"Rich Grise" wrote in message
news
I was doing a thought experiment with this and
when it's completely "flattened out", (180 degrees between the booms),
it looks like a drum roll please Bow Tie!

I think a bow tie's design is actually motivated more by one of the "standard"
ultra-wideband antenna designs, that of a pair of opposing cones touching each
other at their narrow ends, being translated down into 2D.

"Joel Koltner" wrote in message
...
I think a bow tie's design is actually motivated more by one of the
"standard" ultra-wideband antenna designs, that of a pair of opposing
cones touching each other at their narrow ends, being translated down into
2D.

Standard, indeed; this antenna is known as a biconical. It has excellent
wideband response making it ideal for EMC testing. Most good antenna texts
such as that by Krauss give a detailed analysis of the biconical antenna. I
see no relationship between its design and a fractal design other than both
are wide bandwidth antennas.

The bowtie antenna which Rich and others have mentioned is a "flattened"
form of the biconical. If the bowtie is bent along its major axis, it makes
an excellent wideband driven element for a corner reflector antenna.

73, Barry WA4VZQ

Thanks for the details, Barry... tell me though, then, is a discone just a
biconical with a ground plane used to create the (image of the) missing cone?

"Joel Koltner" wrote in message
...
Thanks for the details, Barry... tell me though, then, is a discone just
a biconical with a ground plane used to create the (image of the) missing
cone?

Yes. It will have less gain than a biconical and the bandwidth is slightly
more restrictive too. But it is much easier to construct!

73, Barry WA4VZQ

"NoSPAM" wrote in message ...

"Joel Koltner" wrote in message
...
Thanks for the details, Barry... tell me though, then, is a discone just
a biconical with a ground plane used to create the (image of the) missing
cone?

Yes. It will have less gain than a biconical and the bandwidth is
slightly more restrictive too. But it is much easier to construct!

73, Barry WA4VZQ


All else being equal, a lossless 'ground plane' type antenna, be it a
monopole developed from a dipole or a discone developed from a biconical
dipole, over an infinite ground plane should exhibit 3 dB _more_ gain than
the symmetrical 'parent' form of antenna. This is because its radiation
pattern is limited to half the solid angle of the parent (e.g. only the
space above the ground plane). Then for a given number of watts fed into
the antenna, the power-flux density must be greater in the region where it
can radiate. In practice, the ground plane isn't infinite so there is some
'undercutting' of the vertical radiation pattern, but the gain should still
be somewhat greater.

The apex angle of the cone in a discone can be chosen to yield 50 ohms
terminal resistance over part of its usable bandwidth, and that angle is
different from the apex angle(s) needed in a biconical dipole for the same
impedance. The usable bandwidth is always limited by the limited
flare-length of the cone(s) and usually also by the accuracy of the apex or
apices where termination is made.

Chris

On Fri, 21 Nov 2008 16:26:27 -0000, "christofire"
wrote:

The apex angle of the cone in a discone can be chosen to yield 50 ohms
terminal resistance over part of its usable bandwidth, and that angle is
different from the apex angle(s) needed in a biconical dipole for the same
impedance. The usable bandwidth is always limited by the limited
flare-length of the cone(s) and usually also by the accuracy of the apex or
apices where termination is made.

Hi Chris,

This needs heavy qualification, and probably too much such that a
graphical treatment would outweigh the words for contribution:
http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm

73's
Richard Clark, KB7QHC

christofire wrote:

All else being equal, a lossless 'ground plane' type antenna, be it a
monopole developed from a dipole or a discone developed from a biconical
dipole, over an infinite ground plane should exhibit 3 dB _more_ gain than
the symmetrical 'parent' form of antenna. This is because its radiation
pattern is limited to half the solid angle of the parent (e.g. only the
space above the ground plane). Then for a given number of watts fed into
the antenna, the power-flux density must be greater in the region where it
can radiate. In practice, the ground plane isn't infinite so there is some
'undercutting' of the vertical radiation pattern, but the gain should still
be somewhat greater.
. . .

This is entirely a fictional scenario, although it's the one used by
virtually all the currently available modeling programs. In those
programs you can choose "free space" or "ground plane", where the
infinite "ground plane" restricts the field, as Chris says, to a single
hemisphere while "free space" allows radiation in both hemispheres.

In real life, you can't have either one, except that outer space would
be a reasonable approximation of "free space". The only thing that
matters is whether the field reflects from a large surface like the
Earth on its way to the receiver. If it does, you potentially pick up
field strength from reinforcement of the direct and reflected rays --
but of course you can also lose field strength if the two rays cancel
rather than reinforcing. This is another way of describing the same
phenomenon of increased gain due to a ground plane. Even if you put an
antenna hundreds of wavelengths high, some of the signal will reflect
from the ground -- it's not truly in "free space". It doesn't matter
whether your antenna is a "ground plane" or a dipole -- if a reflection
occurs between the transmitter and receiver, you potentially get that
extra gain; if it doesn't, you don't.

Roy Lewallen, W7EL