Noise Levels at the National Radio Astronomy Observatory
Post on 21-Sep-2016
Embed Size (px)
Findlay: Noise Levels at the National Radio Astronomy Observatory
supporting dipole feeds and special types of invertedhorns. Bipods with guys, tripods, and quadripods ap-pear generally to offer more flexibility. A quadripod willbe built for the 140-foot telescope; this decision wasbased on consideration of stability, symmetry, andadaptability to a variety of feeds and receiver compo-nents. If desired at a later date, another type of feedsupport may be easily substituted.A great deal of thought was given to the side-lobe and
back-lobe effects of the feed support. In general practice,side lobes may be 20 db down from the central beam andback lobes may be 30 to 40 db down. For many installa-tions, e.g., in an aircraft or part way up the mast of aship, there are so many other nearby objects to increasespurious side lobes that attempts to refine the feed sup-port and reduce the inherent side lobes and back lobes
are not warranted. This situation does not pertain tolarge radio astronomy telescopes. If the 140-foot tele-scope will have 20-db side lobes, these will have ap-preciable directive gain. It is therefore desirable to re-duce the side lobes and back lobes to levels below thoseusually accepted. Studies of this problem have beenplanned, the results of which will be reported later.
ACKNOWLEDGMENTMany individuals have contributed to the work re-
ported herein, including those named in the text andreferences, and J. W. Findlay and D. S. Heeschen of theAUI staff. All share credit for bringing the radio tele-scope program to its present status, and we express ourthanks and appreciation to them and to the NSF staffmembers associated with the work.
Noise Levels at the National RadioAstronomy Observatory*
J. W. FINDLAYt
Summary-The measurements which have been made of thefield strengths of radio signals received at the Green Bank site of theNational Radio Astronomy Observatory are described. The site isrelatively free from interference and the measures which are beingadopted to preserve its radio quietness are discussed.
INTRODUCTIONT HE PURPOSE of this paper is to discuss briefly
the levels of man-made noise which exist at thelocation which has been chosen as the site of the
National Radio Astronomy Observatory.The smallest radio signals which can be detected in
radio astronomy are at present determined mainly bythe noise figures, stability, and bandwidths of receiversand by the sizes of the antenna systems used. In thefuture, however, it is reasonable to foresee that the usein receivers of low noise crystals, traveling wave tubes,or masers, and the development of larger antennas, willbring about the state of affairs in which the limit of use-fulness of a radio telescope will be set by the noise at theobserving site. Part of this noise comes from man-madesources. A part which will also become more importantas better receivers are developed, comes from radiationfrom the ground and surrounding objects entering thereceiving system of the radio telescope. This lattersource of noise is not considered in the present paper,
* Original manuscript received by the IRE, November 11, 1957.t Associated Universities, Inc., New York, N. Y.
since current receiver techniques are not yet so good asto make it a limiting factor in the performance of aradio telescope. However, fairly soon it will be an im-portant factor to consider in the design of future radiotelescopes. The present paper discusses first, the approx-imate levels of the smallest signals which have been de-tected so far in radio astronomy, next, the levels of noiseat the site of the National Radio Astronomy Observa-tory are described, and last, the measures which have beentaken to reduce to a minimum any future deteriorationof the site are discussed.
THE SIGNALS RECEIVED IN RADIO ASTRONOMYA survey of the results of various observers over a
wide range of frequencies allows of an order of magni-tude to be stated for the lower limit of the flux densitywhich is detectable with present day techniques and in-struments. This minimum detectable flux is about 10-25watts per square meter in 1 cps bandwidth. This is, ofcourse, a figure which is right only to an order of magni-tude, but it generally describes the results for frequen-cies from about 50 mc up to 10 kmc. Improvements ofan order of magnitude at least are to be expected whenresults from larger antennas and from new receiver tech-niques are available.However, the present figure illustrates the magnitude
of the problem of avoiding interference from man-made
PROCEEDINGS OF THE IRE
sources. A transmitter which radiates isotropically 1 ,.uwper cycle per second of bandwidth gives this minimumdetectable flux at a free space range of nearly 1,000,000km.Every available technique for shielding the radio
astronomy antennas by suitable choice of site and forreducing the side lobes and back lobes of the antennasmust be employed. Sources of interference which areacceptable in communication systems may require muchfurther screening and suppression if they exist near aradio astronomy site.
LEVELS OF MAN-MADE NOISE AT GREEN BANK
The choice of Green Bank, W. Va., as the site of theNational Radio Astronomy Observatory was made part-ly as a result of the low levels of radio signals received inthat area. During the site selection process, measure-ments of field strengths of radio signals were made at anumber of possible sites. These measurements weremade by Jansky and Bailey, Inc., of Washington, D. C.,using the equipment outlined in Table I. The resultsof
TABLE IEQUIPMENT USED FOR FIELD STRENGTH SURVEYS
FrequncyEuipmnt Anenna MinimumFrequency Equipmdent AXnutSendna | detectable fieldrange used used strengths
120 kc-18 mc RCA 308A 3 loop 20 ,v/meterField Intensity antennasMeter
20 mc-400 mc AN/URM-47 Resonant 1IAv/meter todipole 10 ,v/meter
375 mc-1000 mc Stoddart Field Resonant 40 Mv/meter toIntensity Meter dipole 200 ,v/meterNM50A
1.0 kmc-10.75 kmc AN/URM-42 Cone 400 ,v/meter toantenna 2000 ,v/meter
these measurements for Green Bank, in October, 1955,in the frequency range from 20 mc to 10 kmc are shownin Fig. 1.
