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'[EE]:Directional transmitter'
2001\02\20@021857 by Jinx

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Is it possible to have two low power transmitters (different
freqs) next to each other and have one radiate 180 degrees
in one direction and the other radiate 180 in the opposite
direction ? I've been asked to look at making a unit that will
detect when an object goes past a certain point. The object
has a device that is off on this side of a line (influenced by freq1)
and switched on when it crosses a line (influenced by freq2).
It has to be brought back into the influence of freq1 to change
state again. If it goes past the range of freq2 it will stay
switched on. I thought perhaps the transmitters could be
separated by metal plate or mesh shaped to funnel or block
the radiation pattern. The power needed is just enough to
extend perhaps 20ft this side of the line and 10ft the other
side, frequencies unknown at this stage. Any other type of
detection system (IR, US, laser etc) is probably not practical

TIA

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2001\02\20@023617 by Robert Rolf

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Your best bet is to use phase information. It is very hard to
confine an EM emission to a pure hemisphere (unless you're up in
the UHF or microwave bands). The best you can hope for is a
cardioide pattern.

If you told us more about the application, rather than your preconceived
ideas of what solution to use (no offense), we could probably
make more meaningful suggestions.


Jinx wrote:
{Quote hidden}

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2001\02\20@040921 by Roman Black

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Hi Jinx, I think Oatley had some doppler radar
modules pretty cheap, with instructions. These
are quite directional (narrow cone) and would
do what you need I think.
-Roman



Jinx wrote:
{Quote hidden}

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2001\02\20@071029 by Jinx

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> Your best bet is to use phase information. It is very hard to
> confine an EM emission to a pure hemisphere (unless you're
> up in the UHF or microwave bands). The best you can hope
> for is a cardioide pattern.
>
> If you told us more about the application, rather than your pre-
> conceived ideas of what solution to use (no offense), we could
> probably make more meaningful suggestions.

No offense taken, just being cautious.

Imagine it this way. You've got a large area like a sports field in
which you can move this object. But if you try to take this object
past the perimeter of the field it is disabled. This area may be
unfenced (meaning the object could be taken out at any point),
or it may be fenced (meaning the object would have to be taken
out through an exit). To make it work again it has to be brought
back inside the perimeter, which is why the need for two signals

The intention was to mount solar-powered battery transmitters
on posts (as the venue isn't really a sports field, but it is outside,
that's OK). There are many identical objects moving around, so
the effect of the signals must localised to just the immediate
infringer.

I thought radio would be the best choice, mainly because of the
power supply. IR and ultrasound I think may use too much power,
and after sunset or on dull days that may be a problem. Lasers
were thought of too but there's too much chance of the beams
being blocked. The distances involved are too great for RFID,
and inductor loops would mean too much setting up, and there's
a lot of vehicular traffic around which may upset them. So radio
seems to be the only option, the only problem being setting up
distinct fields either side of a boundary

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2001\02\20@083500 by t F. Touchton

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part 1 3317 bytes content-type:text/plain; charset=us-asciiYou can use pulsed magnetic fields in a wire surrounding the property... very
similar to pet containment fences, but with a twist.  Make sure the magnetic
field is pulsed DC... otherwise the current flow in the wire is in one
direction, pulsed on and off to create carrier/modulation characteristics for
reception.  Now.. by watching the intial polarity of the voltage signal on the
receiving loopstick type antenna, you can tell if you are inside of outside of
the wire.


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> Your best bet is to use phase information. It is very hard to
> confine an EM emission to a pure hemisphere (unless you're
> up in the UHF or microwave bands). The best you can hope
> for is a cardioide pattern.
>
> If you told us more about the application, rather than your pre-
> conceived ideas of what solution to use (no offense), we could
> probably make more meaningful suggestions.

No offense taken, just being cautious.

Imagine it this way. You've got a large area like a sports field in
which you can move this object. But if you try to take this object
past the perimeter of the field it is disabled. This area may be
unfenced (meaning the object could be taken out at any point),
or it may be fenced (meaning the object would have to be taken
out through an exit). To make it work again it has to be brought
back inside the perimeter, which is why the need for two signals

The intention was to mount solar-powered battery transmitters
on posts (as the venue isn't really a sports field, but it is outside,
that's OK). There are many identical objects moving around, so
the effect of the signals must localised to just the immediate
infringer.

I thought radio would be the best choice, mainly because of the
power supply. IR and ultrasound I think may use too much power,
and after sunset or on dull days that may be a problem. Lasers
were thought of too but there's too much chance of the beams
being blocked. The distances involved are too great for RFID,
and inductor loops would mean too much setting up, and there's
a lot of vehicular traffic around which may upset them. So radio
seems to be the only option, the only problem being setting up
distinct fields either side of a boundary

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2001\02\20@084704 by Bob Ammerman

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Sent: Tuesday, February 20, 2001 8:27 AM
Subject: Re: [EE]:Directional transmitter


> You can use pulsed magnetic fields in a wire surrounding the property...
very
> similar to pet containment fences, but with a twist.  Make sure the
magnetic
> field is pulsed DC... otherwise the current flow in the wire is in one
> direction, pulsed on and off to create carrier/modulation characteristics
for
> reception.  Now.. by watching the intial polarity of the voltage signal on
the
> receiving loopstick type antenna, you can tell if you are inside of
outside of
> the wire.

