constructing radio telescope for hydrogen emissions … · when doing radio astronomy, ......

WJP, PHY382 (2015) Wabash Journal of Physics v4.3, p.1

Constructing Radio Telescope for Hydrogen emissions at 21cm

Max Millott, Dan Bajo, James Brown

Department of Physics, Wabash College, Crawfordsville, IN 47933

(Dated: May 4, 2015)

A normal telescope which magnifies light using mirrors and lenses will give you

one perspective of the galaxy. However, this is only a small part of the spectrum

of light exists. If you wish to see the other parts of the spectrum like radio waves

you need to use a different setup. We constructed a dish that will magnify incoming

radio and microwaves, and show this by comparing amplitude versus distance with

and without a dish present.


I. INTRODUCTION

The purpose of this lab is to construct a radio telescope that will detect hydrogen emis-

sions at 21 centimeters. A radio telescope is an astronomical instrument consisting of a

radio receiver and some sort of antenna/satellite dish. The radio telescope works by detect-

ing radio-frequency radiation which is emitted by extraterrestrial sources. We will begin the

construction of the process of building a radio telescope and in the long run we should be

able to map out the galaxy. Our group will design and build a dish. We will be using styro-

foam to create the parabolic dish and line in it copper mesh to help reflect the transmitted

waves to our focal point. Once this strep is completed we will attempt to collect conclusive

data and proceed to developing a receiver that will be able to detect the emitted waves from

outer space. After this is completed we should be able to fine tune the system and develop

the desired telescope which we will use to detect hydrogen emissions at 21 centimeters.

II. MODEL

When doing radio astronomy, you need a way to amplify the signals you are receiving in

order to analyze them. This is accomplished using a dish. The dish is shaped like a parabola

in order to focus all of the waves hitting it onto its focal point where the receiver is placed

1.

The shape of the dish is determined by the desired F -number which is the ratio of the

focal length to the diameter. In our case we want an F -number of 0.35. We then use this

number along with material constraints to find the exact size of our dish. We know that we

have a 36” by 36” copper mesh to make our dish so our arc length was going to be 36”. We

also know that the formula in two dimensions for our parabola would be

y(x) = 4fx2 (1)

where f is the focal length. Using this and the arc length equation

s =

∫ d2

− d2

√1 + [y′(x)]2dx (2)

along with our F -number, we can set up a system of two equations and solve for f and d.


FIG. 1. When you have a parabolic dish all of the radio rays that come in will converge at the

focal point as shown. These radio rays are then picked up by the receiver effectively amplifying

the signal.

III. SETUP

Our telescope will have several components to it. There are two ways to set up the

receiver and the components needed to process the signal. The first uses the series of amps

and mixers in figure 2.

The parabolic dish was constructed with two inch thick Styrofoam and lined with copper

mesh. Given the estimated size of the sheet of copper mess and a focal point we found the

estimated parabolic curve. In the process of constructing the dish we used basic boat building

skills which included lofting, and a simple pick up stick. Once the dish was constructed from

the styrofoam we lined it with the mesh and secured it using screws. After determining the

best position for the microwave receiver, we used for testing, as our focal point we constructed

a stationary mount to hold the receiver at the designated focal point.

Also after we continued our testing with the microwave transmitter and receiver, we

determined that we would be required to develop a better feed horn which will allow us to

funnel the transmitted waves to the designated focal point.

The second and probably cheaper option is to use software defined radio as shown in the

block diagram in figure 3. Figuring out how this setup works is something the next group

will need to research. We ran out of time trying to characterize our dish and feed horn and


FIG. 2. The radio signal from our receiver will be very small and at a very high frequency so we

will need a low noise amplifier with a mixer and an oscillator to lower the frequency. Next we will

use a band pass filter in the form of an intermediate frequency amp to filter out the rest of the

noise. From there we convert the signal to DC with a square law detector that converts the signal

from AC to DC and amplify it one more time before we record the final signal.

never quite got to researching this.

