On Thu, 03 Apr 2008 10:41:08 -0600, msg <msg@_cybertheque.org_> wrote:

>Adam Chapman wrote:
>
>> A helical antenna looks the best as it is the most isotropic,
>> although it is also pretty big for our aircraft. However even that has
>> +-45 degree 3dB lobe, so i might have to have the aircraft bank more
>> when closer to the centre of its circular mission area.
>
>How about taking a lesson from radiosonde antennas; I don't know about
>current models, but for a very long time the antenna at 1680 MHz was
>a 1/4 wave stub mounted at the center of an inverted conical reflector
>(about 60 degrees). When suspended from the balloon, the antenna points
>downward and illuminates the ground with a pattern formed from that
>of the stub combined with the reflector, giving horizon-wide coverage
>with no nulls for high inclination either.

Radiosondes have two basic assumptions. The transmitter is always
above the receiver and that antenna is always roughly vertically
oriented. Those assumptions don't work well with an RC airplane,
which can easily be inverted, at odd angles, and at the same elevation
as the controller. Incidentally, the usual RC screwup is losing
control on landing, where the ground (small hills, cars, buildings)
block the signal and the radio link loses control.

>Also, you could mount two such antennas on the fuselage, at 180 degrees
>top and bottom and use a splitter to drive them so that there is
>coverage during maneuvers.

Yep. That's what I'm thinking. Two cardioid patch antennas make a
fair hemispherical pattern. The receive sensitivity won't be affected
much by the splitter, but the xmit power will be equally divided
between the two antennas. Link calcs in each direction will need to
be independently calculated.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558