The HB100 [0] and CDM324 [1] are very inexpensive little CW doppler radar modules at 10GHz and 24GHz respectively. Feed them 5v and send the mixer output to a small microphone amp/input jack and you can record the frequency beat of most objects in motion.<p>[0] <a href="https://www.amazon.com/dp/B00FFW4AZ4" rel="nofollow">https://www.amazon.com/dp/B00FFW4AZ4</a><p>[1] <a href="https://www.amazon.com/dp/B07WH67J9W" rel="nofollow">https://www.amazon.com/dp/B07WH67J9W</a><p>This is what it sounds like when you toss a quarter in front of an HB100. <a href="https://www.youtube.com/watch?v=8riretP8ylE" rel="nofollow">https://www.youtube.com/watch?v=8riretP8ylE</a><p>The bowtie looking part is the actual flight, the other part right after that is it bouncing off the counter.<p>Spinning same quarter - <a href="https://www.youtube.com/watch?v=5lnYvJoxRak" rel="nofollow">https://www.youtube.com/watch?v=5lnYvJoxRak</a><p>Shining it at parts of a ceiling fan - <a href="https://www.youtube.com/watch?v=tIiFvByf1CQ" rel="nofollow">https://www.youtube.com/watch?v=tIiFvByf1CQ</a>
This brings back a lot of memories. One of my proudest technical achievements was as an intern: an assignment to figure out how many unique emitters were in a field of data picked up by some antenna. The data was provided on a CD with not a lot of other info.<p>I ended up finding a program some college researcher had published for matching DNA patterns, and between a little data massaging and software hacking, I was amazed when results actually made sense.<p>As an intern, I never knew exactly who provided that CD or where the program went from there, but it was a fun problem to work on for a while.
Thanks OP, this is a nice resource.<p>One note, though. The presentation of IQ demodulation seems to mix up the real and imaginary components of the analytic signal with the electric and magnetic fields. Even if you skip the analytic signal and just generate a straight up real signal (with zero imaginary component), there will still be a magnetic field.<p>This seems important because the current presentation suggests the complex signal representation is somehow specific to electromagnetic signals, while it is really just a mathematical convenience applicable to any signal (EM, sound, wave on jump rope, etc).
Early nukes used an interesting radar design to trigger at a preset distance above ground: there was a long coil of cable inside the nuke, and the fuze would emit random electromagnetic noise towards the ground. The reflected noise would then be received by an antenna, and the original emitted signal would be delayed by the coil by a certain predetermined time delta. This allowed the nuke's fuze to compute autocorrelation (and therefore detect distance) using entirely analog methods, and in a way that's impossible to jam, because autocorrelation on random signal is pretty darn robust (it's a delta function for ideal white noise).
There was a good talk about how radar (and lasers) are weaponised by police to detect the speed of vehicles (and countermeasures to that) at defcon this year too.<p><a href="https://youtu.be/vQtLms02PFM" rel="nofollow">https://youtu.be/vQtLms02PFM</a><p>Might be interesting too.
It's a good start, but stops short of the most interesting parts. The signal to noise section should at least mention the root of forth power in the radar equation, as this is one of the key limitations. Beam forming with mechanical scanning or phase arrays is also important from the practical standpoint, as so is the relationship between wavelength, antenna size and angular resolution. Finally, at least the concept of how the autocorelation function of a good pulse should look like is worth mentioning, with examples of complementary or pseudorandom sequences .
There is fantastic webinar (consisting of 5 videos) from TI which covers CW and FMCW radar techniques:
<a href="https://training.ti.com/intro-mmwave-sensing-fmcw-radars-module-1-range-estimation?context=1128486-1139153-1128542" rel="nofollow">https://training.ti.com/intro-mmwave-sensing-fmcw-radars-mod...</a>
For the constant velocity case, the graphic shows the transmit pulse as having two cycles, and the received pulse as having four cycles of higher frequency, with the received pulse being the same length as the transmit pulse.<p>Shouldn't the received pulse have the same number of cycles as the transmit pules, and the received pulse be shorter than the transmit pulse (or longer in the case of the target moving away instead of toward the source)?<p>Or does something non-intuitive happen (probably because of special relativity) resulting in the received pulse gaining energy from the reflection, which is reflected (pun intended) as the pulse being longer than you'd expect classically?
To me, the maths of the radar are the easy part.<p>But the physics of it? That's something that seems entirely more complex to me.<p>Is it really? Could any of you explain how you emit the radio wave, and how you detect it back? In terms of hardware?
I’ve always wanted a way to trigger the radar detractors that people use in their cars. I’ve noticed for a while that when a bunch of people suddenly move to the right lane there is usually a cop with a speed gun around, so sending a fake pulse periodically seems a sneaky way to clear the traffic ahead (but would only work when there was something solid to reflect the pulse back, sorry Kansas). I’ve not found anything on Google or Ali Express that looks like it might help me pull this off - nobody wants to build a Rasphberry Pi speed gun it seems.
Of course I knew that radar measured time of flight of radio frequencies, but it never occurred to me to actually look into the math. This was presented incredibly well.
While this is a good intro, a lot of radar work now relies not on square pulses but linear frequency modulated (LFM) waveforms. This greatly expands the number of image formation algorithms you can use, depending on system requirements.
Side note: Same technique can be used to track a cell phone, except GSM frequencies are higher so the tracking distance is considerably shorter then a normal radar. But still good enough to do a city wide track of a person of interest.
Next level: <a href="https://en.wikipedia.org/wiki/Synthetic-aperture_radar" rel="nofollow">https://en.wikipedia.org/wiki/Synthetic-aperture_radar</a>
Thanks, this is a nice introduction.<p>Two typos in this sentence: <i>One way to work around this problem is to use a non uniform spaceing of the pulses which is none as staggering.</i> (spacing, known)