Only if you know the orientation of the loopstick relative to the wire.
(Just turn around and face the other way and this system will think you are
now outside the wire.

There is something here tho....

How about two concentric 'pet fences'?

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\20@085253 by t F. Touchton

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part 1 2676 bytes content-type:text/plain; charset=us-asciiTurning around will not affect it if the antenna is in a vertical orientation,
turning it over will.  So.. you either need to make sure one end stays up, or
you could use an accelerometer to check orientation.

One benefit of working with dogs.. one end is usually up!!!!!

Multiple wires is also a good idea... for those interested in doing either idea,
beware that patents already cover this stuff... maybe not in your exact app, but
it wouldn't hurt to check.


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2001\02\20@100835 by David VanHorn

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>
>Only if you know the orientation of the loopstick relative to the wire.
>(Just turn around and face the other way and this system will think you are
>now outside the wire.

Flux within a solenoid is constant across the area, but outside, it falls
off rapidly.

I'm not sure how well this would work for a really large loop, but it works
well enclosing rooms.

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2001\02\20@101651 by t F. Touchton

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part 1 2335 bytes content-type:text/plain; charset=us-asciiI've looked at this on 11 acre loops, and if you can tolerate 100' or so of
variation, you can make a determination of inside -vs- outside just from the
flux intensity... however metallic structures and secondary currents from
transformer coupling effects can show up and wreak all kinds of havoc.  If the
loop gets much larger... the flux density will drop off inside the loop
signicantly, and not be effective.  If you attempt to crank up the current, you
will run into multiple order resonances due to transmission line effects of
buried wire, and SWR issues.  At Company XYZ we got up to 15,000' using a
resonance design, but did have a flux density taper due to standing waves (this
was at 7.5khz).


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>
>Only if you know the orientation of the loopstick relative to the wire.
>(Just turn around and face the other way and this system will think you are
>now outside the wire.

Flux within a solenoid is constant across the area, but outside, it falls
off rapidly.

I'm not sure how well this would work for a really large loop, but it works
well enclosing rooms.

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2001\02\20@113535 by Don Hyde

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How accurate does the boundary have to be, and how fast do you have to
respond?  GPS units are accurate to a few meters, and are getting smaller
and cheaper by the day.  With one of the OEM GPS units and a PIC, you could
have a perimeter map in memory, and do all the control stuff with no
physical boundary markers at all.

> {Original Message removed}

2001\02\20@153458 by Jinx

face picon face
> How accurate does the boundary have to be,

Pretty well-defined or false disabling will annoy people

> and how fast do you have to respond ?

Within a pace or two at walking speed

> GPS units are accurate to a few meters, and are getting smaller

When I said there are many objects , that could be up to 300 or
so milling around. The cost of outfitting 300 objects with GPS
would be prohibitive in this case, probably worth more than the
objects themselves. If it wasn't my money, I'd spend it on GPS
though IF the resolution was less than a metre

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2001\02\20@154531 by t F. Touchton

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part 1 2323 bytes content-type:text/plain; charset=us-asciiA dual loop magnetic system would probably be the cheapest way to go... you can
implement the loops at different frequencies, or same frequency with different
modulation.. just keep them far enough apart to distinguish (about 2 to 10').
Receiver is dirt cheap.... LC / Transistor / PIC... dump in on RTCC (TMR0) and
analyze frequency... know if you are leaving or coming back.  Transmitters are
really cheap... current sink @ 100mA max for about 15' of detection range...just
modulate the current sink with carrier/modulation info.

Only thing that might be a problem is the coupled magnetic fields... but this
depends on what is in the environment.


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> How accurate does the boundary have to be,

Pretty well-defined or false disabling will annoy people

> and how fast do you have to respond ?

Within a pace or two at walking speed

> GPS units are accurate to a few meters, and are getting smaller

When I said there are many objects , that could be up to 300 or
so milling around. The cost of outfitting 300 objects with GPS
would be prohibitive in this case, probably worth more than the
objects themselves. If it wasn't my money, I'd spend it on GPS
though IF the resolution was less than a metre

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2001\02\20@160102 by Jinx

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> A dual loop magnetic system would probably be

Thanks, I'll look into this

> Only thing that might be a problem is the coupled magnetic fields...
> but this depends on what is in the environment.

Car bodies and people. Lots of 'em

I wondered how far I could get with this, as I remember Tony
Nixon's ID badge problem. I think Peter Peres said that any
radio used would go through walls floors and ceilings, hence
my question as to whether you really can restrict RF to limits
at low cost and low fuss

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2001\02\20@162922 by mmucker

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Is someone trying to keep shopping carts in the parking lot?  :)

{Quote hidden}

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2001\02\20@181708 by Jinx

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> Hi Jinx, I think Oatley had some doppler radar
> -Roman

Had a search but nothing found under "doppler" or "radar"

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2001\02\20@181918 by Jinx

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> Is someone trying to keep shopping carts in the parking lot?  :)

I'll be trying that too now, thanks ;-))))

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2001\02\21@030326 by Mark Whittington

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Have you given any thought to using one or more low powered transmitters
in the "active" area?  You could adjust the output power when you install
them, and the device could disable when it gets out of range of the signal
(or when the signal strength drops below a pre-defined level) and enable
when in range again.

