A Microwave Discone Antenna ...
This page describes a microwave discone antenna that is easy to construct and
has a good response from 1GHz to 4GHz. The origninal source for this design
is unknown so I am unable to give credit where credit is due.
The basic concept of a discone antenna has been documented in many ARRL antenna publications.
This design was scales from an antenna that covered 50 mHz to over 1200 MHz and simply
illustrates a method of construction.
One of my friend's, Mike KD0FW, has used this design to copy the 1.68 GHz signal from
weather balloons while in his truck chasing them. He was the reason I tracked this data down
on the internet. The drawing shown on this page are recreated my myself.
Figure 1 shows what the antenna will look like when completed. The antenna is basically
a funnel turned upside down, with the bottom spout of the funnel removed. A top
hat or disc is electrically connected to the center of the coax cable.
In this case, I used 0.141 OD 50 ohm hardline, quite common in microwave circuits.
FIGURE 1 • Completed Antenna
The apex part of the funnel has a coax cable protruding just
3/32" above the small end of the cone. Figure 3 shows the details of the apex of the cone.
It is the detail of this junction that determines the upper usable frequency of the
discone. The portion of the 3/32" is all insulator, seperating the funnel cone
from the top hat. The center conductor is about 1/4" longer, sticking up above this 3/32"
of insulation.
Mount the top hat horizontally to this center conductor, to the top of the cone and
solder it. directly to the center conductor of the coax cable. The 3/32" space serves
as an insulator and isolates the top disc from the cone.
FIGURE 2 • Cross-section of Antenna
Figure 2 shows ann the important dimensions. The diameter of the top hat and the
base of the cone determin the lower usable frequency while the acuracy of the apex
set is upper usable frequency. The angle of the cone should remain the same if you
are going to scale this antenna to other frequencies.
FIGURE 3 • Apex Details
Please note that the dimensions of the discone are not too critical and there is room
for error. Our antenna was cut out of flashing copper about 0.020" thick and it bends
(forms) quite easily. It alos solders with small wattage irons. Most of the 0.141 solid
sheath comes with Teflon insulation making it somewhat impervious to heat during
construction.
In actual use, the antenna and amplifier functioned well, although no actual measurements
were made on antenna parameteres. The antenna and amplifier have been used in spectrum
analyzer applications to observe low level products from workbench construction projects.
They have even detected some of the microwave ovens in the neighborhood. These ovens operate
on the low 2.2 gHz part of the spectrum. In any case, this conversion should give you good
sensitivity over quite a range of frequencies.
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