Antenna questions

Discussion in 'Wireless & RF Design' started by qbert55ca, Aug 19, 2008.

  1. qbert55ca

    Thread Starter Member

    Aug 19, 2008
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    Hi guys.

    Please don't jump on me to hard as I'm really out of my league in this forum but I need some answers or ideas that I have not been able to find on the WWW.

    I have an autopilot for my RC airplane. It allows me to add some waypoints to the autopilot and the plane, once in the air, will fly the given route. This auto pilot also has a 900mhz spread spectrum radio modem that allows the plane to comunicate with a base station and allows the user to see a moving map of where the plane is, altude, speed etc. Right now the distance is limited by the antenna.

    There is two parts wo what I'm going to ask. a simple yes and this is how or no it can't PERIOD would surfice for the first part.

    Part 1

    Can you take an omni antenna and a patch antenna and tie them together som how so that you get an all around short range signal and a longer directional signal.

    Part 2

    Could you take a patch antenna and using steeper motors or even servos ,to move the antenna, have that patch antenna follow the singnal from the plane.

    Thanks. I would appreciate any help on this I can get. I'm a absolute dim wit when it come to this type of stuff but I do follow instructions well.

    Robert
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    I'm not sure what you're talking about with a patch antenna, but what you're suggesting sounds doable enough. Every time I've ever tried RC aircraft the plane is doomed.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    You always have difficulties combining the feeds from multiple antenna because you have to get them in phase. You would need to do some experimental 'cut and try' work with compensating networks to achieve a contructive combination.


    Yes you can rotate the patch plate assembly independantly of an omni if you can overcome the mechanical construction. Did you want to mount this in the plane or the ground station. ?
     
  4. qbert55ca

    Thread Starter Member

    Aug 19, 2008
    11
    0
    Hi Bill

    I have two antennas. A "Rubber Duckie" 3dbi Omni Antenna and a "Roo 12dbi directional Patch antenna. The Roo is just a flat pannel antenna with a cover and pouch built onto it for enclosing the receiver and other electronics.

    (see pic)

    Robert
     
  5. qbert55ca

    Thread Starter Member

    Aug 19, 2008
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    If I were using the Patch antenna then I would not need the omni as the patch would follow the plane no mater where it went.

    Which of the two methods would be easiest, remember, I know very little about electronics.

    Robert
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    A single omnidirectional antenna would be the easiest. However, that would naturally give you the least range.

    The concept of driving the Roo Patch using stepper motor(s) or servos dependent upon signal strength is a rather advanced topic; something one wouldn't normally get into until they'd spent quite a few months delving deeply into electronics education. I don't have the time to teach you all of that.

    However, you can access updated versions of the materials I studied in years past, right here:
    http://www.tpub.com/content/neets/index.htm

    Start at the beginning. By the time you get up to volume 18, then you should have acquired the knowledge to appreciate and understand the material presented therein.

    It's a very good series, and you can't beat the price.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    I have seen many RC airplanes with omni-directional antennas and they never had a problem with range. They receive signals only, they don't transmit.

    Maybe your range problem is with the transmitter in the plane for the modem, or with its receiver on the ground.
     
  8. qbert55ca

    Thread Starter Member

    Aug 19, 2008
    11
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    This is not a 72mhz RC receiver problem. This is an auto pilot with a long range 900mhz 802.11G modem radio.

    Robert
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Gee Robert, I don't think I'd want something like that in a model aircraft.

    900MHz is in use by a LOT of items, mainly cordless phones and wireless routers. You're going to have a lot of competition on that frequency.

    A simple corner reflector will get you about 10dB of gain in one direction, but it's the aiming of it that will be the problem.

    Being on the same frequency as myriad other units will also make automatic antenna alignment much more problematic. If you were on a unique frequency, it would basically be a problem of monitoring your AGC level and manipulating the antenna until the AGC was maximized. Now, you'll have to differentiate between multiple transmitters on the same frequency. While it still may be do-able, you'll need something like a PIC with a TCP/IP protocol stack capable of interrogating your 802.11g to dig the proper signal strength (your A/C's) out of the melee.
     
  10. qbert55ca

    Thread Starter Member

    Aug 19, 2008
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    You are most likely 110% correct but I have neither the time nor attention span to learn all that just to figure out how to get a antenna to follow a signal. That's why I asked here.

    You would think that some electronic genius has come up with a simple way to get a an antenna to follow a signal. It's used to tune satellite dishes so it can't be that hard to do.

    Robert
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, the initial alignment needs to be performed manually.

