Calibrating the PiRadar range and Doppler

We did a brief PiRadar transmission in the 80 meter ham band today and received on a second antenna. We don’t have the system calibrated yet so we don’t have a quantification on what we were measuring.

Every radar needs calibration to get usable range, Doppler and other measurements. Expanding upon section 3a of the PiRadar functionality test, we describe the materials needed to do an outdoor improvised radar range test. This test can be run independently of the tests from Sections (1) and (2) that are about the radar timing. The idea is to test the system in pieces so that it’s not too complicated testing everything at once.

Summary of materials needed for Red Pitaya radar calibration

Materials needed for one Red Pitaya monostatic radar:

  • wooden dowels to build 4 meter dipole (for 40 MHz, length ~ 3e8/40e6/2)
  • solid wire for dipole elements–something in the 12 to 18 gauge range is probably fine. If you find thinner wire or stranded wire is stiff enough that’s fine.
  • Type 43 snap-on ferrite core for balun (one per dipole)
  • 4-5 or so meters of coax cable, 50 ohms, RG-58.
  • SMA plugs for RG-58.
  • two SMA jumpers, a convenient length perhaps 0.5-1 meter or so between splitter and Red Pitaya In and Out.
  • ANNE-50+ SMA 50 ohm termination
  • ZFSC-3-13-S+ 3-port splitter
  • USB battery pack to power the Red Pitaya

Low-band VHF Dipole construction

Because the dipole antennas will be about 4 meters in length, you also want several meters of coax cable between the dipole feedpoint and the Red Pitaya, particularly to avoid breaking the Red Pitaya SMA connectors. Also, you want enough coax so that you can come out perpendicular to the dipole for a few meters or else the radiation pattern gets goofed up. Solder a 10Kohm resistor across the dipole feedpoint to dissipate static electricity buildup that can zap the Red Pitaya input.

Home Depot has 1.2 meter long wooden dowels for $1, one could connect 3 of them together and then let several centimeters of the solid wire stick past the ends.

Radar duplexing: transmit & receive with one antenna

In order to use one antenna while simultaneously transmitting and receiving, you need a method of combining or duplexing into one antenna. This can be provided by a splitter that combines and provides about 30 dB of port-port isolation such as Minicircuits ZFSC-3-13-S+. The third port needs a 50 ohm termination when not used–it might be handy to have a third port for certain tests and configurations.

Note

There are cheap cables and connectors on eBay, however double-check that they aren’t RP-SMA (doesn’t mate with SMA) or that they’re overseas and take 30 days to ship.