Big Dish Telescope: Original System

Recovered 10-Foot Dish

Support Post and Feed Stalk

Mounting Frame

Feed Stalk Front and Back

The original television receive-only (TVRO) system acquired from the Hurt family consisted of these components:

• 10-foot diameter parabolic fiberglass dish of unknown manufacture (see photos above), with
  • embedded metal foil reflective surface
  • focal ratio of 0.31 (inferred from 2-foot dish depth^*)
  • steel support post and modified polar mount hardware
  • linear actuator for slewing east-west along satellite belt
• 5-foot Chaparral feed stalk (see photos at right), with
  • scalar rings to keep radiation outside dish area from entering feed
  • rotatable resonant element to capture polarized signals at the dish focus
  • radio-frequency (RF) waveguide carrying the signal from the resonant element
  • low-noise RF amplifier (LNA)
  • Uniden single-channel frequency downconverter from 3.7-4.2 GHz received RF to 70 MHz intermediate frequency (IF)
  • direct-current (DC) power (probably 24-volt) and coaxial feed lines (probably RG-6 75-ohm TV coax) through the feed stalk, cut off at the stalk base

No "set-top" box was included, although one was made available by Henry Cantrell, a retired electrical engineer and active radio amateur. TVRO set-top boxes were used not only to demodulate and amplify the IF signal for TV display but also to control the east-west slew motor, polarizer stepper motor, and downconverter local oscillator (called a voltage-controlled oscillator). The original satellite broadcast stations had a bandwidth of about 36 MHz or a bit less, and after mixing down to the IF, one was selected with a bandpass filter in the receiver tuner. Benchtop experiments in Mr. Cantrell's lab were successful in putting both broadband continuum noise and narrowband oscillator signals through the LNA. Unfortunately, the downconverter had a short circuit and underwent thermal failure when tested.

Example Set-Top Box

Fried Downconverter

^*The focal ratio of a parabolic dish reflector can be calculated as f / D = D / (16 × d), where f is the focal length, D is the dish diameter, and d is the dish depth (see Chaparrall Communications Feedhorn Troubleshooting Guide).

Several aspects of the TVRO system required modification to convert it into a functioning radio telescope.

• Dish Integrity: When recovered, the dish had a crack on one side that did not affect its utility as a reflector but reduced its structural integrity and might allow moisture or other contaminants to penetrate its outer surface. This problem was addressed by patching the fiberglass exterior of the dish, strengthening and sealing it from the elements.
• Pointing Ability: The TVRO system used a so-called polar mount designed to track geosynchronous satellites, which all appear along an arc about 5 degrees south of the celestial equator from the latitude of southern Kentucky, and it only needed to slew east and west along this arc. To observe targets elsewhere in the sky, the system must be able to point away from this arc, ideally to any north-south position (called "Declination" in astronomy). To address these issues, WKU engineering students and faculty have investigated a variety of much more versatile mechanical designs in a series of courses (ME 400 & 412, ENGR 490 & 491). Assembly of the redesigned system is in progress.
• Slewing and Tracking: The TVRO system had no ability to track objects at a speed matching that of astronomical sources, which appear to drift from east to west at 15 degrees per hour on the celestial equator. Such tracking is desirable for observing faint sources to add up the received signal over time. The engineering team is also working on this issue.
• Sensitivity and Bandwidth: The original LNA was an early and probably quite noisy model by today's standards, and it required a separate frequency downconverter that is no longer functional. Newer and better subsititutes for both are now available, so we plan to replace the entire feed assembly. We have purchased a Chaparral C-band low-noise block downconverter and feed (LNBF), a single unit that combines the feed, LNA and dowconverter functions. The LNBF has far lower noise temperature (17 kelvins, where the old LNA may have been over 100 K), larger RF bandwidth (3.4-4.2 GHz), and much broader IF coverage (800 MHz "untuned" over 0.95-1.75 GHz); these combined attributes will allow much more sensitive and versatile astronomical observations. We are also planning to install receivers for other frequency bands, such as Chaparral's Ku-band LNBF, or even a 1.4 GHz L-band feed to detect 21-centimeter hydrogen emission.
• Signal Capture: The TVRO electronics had no means of recording the received signal, as is desirable for experiments. While a VCR or DVR could capture demodulated television signals, these have limited bandwidth, and natural signals will be corrupted by the demodulation process. We have instead purchased software-defined radio (SDR) receivers for signal capture, including a simple RTL-SDR and more sophisticated Airspy R2 and SDRPlay RSPdx SDRs and plan to record signals off one of these using either a Raspberry Pi board computer or a laptop PC. WKU physics students in the Data Acquisition Using LabVIEW course (PHYS 318) are working on this project.
• Control Systems: Lastly, the TVRO had no overall control system other than the original set-top box (see above), which is not suitable for astronomical observations. We are working to implement joint controls of both the telescope pointing and data acquisition processes using the LabVIEW interface platform from National Instruments. PHYS 318 and ME 400/412 students are working on different parts of the control system. Our eventual goal is to make the radio telescope remotely operable from the WKU campus for the benefit of students, faculty, and the public.

This project is ongoing. A few highlight images of work on the telescope and some of the people involved are available. Please see the credits page for a complete list of all who have contributed.