On 12/5/2020 9:11 PM, Matt Herron Jr. wrote:
I have a carbon fuselage, and am considering experimenting with a monopole antenna on each side of the canopy. What I am hoping for is the thinnest wire that will work, taped to the inner canopy wall with clear tape on each side. Hopefully this would provide diversity without FOV obstruction. Can any RF engineer out there suggest length of wire, minimum dia. of wire, etc. that might work? Advice on avoiding nodal interference patterns welcome as well.

Thanks,

Matt

I wonder why you're saying "monopole", Matt? An antenna, like a magnet,
needs two poles. The monopole antennas you may have seen sticking out
of metal airplanes use the metal fuselage as the other "pole". You can
achieve the same using several wires in a plane perpendicular to the
"monopole". Those are called ground radials. If the surface you want
to mount it on is made of carbon fiber, which conducts but not as well
as metal, you may want to add a patch of sheet metal (even aluminum foil
may do), or several ground radial wires, flat on the carbon surface and
connected to the ground side of the cable where its internal wire
connects to the "monopole". If it's flat against the canopy then you
can't have the ground plane extending out through the plexiglass, so
it'll be a compromise, but may work well enough.

As far as dimensions, that ground patch should be at least a
half-wavelength across, and the "monopole" should be 1/4 wavelength tall
(minus about 5%). So for 915 MHz (USA FLARM frequency) you'd compute it
as 0.95*300/915/4 = 0.078 meters, i.e. 7.8 cm, or 3.1 inches. It's OK
if the wire is thin, but a thicker wire would work somewhat better
(wider bandwidth - the signals are in a range, 905-925 MHz or so).
There would be no significant interference with antennas on opposite
sides of the canopy (unless they are way forward where the canopy sides
almost meet).

If you can get the bottom end far enough (say 3+ inches) above any
carbon structure, you can use a dipole instead. Those are available off
the shelf complete with cable and connector. Easier than trying to
build a decent connection yourself at those ultra-high frequencies.
This isn't like my shortwave ham radio days... Be sure to route the
cable away from the dipole in a direction perpendicular to the dipole,
for the first few inches. One good dipole may work better than two
compromised monopoles.

There may be monopole+groundplane antennas available off the shelf too,
look around. The fact that we share this frequency with many other uses
makes for better availability of parts.