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I've been reading up a bunch on radar and most applications exploit the long range capabilities (rightfully so), but I'm wondering what is the smallest range a radar system can operate at?

I'm interested in synthetic aperture radar on small scales. Specifically, I would like to measure the ocean waves over a 5m radius (yes 5m, not 5km) and build a "depth map" showing the "instantaneous" wave heights and surface currents (maximum frequency is 10Hz). I know this is done from space satellites, so why can't it be done from a pier or drone?

I'm not a radar person, so excuse my ignorance, but doesn't the concept of radar scale? I would imagine you can build a miniature antenna array that scales to the range of interest? So if my range of interest is 0-100m (yes meters), couldn't the antenna be as simple as a wifi antenna?

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    \$\begingroup\$ It's distance resolution that determines operating frequency in radar. \$\endgroup\$ – Andy aka Jun 10 '17 at 9:01
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Yes, radar scales down; due to diffraction, you need shorter-wavelength radio to image smaller objects. Milimeter-wave radar can be used at airport security.

Antenna complexity is another question. You need either enough antennae to do synthetic aperture, or a physical moving or scanning directional antenna.

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I don't think radar has a minimum distance per se, but I'd imagine it's harder to use lower frequencies for close range applications because as your received signal gets closer and closer to your transmitted signal, it gets harder to tell the two apart. This is why radar uses RF chirps as it allows you to convert distance (well, "time of flight" really) into a low frequency which is easy to measure.

Many radar antennas do share a lot with directional wifi patch antennas (usually buried inside phones and tablets etc.), but not so much with monopole/whip antennas which you'll see on routers and the like. That's not to say you can't use a bunch of whip antennas, it's just harder to do so as they radiate all around their axis normally.

Having said that, Google has a radar system that can track hand gestures so the minimum range of radar can be quite small. What you are probably looking for is an automotive collision avoidance radar as they commonly operate both on the scales you want and on small, slow watery things (i.e. pedestrians).

There are also a near infinite number of microwave motion sensors on eBay, most operate at 2.45GHz but some sit at around 10GHz. With a bit of probing, you might be able to find the demodulated output signal and thus net yourself a functional short-range radar for a couple o' bucks. Good luck.

Shahriar at The Signal Path recently posted a teardown and in-depth explanation of an automotive radar unit if you want to learn more about how these systems work.

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Yes the concept of radar scales. It all depends on the type of radar and the RF frequency. I work on 60 GHz and 80 GHz radars and their range resolution can be less than 1 cm. Also the shorted distance we can measure is a few cm.

For 0 - 100 m I'd recommend to look at radars operating at 10 - 30 GHz. There are products (car radars) that operate at 24 GHz.

No the antenna will usually not be similar to a simple WiFi antenna as a WiFi antenna is mostly designed to transmit in all directions while radar antennas are far more directional. They can be simple designs though but not the same as a WiFi antenna.

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