>|> >Can anyone give me advice on antennas. What are the advantages and
>|> >disadvantages of the different types. Can I build my own? Any
>|> >problems putting the antenna in my attic?
>|> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^This sounds like a job for:
The discone is a good choice for a scanner antenna.
If you're going to put the antenna in the attic,
you don't need the strength and foul-weather
resistance of a commercial discone. The fact that
a discone is a very broadband antenna implies it
should be easy to build one that works well.
The discone is so named because it combines a disk
with a cone (clever, eh?) Scanner-type discone
antennas have disks made of rod stock so ice and
snow don't build up as easily, but you can use solid
aluminum, copper or copperclad PC-board material
instead. All dimensions refer to the _lowest_ frequency
Wavelength = 984/Freq (MHz)
Make the disk diameter about 0.7 wavelength.
The length of the cone side and its maximum diameter
(L) are about 0.25 wavelength. Make the disk diameter
The coax shield goes to the skirt and the center
conductor goes to the disk. The disk and skirt
should be separated by about an inch, but this
dimension, like the others, is not critical.
Spacing between the rods at the bottom should be
about 0.02 wavelength at the lowest frequency.
A discone is vertically polarized. Some commericial
units I've seen have staggered disk-rod and cone-rod
lengths, which is perhaps how they achieve the
extreme bandwidth they quote. You could make the
whole antenna out of 12-ga solid wire. Use a
lower frequency of 150 MHz for the short rods and
30 or 50 MHz for the long ones. The 30-MHz rods
will be about 7.5 feet long, but if your attic is
high enough, why not? Just hang the antenna from
a***hook. As I said, the dimensions are not
critical. Typical frequency range is 4:1, so you
will have to stagger the rod lengths to get the
over-10:1 coverage of the commercial antennas.
The discone has a low angle of radiation; the
higher you can install it the better. I can
envision one outside, covered with a plastic
garbage can for protection from the elements!
This information is from the _ARRL Antenna Book_.
Any _Antenna Book_ published in the last 20 years
has this info; the discone has been around for at
least 25 years!
Here are the dimensions for a 50-225 MHz discone; you can
extrapolate dimensions for higher bands. To cover 50-1300
MHz, I'd use these dimensions for half the rods, and cut
another set for 250 MHz (just divide these dimensions by
5). That should work fine.
Disk Diameter 3.44'
Cone Length/Dia 4.92'