On 4 Nov 2003 11:47:55 -0800, (SpamLover) wrote:

Uh? AFAIK, a cage is not a folded monopole.

The "inside" vertical conductor would be shielded by the cage itself
('cept for its very bottom tip, which is grounded) and would not
radiate ('cept locally, within the cage, hence autocanceled). Apart
from discones I also remember seeing multi-kW 1940's TV dipole arrays
in which arms were huge, bulbous surfaces without slots.

Hi Filippo,

A cage can be a folded monopole (or folded dipole). Has nothing to do
with radiation as this is simply a transformer (not BalUN but
conventional transformer) and shielding the primary or secondary
(depending on which side is driven) does not interfere with either the
transformer action, nor radiation. The trick is to not short out the
skirt to the interior wire (if it exists - and it should to make
sense) at the driven end.

You can also construct a folded mono/dipole by segregating one wire
from the remaining bunch at the driven end (if there is no interior
wire). One of the virtues of a folded mono/dipole is the increase in
apparent radiator thickness causing a wider match range - a cage is
conceptually the same, but typically more extreme in its attempt to
create a thicker radiator.

The discone is another broad band radiator whose characteristics
follow this conceptual sense of building a thicker radiator. Consult:
http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm
to observe how the flare of the skirt affects matching over frequency.

73's
Richard Clark, KB7QHC