Setting Up a 2 Horn Interferometer

Procedure for Setting Up a 2 Horn Interferometer

Set up and run a two-horn interferometer with a Lime SDR and GNU Radio. The GNU Radio spectrometer programs write time-stamped data files once every integration time. The adding spectrometer is Adding interferometer (download GRC). Its data includes spectra for horns A and B, plus power spectra from their summed signals. The multiplying spectrometer is Multiplying interferometer (download GRC). Its data includes spectra for horns A and B, plus interference magnitude and phase.

List of Equipment
- 2 Horn telescopes, each consisting of:
  - horn and can antennae
  - cradle
  - stand
  - bolts for cradle axle
  - clamps - recommended to secure the cradle position after the horn is aligned
- Electronics
  - computer with the GNU Radio spectrometer program on it:
    - adding spectrometer program: Adding interferometer (download GRC)
    - multiplying spectrometer program: Multiplying interferometer (download 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
- 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
- Assemble 2 horns as would be done for a single horn observation.
- For an east-west baseline, align the horns in parallel toward the same sky position (e.g. the transiting Sun).
- Here are some tips:
  - Sometime before doing interferometry, establish the N-S direction marking the shadow of a vertical pole when the sun transits.
  - For an east-west baseline, use a large compass or square. Mark the east-west direction perpendicular to north-south. A tape measure or string can be useful for doing this.
Setting up the electrical connections - REFER TO THE PHOTO ABOVE
- The cable connected to RX1 on the Lime SDR collects “Horn A” data. RX2 collects Horn B data.
- Plug in the 5 V dc transformer that powers the bias-T’s. Check that the power strip is on.
- The “RF” terminal of each bias-T is connected to the respective RX1 and RX2 on the Lime.
- 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:
- Run the spectrometer program limesdr-adding.grc
- Plug the Lime SDR into a USB port.
- 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 GNU Radio: $ gnuradio-companion
  - Open the Variable boxes described below in limesdr-adding.grc. They are at the upper left of the GNU Radio canvas:
    - Open the “prefix_hornA” Variable block (double-click it). For “Value”, enter the quoted path to the folder for Horn A spectra. In this example, it is “hornA_july21”. That folder is inside “sun_july21”, which is inside “interferometer_data”. 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.
- If all is good, you are ready to go. Delete the test files in each folder (not required but recommended).
- Start the program. Select “Long Integration” and “Writing to File”. If all is good, the graphs should show signals being collected.
For Multiplying Interferometry:
- Run the spectrometer program limesdr-multiplying.grc
- Plug the Lime SDR into a USB port.
- 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 GNU Radio: $ gnuradio-companion
  - Open the Variable boxes described below in limesdr-multiplying.grc. They are at the upper left of the GNU Radio canvas:
    - Open the “prefix_mag” Variable block (double-click it). For “Value”, enter the quoted path to the folder for interference magnitude. In this example, the data goes into “magnitude_july21”. That folder is inside “sun_july21”, which is inside “interferometer_data”. This will be written in the prefix-mag Variable box as: “/home/john/dspira_2021/interferometer_data/sun_july21/magnitude_july21/”
    - Set up similar file pathnames in the Variable blocks “prefix_phase”, “prefix_hornA”, and “prefix_hornB”.
    - Be sure these sub-folders on the computer have been created.
- If all is good, you are ready to go. Delete the test files in each folder (not required but recommended).
- Start the program. Select “Long Integration” and “Writing to File”. If all is good, the graphs should show signals being collected.

Check recording before a long run

Briefly select Writing to File, then stop the program and inspect each output folder. Confirm that new, time-stamped files contain data for the selected spectrometer. Keep test observations separate from the science run. Use your browser’s Print command if you need a field copy of these instructions.