Toy radiolocation and LORAN envelope
Hello!
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
Hello Andrea,
The angle could be found with a directional antenna or inerferometry, but about
the distance?
About the only reliable way to locate hidden transmitters is by field
strength. The problem in built-up areas is reflections.
At one time Doppler DF systems were popular, but anybody who has used
one will tell you they are usually more trouble than they are worth.
A simple beam or dipole is useful, but only when out in the open country
By simply gridding the area, and drawing a field strength map, you will
soon cut the location down to a small area. Then with a field strength
meter you can walk right up to it.
When doing professional DF work, I would use a scanner with an external
S-meter and an effective attenuator in the antenna lead.
Regards........... Zim
At 06:40 AM 5/27/2014, Graeme Zimmer wrote:
A simple beam or dipole is useful, but only when out in the open country
I have seen some of the equipment used by FCC field agents when DFing deliberate interference to aircraft communications. They used three transportable fixed mountaintop installations ringing the area of interest that measured field strength and also recorded transmitter spectra for later identification of the suspect transmitter, and three mobile equipment sets employing directional antennas used in a small-city urban area.
Dale H. Cook, GR / HP Collector, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcity/radios/index.html
Hi Andrea:
There are a number of object location schemes. I got interested after seeing the "blue dot" in the movie "The Da Vinci
Code".
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:
http://www.prc68.com/I/ObjectLocationTags.html
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
antenna size.
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.
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
Andrea Baldoni wrote:
Hello!
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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and follow the instructions there.
You don't need much power because the device in the toy only needs to
transmit for a few milliseconds then shut off. Some transmuter you
have pings the device in the toy that acts like a transponder. It
only transmits in response to a "ping". So limey it can be powered
by a button battery. How to power the receiver? RFID tags are
devices that are powered by rectifying the RF energy from the antenna
. Crystal sets work this way also. as for logic, the TI MSP430 can
run on nano amps of current. and sleep modes use pico amps. So it
could be powered by a capacitor that is charge from some powerful AM
broadcast station.
It turns out that "energy harvesting" (getting power from the
environment) is a hot tops right now. The simplest method would be
the solar powered calculator. That tiny little cell runs a uP and an
LCD screen. The old mechanical self winding watch was another example
of harvesting energy. You could run the receiver and logic on
harvested energy and then use the coid cell battery for a transmitter.
Your locator device would measure time of flight to the toy and its
own location via GPS. As you move the locator it gathers data used
for a solution. All the "smarts" is on the hand held locator that
has a high power transmitter, sensitive receive, GPS and a computer.
The device in the toy is a simple transponder.
In "monitor mode" the locator would request a micro-power ping (not
using the coin battery, just the capacitor. from the toy and use a
large non-mobile antenna and would alarm if the time of flight
changed.
You'd want some frequency that would go through walls and trees and in
some band that is legal for such stuff.
...
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,
--
Chris Albertson
Redondo Beach, California
Hi Chris:
The trouble with a transponder is the receiver is a power hog.
One way to mitigate that is to have the receiver operate on a low duty cycle and use a transmit signal that's longer
than the receive period.
The military PRC-68 squad radio used this method and the squelch opened so fast you would not know it worked that way.
http://www.prc68.com/I/PRC68.shtml
PS that radio is the basis of my web page URL.
In "Leash" mode the location tag transmits a pulse (has no receive function) with a low duty cycle and the smart phone
looks for some number of missing pulses in order to sound an alarm.
This is one way the Bluetooth tags work. The smart phone stores it's location every time it receives a ping from the
tag, not all of them just the last known location, so that when the tag goes missing you know it's last location. Some
proprietary smart phone software allows other smart phones to report hearing a tag that's been reported as stolen.
Another mode is "Find or Locate", i.e. where are my keys? This requires the tag to act as a Bluetooth receiver and beep
when queried. This may be the most popular Bluetooth tag mode.
http://www.prc68.com/I/ObjectLocationTags.html
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
Chris Albertson wrote:
You don't need much power because the device in the toy only needs to
transmit for a few milliseconds then shut off. Some transmuter you
have pings the device in the toy that acts like a transponder. It
only transmits in response to a "ping". So limey it can be powered
by a button battery. How to power the receiver? RFID tags are
devices that are powered by rectifying the RF energy from the antenna
. Crystal sets work this way also. as for logic, the TI MSP430 can
run on nano amps of current. and sleep modes use pico amps. So it
could be powered by a capacitor that is charge from some powerful AM
broadcast station.
It turns out that "energy harvesting" (getting power from the
environment) is a hot tops right now. The simplest method would be
the solar powered calculator. That tiny little cell runs a uP and an
LCD screen. The old mechanical self winding watch was another example
of harvesting energy. You could run the receiver and logic on
harvested energy and then use the coid cell battery for a transmitter.
Your locator device would measure time of flight to the toy and its
own location via GPS. As you move the locator it gathers data used
for a solution. All the "smarts" is on the hand held locator that
has a high power transmitter, sensitive receive, GPS and a computer.
The device in the toy is a simple transponder.
In "monitor mode" the locator would request a micro-power ping (not
using the coin battery, just the capacitor. from the toy and use a
large non-mobile antenna and would alarm if the time of flight
changed.
You'd want some frequency that would go through walls and trees and in
some band that is legal for such stuff.
...
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,
On Tue, May 27, 2014 at 11:32 AM, Brooke Clarke brooke@pacific.net wrote:
Hi Chris:
The trouble with a transponder is the receiver is a power hog.
Why does a receiver have to be a power hog? I remember building a
receiver once that did not use any power other then the energy coming
in from the antenna. RFID chips work like that, with no power. I
think you could build a receiver that uses almost no power.
--
Chris Albertson
Redondo Beach, California