FIG. 3. The signal in this setup comes through the dish and is connected to the hardware’s front

end where it is filtered and amplified for the ADC. After leaving the ADC the signal goes through

a Field Programmable Gate Array or FPGA which extracts the components from the signal and

feeds them to the USB controller which stores the data for the PC to interpret.

IV. DATA

We haven’t been able to get high quality data from the dish yet, but we were able to get

data using our microwave emitter receiver pair. From the data we can see that we have what

resembles an inverse square relationship between distance and amplitude which is what we

would expect. While waiting on parts to come in we also experimented with what would

happen if we took the feed-horn off of the receiver. The results can be seen in Figure 5.

When we place the emitter and receiver facing each other and compare with a feed-horn

and without, we see there is a significant difference in not only the amplitude but also the


FIG. 4. Since our emitter receiver combo creates cavity resonance, the peaks will be located λ2

apart.

FIG. 5. The red curve is with a feed-horn and the blue is without.

wavelength. Since there isn’t a feed-horn to create cavity resonance in the receiver the

wavelength increases and the amplitude drops significantly.

Also after experimenting with and without the feed horn, we determined that the receiver

functions better when the waves were tunneled back to the focal point. The receiver is

highly sensitive the and as we mentioned before the results were drastically different with

and without the feed horn.


V. ANALYSIS

We see that the copper mesh dish does amplify signals and gives us a signal at distances

where the emitter/receiver combo didn’t without the dish. Also after experimenting with and

without the feed horn, we determined that the receiver functions better when the waves were

tunneled back to the focal point. The receiver is highly sensitive the and as we mentioned

before the results were drastically different with and without the feed horn as you can see

in Fig.5.

VI. CONCLUSION

We most likely won’t be able to do any actual work with radio waves with our dish since

parts haven’t come in and probably won’t before the year is over, so our conclusions are

that the dish does amplify the signal (so much that we weren’t able to get a reading less

than saturation at distances farther than were it fell off without the dish) and we set good

grounds in the beginning process of constructing a radio telescope in the future. If the

construction of the radio telescope is continued the next group should building a feed horn

which will help tunnel the reflected wave back to the desired focal point. The feed horn

should be constructed as a cone shaped funnel and out of metal. The next group should

also tune and perfect the dish in order to minimize the sensitivity of the waves. A problem

that occurred with us was that the system of the dish was not completely parabolic and

therefore all the reflected waves were not traveling to the designated focal point. After an

ideal dish is created a radio receiver will be needed, whether built or purchased, and that

will be the next step needed to be taken. This will help receive the transmitted waves from

space in order to using the radio telescope for mapping out the stars, comets, etc. in outer

space.

[1] Laffineur, Marius, Radio Astronomy Am. J. Phys. 29, 198 (1961);

http://dx.doi.org/10.1119/1.1937718

[2] Mark Kac, A. J. F. Siegert, On the Theory of Noise in Radio Receivers with Square

Law Detectors. J. Appl. Phys. 18, 383 (1947); http://dx.doi.org/10.1063/1.1697662


[3] Maurizio Tinti,Construction Of A 12 GHz Total Power Radio Telescope For Teaching

Purposes, Suitable, For Noisy Environments, Using Satellite TV Devices.Progress

In Electromagnetics Research C, Vol. 37, 159170, 2013

[4] A. HOPKINS1, R. WINDHORST, L. CRAM and R. EKERS, What Will The Next Gen-

eration Radio Telescope Detect At 1.4GHz?Australia Telescope National Facility, P.O.

Box 76 Epping, NSW 1710, Australia, Department of Physics and Astronomy, Arizona State

University, Tempe, Arizona, USA, Astrophysics Department, School of Physics, University of

Sydney, NSW, Australia. Experimental Astronomy, 10, no. 4 (2000): 419-437

[5] Stefan Leinen; Matthias Becker; John Dow; Joachim Feltens; and Knud Sauermann, Geodetic

Determination of Radio Telescope Antenna Reference Point and Rotation Axis

Parameters. Journal of Surveying Engineering. May2007, Vol. 133 Issue 2, p41-51. 11p. 2

constructing radio telescope for hydrogen emissions … · when doing radio astronomy, ......

Documents