[time-nuts] Toy radiolocation and LORAN envelope
brooke at pacific.net
Tue May 27 17:14:24 UTC 2014
There are a number of object location schemes. I got interested after seeing the "blue dot" in the movie "The Da Vinci
It is a totally fictional device.
A lot of new products like a "blue dot" are coming on the market based on Bluetooth and the use of smart phones.
But there are many other methods used for things like pets, see:
Note the various modes of operation.
The bandwidth of low frequency antennas is very narrow (high Q) since there is a direct tradeoff between bandwidth and
See the Wiki page for Chu–Harrington limit: http://en.wikipedia.org/wiki/Chu%E2%80%93Harrington_limit
At 100 kHz a wavelength is 3,000 meters.
So the waveform needs to have minimal bandwidth.
Andrea Baldoni wrote:
> Years ago, I sometimes played the transmitter hunting game, probably known to
> most of the members of this list.
> A friend of mine recently suffered a theft so I thought about the opportunity
> to embed little marker transmitters in some object usually left in the yard
> (like bicycles for example), and have inside the house a system that
> constantly monitors them for unwanted movement, while at the same time be
> eventually able to recover them once "disappeared" using portable equipment.
> I think that the very simple RF power way to monitor movement is inadequate,
> but at the same time I would like to keep things very simple, little and really
> cheap, so no things like gps receivers on it: maybe just a 8 pin
> microcontroller and a crystal.
> Also, the battery life should be the maximum possible, so the RF power will be
> little and this also exclude to put a transponder inside the gadget, unless
> its on-time could be limited to short and precise slots, because a
> constantly-on RX will consume more power than a low duty cycle (say, 0.1pps)
> pulsed TX.
> Obviously, the goal is to reach the best positional accuracy possible within
> the constraints.
> The frequency is yet to choose, but I think it should stay in the ISM band
> around 40MHz.
> The angle could be found with a directional antenna or inerferometry, but about
> the distance? There is the need of a very good accuracy of the transmitter
> pulses. It's obtainable at least in the short term (the time needed to
> recover the object, for instance) for low power and low price? The long term
> inaccuracy could tracked and offset when the system is aware that the target
> is not moving.
> By the way, the LORAN envelope, was so shaped just to limit harmonics, push
> up energy efficiency, or there were other reasons not to transmit a square one?
> Best regards,
> Andrea Baldoni
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