Hello all,

I'm trying to make a carfinder, where a pocket-sized, keyring-attached
transmitter sends a signal to a car, the car sends a reply, and the
carfinder shows some kind of signal strength indication. Do this as
the operator turns in a circle, and he should have an idea of where he
left his car in the carpark.

In Australia, it is legal to use 150MHz for any purpose, so long as the
output is limited to 100mW. I've found a pre-made transmitter unit for
that frequency and power (which gets 10km with a good antenna), but I'm
a novice to radio in general and am looking for pointers to information
on choosing the right kind of antenna to that is small enough to fit.

In the car, I don't have much more space (I could use the car's antenna
instead), but it would need to be omnidirectional.


Thankyou for your time.
--

On Mon, 26 Mar 2007 02:35:51 GMT, "David Hooker"
wrote:

Hi David,

Presumably, you are speaking of
a pocket-sized, keyring-attached
receiving antenna too, such that it can resolve the point of
transmission to a fairly narrow region.

It would take an especially large pocket to accomplish (not even a
'roo's). Such a design demands complexity in the form of volume at
the scale of 2 meters. You are off by a scale of 100 or so. Perhaps
if your frequency was set to 15GHz....

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Mon, 26 Mar 2007 02:35:51 GMT, "David Hooker"
wrote:

Hi David,

Presumably, you are speaking of
a pocket-sized, keyring-attached
receiving antenna too, such that it can resolve the point of
transmission to a fairly narrow region.

Yes, I forgot to mention that. I always forget something when Usenet
posting. Sorry.



It would take an especially large pocket to accomplish (not even a
'roo's). Such a design demands complexity in the form of volume at
the scale of 2 meters. You are off by a scale of 100 or so. Perhaps
if your frequency was set to 15GHz....

73's
Richard Clark, KB7QHC


Ahhh. OK, if I can legally call this a "radiodetermination
transmitter" then I can use up to 25W (!) at 7.6-7.7GHz; otherwise the
highest frequency I can use is 2.4-2.425GHz at 100mW.

But if I'm in a carpark and I'm using such a high frequency (please
excuse me, I'm a novice) but wouldn't I have trouble with reflections
from the metal of the cars and concrete around me?

--

On Mon, 26 Mar 2007 04:27:01 GMT, "David Hooker"
wrote:

Ahhh. OK, if I can legally call this a "radiodetermination
transmitter" then I can use up to 25W (!) at 7.6-7.7GHz; otherwise the
highest frequency I can use is 2.4-2.425GHz at 100mW.

But if I'm in a carpark and I'm using such a high frequency (please
excuse me, I'm a novice) but wouldn't I have trouble with reflections
from the metal of the cars and concrete around me?

Hi David,

100 mW in the 2M region is enough power to communicate with the space
shuttle. You are not lacking for power, you are lacking for scale.
Your inspiration of 7+ Ghz comes close enough to be achievable (and
using Gunn Diodes as transmission sources). However, scale will still
make the product about as long as a pencil, or the entire structure
the size of a business card. Not quite the key fob, but closer.

73's
Richard Clark, KB7QHC

I'm
a novice to radio in general and am looking for pointers to information
on choosing the right kind of antenna to that is small enough to fit.

In the car, I don't have much more space (I could use the car's antenna
instead), but it would need to be omnidirectional.



Getting any kind of directivity at 150MHz with small aerials is very
difficult. The wavelength at 150MHz in 2m so you can see that element
lengths will be quite long. One possibility might be a ferrite rod type
antenna if you can find some vhf ferrite. Some older pagers used them so
that may be a source. However you may still struggle to get very good
directivity.

Perhaps an active solution may be a better bet. Try Googling for
Handifinder, which is basically 2 small antennas that are switched at a fast
rate, this produces a tone in the receiver which vanishes when both antennas
have equal path length to the source.

Regards
Jeff

In message , Jeff
writes

I'm
a novice to radio in general and am looking for pointers to information
on choosing the right kind of antenna to that is small enough to fit.

In the car, I don't have much more space (I could use the car's antenna
instead), but it would need to be omnidirectional.



Getting any kind of directivity at 150MHz with small aerials is very
difficult. The wavelength at 150MHz in 2m so you can see that element
lengths will be quite long. One possibility might be a ferrite rod type
antenna if you can find some vhf ferrite. Some older pagers used them so
that may be a source. However you may still struggle to get very good
directivity.

Perhaps an active solution may be a better bet. Try Googling for
Handifinder, which is basically 2 small antennas that are switched at a fast
rate, this produces a tone in the receiver which vanishes when both antennas
have equal path length to the source.

Regards
Jeff



I found that, when using a 2m handheld (Trio 2200, therefore both
hands!), using your body as a shield can be used to give a useful
indication of where the signal is coming from.
Ian.
--

Richard Clark wrote:

On Mon, 26 Mar 2007 04:27:01 GMT, "David Hooker"
wrote:


Ahhh. OK, if I can legally call this a "radiodetermination
transmitter" then I can use up to 25W (!) at 7.6-7.7GHz; otherwise the
highest frequency I can use is 2.4-2.425GHz at 100mW.

But if I'm in a carpark and I'm using such a high frequency (please
excuse me, I'm a novice) but wouldn't I have trouble with reflections

from the metal of the cars and concrete around me?

Hi David,

100 mW in the 2M region is enough power to communicate with the space
shuttle. You are not lacking for power, you are lacking for scale.
Your inspiration of 7+ Ghz comes close enough to be achievable (and
using Gunn Diodes as transmission sources). However, scale will still
make the product about as long as a pencil, or the entire structure
the size of a business card. Not quite the key fob, but closer.

73's
Richard Clark, KB7QHC

You could always use a fractal antenna! :-)

Irv VE6BP

On Mon, 26 Mar 2007 02:35:51 GMT, David Hooker wrote:
Hello all,

I'm trying to make a carfinder, where a pocket-sized, keyring-attached
transmitter sends a signal to a car, the car sends a reply, and the
carfinder shows some kind of signal strength indication. Do this as
the operator turns in a circle, and he should have an idea of where he
left his car in the carpark.

You realise, don't you, that in a large parking lot filled with cars
(that's why you can't see yours - isn't it) you will be surounded by a
multitude of large RF reflectors?

Use your ears.
Honk the horn.
Your ears will be more directional than any
RF-with-tiny-directional-antenna-at-150mcs solution could be.

Or, get an amateur radio license and mount a large, honkin' 80M mobile
whip on it.
Use your eyes -- the *most* directional.

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
*** Killfiling google posts: http://jonz.net/ng.htm

Thankyou all for your replies

Richard Clark wrote in
:

On Mon, 26 Mar 2007 02:35:51 GMT, "David Hooker"
wrote:

Hi David,

Presumably, you are speaking of
a pocket-sized, keyring-attached
receiving antenna too, such that it can resolve the point of
transmission to a fairly narrow region.

It would take an especially large pocket to accomplish (not even a
'roo's). Such a design demands complexity in the form of volume at
the scale of 2 meters. You are off by a scale of 100 or so. Perhaps
if your frequency was set to 15GHz....

Why not, due to the close range, think inverse square law instead of
directivity? Could still be a bit confusing in an underground carpark,
but basically, the source and the receiver are close enough that a few
steps ought to make a readable difference in strength. Just walk in a
circle and keep widening it on the side where the signal is strongest and
narrowing where it is weakest. The closer you get, the less sensitivity
you need, so have some method of turning that down. And 100mw is LOTS of
power for this!


--
Dave Oldridge+
ICQ 1800667