Re: Finding offset using AM
- From: "john" <j.alexander.hawkins@xxxxxxxxx>
- Date: 25 Jun 2006 19:06:36 -0700
erik.axdahl@xxxxxxxxx wrote:
I'm thrilled that so many ideas have been put forward! I suppose IHi
should have mentioned that this design will be mounted on a small UAV,
so weight is definitely a consideration at my end--I can't just pick up
a box and throw it in the cargo hold!
I've modified my design somewhat from my original posting, it's as
followed:
The transmitting craft has two transmitters operating at two different
FM-band frequencies (say 88MHz and 100MHz). The wing of the aircraft
has a directional, rearward-facing antenna on one wingtip transmitting
one signal, with the other antenna on the other wingtip broadcasting
the other signal.
The receiving aircraft has two forward-facing antennas in a mirror
configuration. When the two signals are of equal strength, the
aircraft knows it's lined up.
I have two main concerns of this design, however. For example, how
will the transmitting craft know to which side it's drifting? Also, I
realize I will need to take great care in designing a (light, small)
Yagi antenna of some 1/2^n size to the wavelengths I'm operating on.
Any thoughts?
I dont think your design is going to work. There is no way to descern
left from right with your current configuration, and theres no
modulation trick (i can think of) that will get you a left or right.
Im also worried about the distance your aircraft are following each
other.... 4ft or 1.29m (for the frequency your talking about) The
radiation characteristics of an antenna are much different in the near
field, (less power transfered to load among other effects), vs. the far
field. I dont know what the radiation pattern of the antenna is that
close. but at the frequency your talking about, (for a wavelength of
3m) its usual practice to have
(2*pi*r)/wavelength >>1, r being the distance between the
leading aircraft and the one you want to control, to insure your in
the far field, (frauhnhoffer) region of the radiator.... for your
frequency(s) that would be r>>0.478m or 1.567ft. As well the paracitic
elements, (director and reflector's) of yagi-uda have to be placed at
significant fractions of a wavelength to work right. i still think
your frequency is to low.
furthermore the radiation pattern of a yagi uda antenna is symetric
about the axis of maximum power transmited (or recieved,) so you wont
get a left or right signal with your current design.
in summary i think you need to go up in frequnecy, and drift farther
away from the AM thing, and more twords relying on the radiation
pattern of the antenna to determine your error signal. here's an
idea...
plane 1 TX (at centerline of
aircraft one, directional)
( with lobe
maximum pointed directly back)
4ft or so ? RX RX
on each wingtip, both RX antennas are directional (with no sidelobes)
directed straight in front. you could then get a left or right error
signal as follows.
RX (left) RX (right) are calibrated to match their recived
signal strength's to the centerline of airplane one. then you should
have decent dropoff in the recieved signal strength of say RX (left)
while RX (right) goes up. then your following aircraft is drifting
right. etc...
if you work at some big evil military industrial complex type place,
you should be able to get some big evil RF/Antenna guy to help you...
if no one there is evil enough to assist you in RF stuff where you
work.....im looking for a job ;)
p.s. concerning radiators... you might want to take a look at a MPA
array, patch antennas are probably really good for you as they are low
profile (some are even conformable) so theres not the aerodynamics
problems, light weight etc.. the directivity of one isnt good enough
for the solution proposed above, so you will need an array. there are
other microwave antennas you might consider as well.
good luck,
john
.
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