Al Klein schrieb:

On Sat, 30 Oct 2004 11:55:50 +0200, Volker Tonn
said in alt.radio.scanner:


Al Klein schrieb:

On Fri, 29 Oct 2004 22:45:21 +1000, "Troy R."
said in alt.radio.scanner:


They don't seem to know much about it. Discones aren't resonant
antennas, but they do pick up signals from the design frequency to
daylight..


Not having read the linked page....
Discones ARE resonant on an extremly wide range.
Therefore they have nearly no gain above the (theortical) isotropic
antenna. This means they have a loss of around 1/2 S-unit against a
dipole or a half-wave vertikal antenna on a specified frequency. Not too
bad for a compromise.
The resonant range of a well designed and built discone is spread 1:10
on wich you can transmit. This is 50 to 500 Mhz or 100 to 1000Mhz for
example, covering the most wanted frequencies for scannists and is also
usable on ham-bands or FRS to transmit.


We're using different definitions of "resonant". The definition I use
says that an antenna can only resonate on a frequency. (Pretty much,
the frequency a dip meter will indicate when coupled to the antenna.)
It can have a bandwidth over which it can be used, but the resonant
point is (usually) one single frequency. Discones look like dipoles
with the 2 legs being different lengths, so the best one could say -
using my definition - is that they "resonate" on the frequency at
which the disc is 1/2 wavelength in diameter and the frequency at
which the cone element length is 1/4 wavelength.

To say so, a discone has it's "middle frequency". But impedance curve
stays constantly over a very wide range around. So in theoretics you can
get a frequency modulated signal with center frequency of let's say
500Mhz and a bandwidth of 800Mhz through a discone with a straight power
curve wich has no loss by mismatching of the antennea.
I don't know of any TRX providing such a signal :-)

As I understand the principle of a discone the disc is a load plate wich
spreads the bandwidth in conjunction wich the conical "antenna wire". So
the "resonant" wavelength is more related to the length of the cone
measured from the the top plate in 90 degree angle to the bottom.
You may correct me if I'm wrong :-)
Have a nice weekend.