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5.2 Meter Radio Astronomy Project for 1420 MHz

 

This is a description of my 1420 MHz radio telescope project for observing the natural radio emissions of neutral hydrogen atoms found throughout space. Specifically, the study of the radio spectra of these emissions is used to determine the distribution and dynamics of hydrogen throughout our galaxy.

 

 

Hello, Everyone!

I have been interested in radio astronomy for years but have only recently been able to find the time to assemble my own radio telescope. I have completed construction of a 5.2 meter antenna which I use to detect the 1420 Mhz radio emissions of neutral hydrogen found throughout the galaxy. I have included some information on construction and sources of material, and there is also a description of how I am using it and what type of information can be gained from these types of observations. It is my hope that this information will be helpful to others who are interested in radio astronomy or who may even be considering a project of their own.
The antenna has a computer controlled Az-El mount and was built mostly from recycled C-band satellite TV components. The elevation drive was once a horizon-to-horizon drive for a 16-foot satellite TV antenna and has an integral worm drive. It is driven by a 90-vdc linear actuator motor/gearbox. The azimuth drive is a roller-chain arrangement driven by another worm drive. This worm drive is also driven by a linear actuator motor/gearbox. The antenna uses a potentiometer for azimuth position feedback which produces a voltage ratio read by a "data acquisition" board on an old 286 PC. Elevation position is obtained by using a digital inclinometer connected to one of the 286 PC COM ports.
Electronics include a 1420 Mhz GaAsFET preamp, an ICOM R-7000 receiver (AGC disabled), and a Tektronix 2710 digital/storage spectrum analyzer (10 KHz-1800 MHz). The spectrum analyzer can be controlled by an old HP-87 microcomputer which I use as a low cost IEEE-488/GPIB interface. The HP-87, 286 PC and an Apple Mac (used for data collection and reduction) all communicate via RS-232 links. More infomation can be found on the block diagrams shown below.


CONSTRUCTION OF THE ANTENNA (click on images for larger views):

 

Radio Astronomy Telescope Az-El mount and platform

 
The steel Az-El mount and antenna assembly platform.
Radio Astronomy Telescope elevation drive
 
The elevation worm drive.
Radio Astronomy Telescope elevation drive
 
A closer look at the elevation worm drive.
Radio Astronomy Telescope elevation drive counterweights
 
A closeup of the elevation limit switches and part of one of the counterweights. A total of 400 lbs. of lead and steel is used for the weights.
Radio Astronomy Telescope rib attachment
 
Ten ribs attach to the central steel hub.
Radio Astronomy Telescope Antenna rib arrangement
 
Arrangement of ribs from top of assembly platform.
Radio Astronomy Telescope rings
 
Arrangement of rings between ribs.
Radio Astronomy Telescope structural rings
 
Structural rings attached to a rib.
 
Closeup of the underside of the antenna.
Radio Astronomy Telescope feed supports
 
Method of attachment of the feed supports.
Radio Astronomy Telescope azimuth drive
 
Another look at the azimuth drive.
Radio Astronomy Telescope azimuth drive positioner
 
Another look at the azimuth drive. This view shows the
10-turn potentiometer used for azimuth position control.
Radio Astronomy Telescope limit switches
 
Another look at the azimuth drive. This view shows the limit switches and chain-tensioning spring.
Radio Astronomy Telescope near complete
 
Near-complete antenna with aluminum mesh installed.
Radio Astronomy Telescope azimuth drive
 
Closeup of the azimuth drive showing the drive motor (from a satellite TV linear actuator), the potentiometer and shaft coupling to the worm gear, and the two limit switches.
Radio Astronomy Telescope feed cradle
 
A cradle was constructed to hold the feed. The threaded rod extending out the back is used for securing the plastic cover.
Radio Astronomy Telescope feed cradle
 
Another look at the feed cradle support assembly.
Radio Astronomy Telescope feed
 
The feed lays in the cradle and is secured with hose clamps.
Radio Astronomy Telescope feed support
 
A view of the feed and support assembly as seen from the work platform.
Radio Astronomy Telescope feed
 
The feed with the antenna switch and preamp installed.
Radio Astronomy Telescope overall
 
Overall view of the antenna showing the 8-foot work platform and the plastic cover. The antenna is pointed approximately west in this view. As you can see, my view of the sky is a bit restricted in some directions!
 
The completed antenna.

 

SYSTEM DESCRIPTIONS:

 

 
Block diagram and description of system at antenna site. Estimated costs and sources of supply included.
 
Block diagram and description of system in the radio room. Estimated costs and sources of supply included.

 

 

Radio Astronomy Telescope electronics  
A view of the radio room showing the equipment.
Radio Astronomy Telescope electronics  
A close-up of the R-7000 receiver and the Tektronix 2710 Spectrum Analyzer. A Telonic 1-2 GHz tunable bandpass filter can be seen just to the right of the analyzer. The trace on the analyzer is a spectra being recorded at 70 degrees Galactic Longitude.
Radio Astronomy Telescope builder  
A view of me in the radio room.