    For a given spot on the globe, a satellite will have a relatively fixed azimuth and elevation. A technican gets that information, manually aligns the antenna while observing the signal strength, then clamps it in place and re-checks the signal level to ensure that it's within specifications.

    There are satellite antennas which can be steered around - but they had to be set up initially. Once the AZ/EL is established for a pair of satellites, it's just a math problem to determine the AZ/EL setting for any other satellite in the constellation.

    Satellites generally don't move around, unless they're not in a geosynchronous orbit. Australia and NZ have a few like that; they're too far south to use a geosynchronous orbit satellite. And there is no competition with other signal sources from those kinds of elevations at those frequencies (around 1.5GHz-4GHz)

    Your situation is really quite different. You have a moving A/C and a moving (portable) base station, with lots of interference from multiple trancievers on the same frequency. The problem you're facing has more in common with a tactical radar system in a fighter aircraft than it does with a satellite dish/receiver.
     
    Last edited: Aug 20, 2008
  12. qbert55ca

    Thread Starter Member

    Aug 19, 2008
    11
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    Just thinking!!! (ya, I know, I smell smoke too) During the start up procedure for the plane it sets it's originial coordinates so that in case of a lose of signal, low battery or other problem it turns towards home. Using these startup coordinates, place the antenna on a tripod beside the plane. Once the plane is in motion it starts sending gps and other data to the computer via the modem. Since the computer has the initial sart point and the planes new position a simple program should be able to run a program to follow the plane via the gps location. It's actually the reverse of what you said about the satalites. we find them because they don't move. we the antenna don't move except to swivel. As long as the antenna is pointed at the plane and sitting on or very near the spot the the plane set as it's origin then when you start flying the plane the antenna should follow the plane. (I think) :)

    Robert
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    The art of RC, it be achangin. Why not have the antenna on the joystick box, so when your facing the place the antenna is aimed that way too?
     
  14. qbert55ca

    Thread Starter Member

    Aug 19, 2008
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    That would work too, and simple, but if I could get it to do it iteslf:D:D:D.

    Robert
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Well, for such a contraption to work decently, the base station (your control box) would not only have to know it's GPS position, but also it's orientation (which direction it is facing) fairly accurately; within a few degrees.

    If the A/C were within a certain distance (say, 100 yards) you'd want to switch to an omnidirectional antenna, as otherwise the antenna may lag behind the A/C's current position.

    Then it would be a matter of interpolating the coordinates transmitted from the A/C with the coordinates of the base station to slave the antenna using servomotors or stepper motors.

    Sure would be easier to just have the antenna mounted on your control box :D
     
  16. Papabravo

    Expert

    Feb 24, 2006
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    Maybe someone has, but they're not sharing the knowledge. Shame on them! BAD Netizens.
     
  17. beenthere

    Retired Moderator

    Apr 20, 2004
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    Actually, it's been done. It takes a parabolic antenna with the interior divided into quadrants that feed separate leads to receivers. Differences in received signal strength show you which way to slew the antenna.

    It is kind of a 'price no object' solution. In addition to the custom made antenna and 4 receivers, you need a robust servo system driven by an amplifier that can successfully integrate the receivers' signals and produce steering commands.

    You might be able to skip the quad-divided parabolic dish with 4 Yagis in cells. Think about 5 or 6 directors for gain, and some way to adjust antenna output. Then you have to slew that mess in 2 dimensions as well.
     
  18. Papabravo

    Expert

    Feb 24, 2006
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    A good amateur station like the the one at W8UM can do the job of tracking satelites in low Earth orbit. It is true that the system was not cheap, but it does a helluva job.
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    Tuning a satellite dish to a peak is easy. There is only one at that frequency up there at that angle.

    You have millions of signals on your frequency unless you are on Mars.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    It can also be done using a single receiver by nutation of the parabolic reflector, or feedhorn nutation. Some early fire control radars used parabola nutation, but that led to obvious difficulties with gyroscopic precession, parabola deformation due to centrifugal force, etc.

    As long as the antenna control was synchronized with the current offset angle of the nutation and was constantly fed the instantaneous receiver signal level, it was relatively easy to keep the parabola focused on the target; simply move it in the direction of the strongest return.

    But, such a thing would be rather difficult for a hobbyist to put together, even if they were pretty well-versed in electronics - and our OP has stated that they have neither the experience nor the focus to acquire the knowlege necessary. So, it's rather pointless to persue such an avenue.
     
    Last edited: Aug 20, 2008
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