At 09:01 PM 8/4/01 -0400, Sean Breheny wrote:
>Hi Dave,
>
>I've got to ask: why do people use marine radar on cars?!

Because they don't make radar units for cars.
:)


>First of all, my primary interest was toward the aircraft side (I think
>it would be REALLY amazing to have a display on your computer screen in
>real time of all the aircraft in, say ,a 30 miles radius!) so my ideas
>may not be as relevant to the weather version, although that interests
>me, too.

Probably similar application.
I think doppler is out of the reach of the hobbyist, but a good look at
storm structure would be achievable.

>I was thinking of using a bunch of Mini circuits ERA amplifiers in
>parallel (with hybrids to combine power) as the output amp, providing
>only around 100mW depending on how many I wanted to parallel.
>Since these can be had for only a few dollars a piece and go up to about
>20mW power. Some similar parts from minicircuits would also
>make a nice LNA for the receive side.

I can check with my local microwave ham, I'm sure there are simpler devices
to hack.


>I would mount an upconverter and a downconverter right up on the back of
>the antenna (probably a small dish). I would feed the TX signal and RX
>signal to/from the converters with regular coax, using a baseband of
>about 1 MHz.

I'd be inclined to go with 70 MHz, and use TV receiver components

>  Since A to D and D to A converters can easily sample at several
>megasamples/sec, the outgoing signal could be generated by a D to A
>connected to a high-speed DSP chip, and the incoming signal could be
>processed by an A to D and another DSP chip. This way, a change of coding
>scheme or decoding algorithm would be only a software change. I'm fairly
>certain this would work for simple pulses, but I'm not sure if DSP chips
>have enough horsepower for doppler extraction from pulse compressed
>coding schemes (like Barker codes) which we might need to make up for the
>low output power.

Interesting.


>Finally, some PICs could be used to control steppers or other servos to
>tilt the antenna horizontally/vertically. I was thinking of using
>something around 5GHz, so a relatively small dish (perhaps a 2 to 3 feet
>diameter) could be used.

Makes sense.
I'd like a small beamwidth to get high resolution.
This is one problem with marine radars. They have a thin but tall fan beam,
because ships roll around a lot. That makes them less than stellar for
mapping clouds, where things change with height.

10 or 24 would shrink that antenna, but I don't know yet what the rain
attenuation would be like.



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