 


 

What this Project is All About:

 

OBJECTIVES

This project is a hobby that combines mechanical, electrical, electronic, radio frequency, astronomy, physics, math, and software challenges. Some proficiency is required in all of the areas in order to obtain meaningful results. It has been fun and VERY educational!
I find it interesting to observe and confirm some of the original observations made by radio astronomy professionals in years past. It is gratifying to compare my results with what I find in the reference books, and in a small way share the excitement of the early researchers. I read their books and can duplicate some of their experiments. Who knows? Maybe I will stumble across something they missed!
I never get bored and am always looking for ways to improve the quality of the data, such as through improved receiver sensitivity, better antenna tracking accuracy, or better software.
The bad news is that I have no external funding. No grants or advances. Everything is bought out of the family budget! The good news is that I have no exernal funding! This means no deadlines, no publication dates, no pressure. I can take my time, focus on the areas that interest me the most, and enjoy!

 

(click on images for larger views and more detailed explanations)

 

A typical spiral galaxy (M100 in Coma Berenices) similar to the Milky Way. If this were our own galaxy the sun would be located near the outer edge of an arm. Hydrogen is the most abundent element in the galaxy. (NASA/HST photo)

 

 

A simplified diagram of the Milky Way galaxy. The sun is located about 30,000 light years (approx. 9 kiloparsecs) from the center. (Ref. 1)

 

 

After being excited, a neutral hydrogen atom will spontaneously decay, on average, after about 12 million years and emit a photon on 1420.40575 MHz. Fortunately, hydrogen is found in great abundance throughout the galaxy and these emissions are sufficient to be detected and studied. This is a crude map of neutral hydrogen in our galaxy derived from radio telescope observations. (Ref. 1)

 

The motion of different parts of the galaxy relative to the sun caused by the spiral rotation. Studies of the galaxy usually refer to galactic latitude/longitude coordinates. By convention, galactic longitude (l) is zero towards the center and 180 degrees directly away from the center. (Ref. 1)

 

The rotation curve for the galaxy showing rotation velocity, v (km/sec), and rotation rate, w, (radian/sec). This motion introduces significant doppler shifts in original emission frequency of 1420.40575 MHz. The frequency spectrum of these emissions reveals information on the structure and dynamics of the galaxy, as shown below. (Ref. 1)

 

Example radio spectra of hydrogen emissions (frequency vs. strength). (a): If a source were stationary emissions would be received only on a single frequency (vertical line in center). Practical receiver bandwidths broaden the spectral line into a typical curve. (b): Effect of a cloud moving away from the observer. The received spectrum is shifted down in frequency. (c): Turbulence within a cloud broadens the received spectrum. (d): A typical hydrogen spectrum is composed of emissions from multiple sources moving at different rates. (Ref. 1)

 

Typical hydrogen emission spectra from four different galactic longitudes (l). The received frequency (relative to the rest frequency) and corresponding relative velocity of the sources can be determined. (Ref. 1)

 

Simplified analysis of a received spectrum (l=75 deg). If the motion of hydrogen in the galaxy was perfectly circular, and followed the distance-velocity (solid curve) shown in (a), the expected idealized received spectrum would be the solid curve in (b). If the hydrogen were of uniform density, temperature, and dispersion the expected spectrum would be the solid curve in (c). The dashed lines show the effect of variations in local velocities of a few km/sec. The dotted curve in (c) shows an observation. (Ref. 2)

 

Typical Results:

Typical scans in the frequency range of 1420 to 1421 MHz plotted from the spectrum analyzer. These spectra of hydrogen line emissions show predominant peaks at different frequencies depending on the galactic longitude where taken. All of the emissions originated at the rest frequency of 1420.40575 MHz and have their spectrum broadened by turbulence.
In addition, observed spectra are shifted due to motions of the Earth's rotation, the motion of the Earth around the Sun, and the motion of the Sun through the galaxy. These motions must be calculated and taken into account in order to convert frequency into relative velocity.
A doppler shift of +100 KHz represents a closing velocity of the source of about 21.1 Km/sec.
Many of these scans will be necessary to form a complete database for the galaxy. The plots shown are actually 16 or 32 individual spectra averaged. This process helps to cancel random noise in the data.
Click on the individual images for larger views. The value of "l" indicates the galactic longitude of the observation.

 


Chronology of Events (including photos of the antenna smashed by the wind!)


More Information...

 

ELEVATION TRACKING SYSTEM DESCRIPTION:

A description of the digital inclinometer elevation tracking system is now available (14 Mar 99).

 

DISABLING THE AGC ON R-7000 and R-7100 RECEIVERS:

A description of my mod to disable the Automatic Gain Control (AGC) of ICOM R-7000 and R-7100 receivers is now available (14 Mar 99).


References:

1. Radio Astronomy, 2nd Edition, by John Kraus. Cygnus-Quasar Books, Powell, Ohio, 1986

2. Galactic and Extragalactic Radio Astronomy, Second Edition, by G.L. Verschuur and K.I. Kellermann, Springer-Verlag, New York, 1988


 

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