-Mark Whittington

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2001\02\21@034101 by Jinx

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> Have you given any thought to using one or more low powered
> transmitters in the "active" area?  You could adjust the output
>
> -Mark Whittington

That's a possibilty. Flood the area within the perimeter instead
of just at the perimeter itself

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2001\02\21@075721 by t F. Touchton

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part 1 2037 bytes content-type:text/plain; charset=us-asciiBeware of the frequency you do this at... you can get a 20dB drop through a
human body... so if you are doing signal strength this can greatly impact you.
I checked this figure at 1ghz and 500Mhz.... and found it originally in
reference to pager designs.


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Have you given any thought to using one or more low powered transmitters
in the "active" area?  You could adjust the output power when you install
them, and the device could disable when it gets out of range of the signal
(or when the signal strength drops below a pre-defined level) and enable
when in range again.

-Mark Whittington

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2001\02\21@080801 by Roman Black

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Jinx wrote:
>
> > How accurate does the boundary have to be,
>
> Pretty well-defined or false disabling will annoy people
>
> > and how fast do you have to respond ?
>
> Within a pace or two at walking speed
>
> > GPS units are accurate to a few meters, and are getting smaller
>
> When I said there are many objects , that could be up to 300 or
> so milling around. The cost of outfitting 300 objects with GPS
> would be prohibitive in this case, probably worth more than the
> objects themselves. If it wasn't my money, I'd spend it on GPS
> though IF the resolution was less than a metre


Jinx, have you seen the miniature robot soccer (table top
field, fist sized bots)??

Many of the teams use one overhead colour CCD camera,
and coloured dot(s) on top of each bot. With a bit of
simple processing they can track the whole team from
the one camera. :o)
-Roman

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2001\02\21@081147 by Roman Black

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Jinx wrote:
>
> > Hi Jinx, I think Oatley had some doppler radar
> > -Roman
>
> Had a search but nothing found under "doppler" or "radar"

They definitely had some beacuse I remember talking to
someone there are asking about them. Maybe try ringing
them? They don't put all the leftovers on the web page
I think.
-Roman

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2001\02\22@061826 by Alan B. Pearce

face picon face
>> Have you given any thought to using one or more low powered
>> transmitters in the "active" area?  You could adjust the output

>That's a possibilty. Flood the area within the perimeter instead
>of just at the perimeter itself

Check out the archive for comments I made some time back on setting up a hearing
loop for use with hearing aids in a building. What is being proposed here is a
variation on that. By my experience the coverage outside the loop could be about
the linear distance from the centre of the loop to the loop, i.e. your total
coverage could be about 2 loop radii.

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2001\02\22@121306 by Peter L. Peres

picon face
Hi Jinx,

I think that you can use two 433 MHz modules keyed with different codes
(simplest: two different frequencies) and a corresponding receiver. You
don't really need to make the radiation patterns entirely separate, just
detect when the two signals mix equally. The transmitter antennas will be
two rubber ducky antennas for 433 MHz spaced by lambda/4. Each will act as
the reflector for the other. I suggest ILS modulation pattern: One
transmitter sends 50% duty cycle and the other 20-30% in the gaps of the
first. This assumes that the receiver is peak-detecting. Watch out for
continuous beat from residual carrier (since you are using two
transmitters with different reference clocks).

Peter

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2001\02\22@121314 by Peter L. Peres

picon face
>I wondered how far I could get with this, as I remember Tony
>Nixon's ID badge problem. I think Peter Peres said that any
>radio used would go through walls floors and ceilings, hence
>my question as to whether you really can restrict RF to limits
>at low cost and low fuss

For such a short distance (20ft) the best range limiting is to attenuate
the signal as much as you need and use desensitized reecivers. You will
have to use some form of signal quality detection system in the receiver.
See my other posting on this in this thread. You could detect S/N by
comparing the shape ('cleanness') of the received pulses in that system,
in the time domain. I'd like to remind that a simple technique to decide
whether a digital signal is 1 or 0 with noise over it, over a period of
time, is to sample it at a constant fast rate and use two accumulators for
0 and 1. Then (or rather meanwhile) you normalize them. The result is the
average signal level and a S/N number that can be used as a threshhold for
squelch (the S/N number can be obtained by incrementing a different set
of accumulators for too short and too long pulses in the input).

With the ILS style modulation I described in the other posting you should
end up with 25% 1's on one side 50% 1's on the other and 75% in the
middle. Beware reflections etc at short range from people and car bodies
and fences and garbage cans and posts and whatever else Murphy decides to
throw at you. A large van with metal cab parked in the wrong place is a
classic showbuster in my experience.

good luck,

Peter

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