Making a 9GHz doppler radar

[Kalle] is currently building an FMCW radar, but as he doesn’t have all the parts finished he decided to build a 9GHZ doppler radar in the mean time. The H-plane horn antennas were made from brass sheet and soldered together. [Kalle] checked the matching between the emitter and the antenna by inserting a directional coupler between the two and measuring the intensity of the reflected signal (approximated return loss). At 9Ghz, the Doppler shift for a 1 meter per second speed is about 30Hz so he connected the radar’s output signal to his soundcard.

A quick explanation of the Doppler effect that a radar uses: if you send an RF signal at a given frequency to a moving target, the reflected signal’s frequency will be shifted. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession. Hackaday featured plenty of projects using this effect: a small doppler motion sensor, gesture control using doppler shift, hacking an old radar gun

22 thoughts on “Making a 9GHz doppler radar

    1. No chance to do that officially, it’s not the purpose of amateur radio.
      Amateur radio is experimentation, no daily exploitation.

      Sending microwaves and receiving echos is possible with this kind of experimental device, which is very cool.
      However sending megawatts/kilowatts of RF kilometers away is not something the FCC et al would be happy to see us doing. In France we have an official 125W tx limit, I assume some kind of limitation also applies in other countries.
      Not to mention the extreme danger of high power RF radiation, and the difficulty to get meaningful results.

    2. more details: 125W limit for these MW bands. lower bands have higher limits.

      And people trying to receive small signals would have reason to become mad if some device was sending such strong signals 360 degrees away!

      1. I would have to look at the US regulations more.
        I know the 1.5kW limitation of Hf/UHF/VHF but MW I’ve not looked into much as the very little I do with MW is in the 3-5mw range

      2. 9gig has insane directivity, he will NOT be sending it 360, but at 1 degree on a rotating platform.

        Second sending it into the sky will not bother anyone. Perfectly acceptable to make ham radio weather radar. Except for the power limit.

        now at 10 watts it is very useable as ground radar. and if you bring it down to 100ma, you just invented the sensor needed for anti collision radar used in cars.

        1. It certainly may bother someone if he hits an airplane and causes problems for their radar; he’s responsible for making sure this doesn’t happen. There are also additional restrictions in part 97.305 – only designated bands allow pulse as an emission type (no bands lower than 33cm allow it, for example).

          1. You know, now I’m starting to doubt that it’s legal, and there’s a lot of confusion among hams online. One way emissions, unless of a type specifically authorized (i.e., control of a model craft) are against the rules per 97.113(5)(b). Radar is not a specific exception. Then again, I’m not sure if it qualifies as “one-way” since *something* is receiving the signal: the same station who sent it. Also, what could the pulse emission type be for if not for radar? Anyone know for sure?

          2. I don’t know for certain whether this is kosher or not under amateur rules but to me it is analogous to moon bounce which is ham kosher (I see what I did there). Instead of bouncing off a natural satellite, you are bouncing off the car in front of you or nearby clouds.

          3. The thing with moon bounce is it’s communication – it’s not a one-way transmission, which is allowed by part 97. One-way communications are banned, except for a specific list of exemptions. I’ve gone through all of part 97 and it’s silent on the topic, but I wonder if there have been any FCC memos, etc, clarifying this. I’ve found the question asked on ham messageboards, but no one has a definitive answer.

    1. Yes. He chose a poor frequency also. 9GHz is used for military radionavigation in europe. But, if you retunes the YIG sligtly (goes from 8 to 10GHz) to 9.2-9.975GHz it will be in a band designated for movement detection (power limit 25mW radiated). Not sure if this is in whole Europe or my country only.

      1. Hey! Yes my Doppler radar is technically operating on an illegal band, sorry about that :) It was just for a couple of minutes and not continuous operation so I’m fairly certain a low power like this didn’t interfere with anyone. Like Dodo said the 9.2-9.975 GHz band would be better for a radar like this. I ran out of power supplies for the YIG tuning current so I couldn’t tune it to the right band. A better version is coming up…

          1. No, you don’t need a license to operate radar on a boat, as long as the radar is of a type that can be operated exclusively from the controls (no internal adjustments allowed). You cannot operate the same type of radar on land, though.

    1. These are kind of…crappy… makes similar modules, but with the chance to modulate the output frequency, so it’s the perfect basis for a small DIY FMCW radar, the module has all the microwave woo-doo stuff, all you do is connect a microcontroller of your choice.
      They also have a long-range version.

      Now if they only weren’t so expensive :-(

  1. I just want to point out that doppler effect work in a manner not completely explained in the article. The frequenci gets shiftet uppwards when the target gets closer. And gets shiftet downwards when the target moves away. The frequenz stays the same when target is at a constant distance

  2. If anyone is interested in Antenna design and simulation (as well as other RF simulations) there is a software called OpenEMS.
    It has a utility to convert eagle to a .hyp format it can use. Many other circuit board programs (Altium, Pads, etc.. ) can export to this format too.

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