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I am currently working on a student project that involves having a UAV fly an orthogonal path over a series of transmitters -- taking in data (through an arduino-based system for data processing).

The problem is -- I can't seem to decide on the right antenna(s, if more are required) in order to efficiently receive these signals (from these various trackers and transmitters in the canopy). A suggestion was to use a series of omni-directional whip aerial antenna linked to a single receiver. Most of the recording would be done on board. The drone should theoretically fly a .5 grid over the canopy area, sweeping for signal strength throughout the channels (operating in the 142 - 150 MHz range).

Unfortunately, I am a bit directionless in the area of hardware -- what receiver and corresponding antenna would be optimum for logging signal strength over a forested area. I've looked into smaller Yaesu receivers, but I am a bit clueless in regards to what antenna attachment to look into -- perhaps a series of omnidirectional whip antennas linked to a receiver. An alternative was also taking a Baofeng radio and hacking the thing. The bird itself would be reasonably high above the canopy, but I'm told that the rule of thumb is basically to be able to sweep the area as far as possible while maintaining a good gauge of signal strength from each transmitter beneath the canopy.

Would any of you be able to point me in the right direction in terms of hardware? I work mostly on the software end, so this is a rather new horizon for me.

Thanks!

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  • $\begingroup$ For good advice, you need to tell us about the transmitters as well! $\endgroup$ – AndrejaKo Jul 1 '15 at 18:57
  • $\begingroup$ Yes; in particular, can you control what they transmit, or are they a specific type (e.g. animal trackers) that your receiver needs to be compatible with? $\endgroup$ – Kevin Reid AG6YO Jul 1 '15 at 20:48
  • $\begingroup$ The transmitter sends a signal in .01 second pulses spaced about 1.5 seconds apart. This transmitter signal varies from a frequency of 142 to 150 megahertz with steps of .01 megahertz. The transmitter pulse current is 4.2 milliamps. They are animal trackers. $\endgroup$ – Richard Long Jul 1 '15 at 20:49
  • $\begingroup$ @Richard Long What is actually useful information for us is the output power of the transmitters and the antenna gain. If you would give us voltage at which the transmitters are operating, we might be able to somehow get a very rough idea of output power. Also, information about transmit antenna shape and size would be helpful in order to estimate the gain of the antenna. $\endgroup$ – AndrejaKo Jul 3 '15 at 11:08
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First of all, the antenna on the aircraft needs to have a good radiation pattern downward. This rules out a whip, though a horizontal dipole may work.

As the polarisation of the source is unknown, for most reliable detection you need a circularly polarised antenna.

This could be a pair of dipoles fed 90 degrees out of phase, called a turnstile antenna.

It could also be a pair of loops, which have the advantage of a wider radiation pattern. The two loops fed 90 degrees out of phase are called an eggbeater antenna. It will be about 50 cm in diameter.

The frequency range you list is quite broad,you'll struggle to cover it in one go. Surely you know what the transmitters are set to? A standard two-element yagi for animal tracking will be a very narrow band antenna, just a few MHz, so no-one would set the collar transmitters all over the place.

The last antenna you might consider would be a quadrafilar helix, as used for GPS and iridium phones, and many satcom applications. A QHA for your frequencies might be 0.6 m tall, 0.3 m diameter.

Both the quadrafilar helix and the eggbeater are commonly constructed by amateurs for receiving weather satellites, in a very similar frequency band, so you might find good information on building them.

One last idea - it might be lighter and lower wind resistance to just use two receivers, connected to two dipole antennas, one in each polarisation, and combine their detection results in software later.

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  • $\begingroup$ There's one thing I don't understand here: Why would a properly attached whip be any worse than a dipole? If we could attach a whip to the bottom of an airplane, wouldn't its whole radiation pattern be pointed downwards, at our targets? If the UAV is helicopter-shaped, then this might be an even smaller issue. $\endgroup$ – AndrejaKo Jul 3 '15 at 11:16
  • $\begingroup$ The patterns of a whip are all around, broadside to the whip, and not off the end. So you'd need to mount it horizontally. What would it use as a ground plane? Then, if it is (say) oriented along the line of flight, what if the transmitter is perpendicular to that. You will suffer a 10 dB loss of signal, or more. $\endgroup$ – tomnexus Jul 3 '15 at 11:32

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