type. An easy to follow brief
description of folded unipoles, but including equations and a graph
(computed by NEC-3) and with reference to more detailed analysis is given
in my chapter on "VLF, LF and MF antennas", in The Handbook of Antenna
Design, Volume 2 (editors Rudge, Milne, Olver and Knight), published by the
IEE, London, 1983, pp. 604 - 606.

As others have noted this antenna has two characteristic modes, a
symmetrical mode characterized by equal (radiating) currents in the two
conductors; and an asymmetrical mode characterized by equal and oppossite
currents (transmission line mode) in the two conductors.

If the antenna is made out of 300-ohm ribbon, or coaxial cable, there is a
significant difference between the electrical lengths for the transmission
line mode (characterized by velocity of propagation) and the radiating mode
(characterized by an antenna factor). Certainly the short at the end of
the monopole should be determined by the velocity of propagation factor for
the transmissionn line mode down the two conductors, and since v (as a
length factor) is always less than k the antenna factor, the short is in
from the end of the monopole.

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 Tue Feb 13 1996 - 16:45:00 EST