A ground plane has 5 to 8dB more gain than a Discone? Where do these
numbers come from? If you look at the gains and plots from any
commercial/military Discone you will see it is about on par with a 1/4
wave groundplane across most of the Discone bandwidth. I don’t mean
inflated scanner Discone specs, look at commercial companies that
reference to isotropic or 1/2 wave dipole for measurements. At the upper
end of the Discone’s range the pattern can get a bit squirrelly, but
then a ground plane with only 3 or 4 radials does not put all of it’s
energy at the horizon either, the mail lobe is at least several deg
higher than the horizon. A “scanner” type Discone will perform within 1
or 2dB of a 1/4 wave groundplane on the VHF and UHF bands, which is the
lower end of its useable range. What does this really mean? You will
probably not notice any difference between a good quality Discone and a
ground plane cut to frequency on the VHF or UHF bands. 800 MHz would
benefit from a gain type antenna if your in a fringe area.
Mike


Colic wrote:
wrote in message
oups.com...

I keep reading discones cover a very wide frequency. Are these truly
the best scanner antenna to have? Or would a ground plane tuned to the
freqency you are listening to be better?



If you have one specific frequency, or a narrow frequency band of less than
about 10%, then a properly designed and built ground plane antenna will have
significantly more gain. It would be easy to get 5 to 8 more dB of signal
from the ground plane over the discone. It is possible to get 10 to 12 dB
or more from the ground plane (over the discone only, I am not saying 10-12
dB gain over an isotropic), but this starts to narrow up the bandwidth below
the above mentioned 10%.

What is 5 to 12 dB more signal? Every 3 dB is double the signal strength.
A gain of 12 dB would be a signal about 17 times the signal of the discone.

What does 10% of bandwidth mean? If you design a ground plane for a center
frequency of about 150 MHz it will work across about a 10% bandwidth (this
is a rule of thumb, but not hard and fast). 10% means the antenna will have
a 15 MHz band width of optimum performance. Or about 142 to 157 MHz
(rounded off). The antenna will work outside this range, naturally, but the
gain will fall off rapidly.

For broad band applications it is very hard to beat the discone antenna.
This is why the discone is so popular with the military and countermeasures
community. The discone is not best at any one thing, but it takes the place
of a multitude of other antennas and does it reasonably well.

C