Setting Up a 2 Horn Interferometer

Procedure for Setting Up a 2 Horn Interferometer

These are instructions for how to set up and run a 2 horn interferometer using a Lime SDR with GNURadio. The GNURadio spectrometer programs write time-stamped data files once every integration time. For the adding spectrometer, called Interferometer_SimpleSpectrometer_Adding.grc, the data includes horn A spectra, horn B spectra, and the power spectra from the sum of their signals. For the multiplying spectrometer, called Interferometer_SimpleSpectrometer_Multiplying.grc, the data includes horn A spectra, horn B spectra, the magnitude of the interference, and the phase of the interference.

List of Equipment

a. 2 Horn telescopes, each consisting of:

  ▪ horn & can antennae
  ▪ cradle
  ▪ stand
  ▪ bolts for cradle axle
  ▪ clamps - recommended to secure the cradle position after the horn is aligned   b. Electronics

  ▪ computer with the GNURadio spectrometer program on it:
      - adding spectrometer program:  [Interferometer_SimpleSpectrometer_Adding.grc](https://github.com/WVURAIL/dspira-lessons/blob/master/FilesUploaded/interferometer_simpleSpectrometer_Lime_adding.grc)
      - multiplying spectrometer program: [Interferometer_SimpleSpectrometer_Multiplying.grc](https://github.com/WVURAIL/dspira-lessons/blob/master/FilesUploaded/interferometer_simpleSpectrometer_Lime_multiplying.grc)
  ▪ Lime SDR with USB cable to connect to a computer – This SDR can digitize 2 channels.
  ▪ 2 bias T's for powering the LNA's; the Lime does not provide power to the LNA’s.
  ▪ 5 V transformer to power the bias T's
  ▪ 2 SMA cables to connect the Lime Rx outputs to the "RF" inputs of the bias T's
  ▪ 2 long SMA cables to connect the "RF+dc" outputs of the bias T's to the LNA's
  ▪ 2 LNA's

c. Accessories for setting up

  ▪ duct tape - recommended for taping the devices and cables in place
  ▪ inclinometer - recommended for setting the telescope angle
  ▪ E-W (N-S) line - critical!
      - best method: determined, prior to a run, from the shadow of a vertical pole or string at sun's transit
      - or determine using an accurate compass
  ▪ Large protractor to help orient the telescopes
  ▪ pliers for tightening the horn rotation axis
  ▪ string - can be useful for aiding the alignment of the horns relative to each other
  ▪ tape measure
  ▪ tent in case of rain

Schematic Diagram and photo of the electrical connections
Setting up the horns

a. Assemble 2 horns as would be done for a single horn observation. b. For horns with a baseline along the east-west direction, set them parallel to each other, pointing at the same spot in the sky (e.g. the sun when it transits). c. Here are some tips: ▪ Sometime before doing interferometry, establish the N-S direction marking the shadow of a vertical pole when the sun transits. ▪ If the horns are to be set up with an E-W baseline, use some sort of large compass or -square to mark the E-W direction perpendicular to the N-S direction. A tape measure or string can be useful for doing this.
Setting up the electrical connections - REFER TO THE PHOTO ABOVE

a. Data for “Horn A” is collected on the cable that is plugged in to RX1 on the Lime SDR; RX2 is for Horn B. b. Be sure that the 5 V dc transformer for powering the bias-T’s is plugged in, and check that the power strip is on. c. The “RF” terminal of each bias-T is connected to the respective RX1 and RX2 on the Lime. d. The “RF + dc” terminal of each bias-T is connected to the respective LNA.
NOTE ABOUT COMPUTER POWER: Be sure the computer power settings are such that it does not go to sleep. Also, for longer runs it is recommended to have the computer plugged in during the run.

For Adding Interferometry:

a. Run the spectrometer program Interferometer_SimpleSpectrometer_Adding.grc

b. Plug the Lime SDR into a USB port.

c. On the computer set up the file directories for data collection:

  ▪ Create a folder appropriately titled for the data collection, e.g. "Sun_scan_July21". Be sure any folders and filenames have no spaces in them.

  ▪ Run GNURadio: $ gnuradio-companion

  ▪ On the canvas in the Interferometer_SimpleSpectrometer_Adding.grc program, open the Variable boxes described below; these are in the upper left side of the GNURadio canvas:

      • Open the "prefix_hornA" Variable block (double-click it). For "Value" type the pathname to the subfolder that will be used to collect the spectrum of Horn A. This must be in quotes. In the example below horn A data is written to the sub-folder "magnitude_july21" that is in the subfolder "sun_july21" which is a subfolder of "interferometer_data", etc. This will be written in the prefix-hornA Variable box as: "/home/john/dspira_2021/interferometer_data/sun_july21/hornA_july21/"

      • Set up similar file pathnames in the Variable blocks "prefix_hornB" and "prefix_adding".

      • Be sure these sub-folders on the computer have been created.

d. If all is good, you are ready to go. Delete the test files in each folder (not required but recommended).

e. Start the program. Select “Long Integration” and “Writing to File”. If all is good, the graphs should show signals being collected.

For Multiplying Interferometry:

a. Run the spectrometer program Interferometer_SimpleSpectrometer_Multiplying.grc