It will be realized readily that the signals recorded inFig. 1 are very large compared with the low levels atwhich radio astronomy measurements are made. Never-theless, the primary purpose of the survey, which was toinsure that the site chosen suffered a minimum of inter-ference from signals generated at a considerable distancefrom the observatory, was very adequately fulfilled.Other measurements have been made at Green Bank
since the site has been occupied. Locke and Argyll fromthe Dominion Observatory of Canada, made measure-ments at Green Bank in March, 1957. The results of theirsurvey are given in Fig. 2. These measurements weremade with equipment which was capable of detecting asignal level approximately 20 db lower than the earliersurvey. This increase in sensitivity was acquired by us-ing an antenna of about 10 db gain and since this gain
was not very accurately known the increased sensitivityof the equipment was achieved at some loss of absoluteaccuracy in the results.An attempt was made in June, 1957, to detect radia-
tion from the overhead power lines in the neighborhoodof the radio telescope sites. A group of engineurs fromthe Westinghouse Electric Corp., Baltimore, joined withrepresentatives of the Monongahela Power Co. in mak-ing measurement near various typical overhead lines.The lines tested were carrying either 7200-v single-phaseor 13,200-v three-phase current on wooden poles about25 feet from the ground. Receiving equipment coveringthe frequency ranges 950 mc-2040 mc and 7.3 kmc-11.26 kmc was operated about 5 feet above the groundbelow the lines with a directional antenna pointed to-ward the lines. The minimum detectable field strengthswere about 50 microvolts per meter at 1200 mc and 500microvolts per meter at 11 kmc. No noise above theselimiting fields was detected, except in the special casewhen an arc was intentionally drawn from a 7200-voltline to earth. This produced a small signal, just audiblein the receiver noise, at frequencies near 1150 mc.The results of the foregoing experiments cannot, of
course, determine the limiting noise levels at the GreenBank site. However, all the evidence so far goes to sup-port the view that the site chosen is very free from elec-trical noise. Now that the site is occupied, more meas-urements will be made using the techniques of radio as-tronomy to study more closely the noise levels and todetermine, if possible, the sources of any noise that isfound.
THE PROTECTION OF GREEN BANKFROM FUTURE INTERFERENCE
A number of steps have been taken to insure that theobservatory site will be protected as far as possibleagainst any future increase in the levels of radio noise.Indeed, it is hoped that a continued program of inter-ference reduction will result in the site becoming morequiet to match the probable improvements of radioastronomical techniques.A total area of land of about 2700 acres is being ac-
quired for the observatory site. This quite large areawill give very adequate facilities for large antenna sys-tems, and it has the additional advantage of allowingparticularly sensitive instruments to be located wellwithin the perimeter of land under the control of the ob-servatory, and thus be isolated from local noise.
Starting from the perimeter, and working outwards,restrictive covenants are being acquired on the land ofadjacent land owners. These covenants are an agree-ment that no electrical equipment will be operated onthe land that causes interference to the instruments atthe observatory. The actual land area which will becovered by these covenants is not yet determined, butit is certain that very valuable additional protectionagainst interference will be obtained in this way.An important factor in deciding on the site for the ob-
Findlay: Noise Levels at the National Radio Astronomy Observatory
BRQIEN LINE INDICATES MINIMUMMEASURABLE SIGNAL_1
~tr1 h.IIII-e *a III* ..,,..I10 50 100 500
FREQUENCY IN MEGACYCIES
~tT I1K00 5000 s,0O
Fig. 1-Measured field strengths of radio signals received at Green Bank on a weekday in October, 1955.
t f -
Fig. 2-Approximate field strengths of radio signals received at Green Bank in March, 1957.
PROCEEDINGS OF THE IRE
servatory was that the State of West Virginia was pre-pared to introduce legislation for the protection of thesite from interference. A bill, known by short title as the"Radio Astronomy Zoning Act," was passed on August9, 1956, by the State of West Virginia. This bill restrictsthe operation, within 10 miles from the observatory, ofany unlicensed electrical equipment which generates in-terfering field strengths greater than certain prescribedlimits.The site of the observatory lies in a flat valley, at an
elevation of about 2700 feet, surrounded by ranges ofmountains rising about 2000 feet above the level valleyfloor. This topography was chosen since it provides thebest possible screening of the antennas from radiationwhich might reach them from distant radio transmit-ters. Although some such radiation does exist, as is evi-denced by Fig. 1 and Fig. 2, its effect in obscuring con-siderable parts of the frequency spectrum is not serious.However, to insure that the present situation should notdeteriorate in the future, specific measures to limit theincrease of licensed radio radiation into the valley areessential. Submissions, therefore, have been made to theFederal Communications Commission for the limitationof radiation from licensed radio transmitters. These sub-missions, which were presented by Dr. Lloyd V. Berk-ner, President of Associated Universities, Inc., and byDr. Alan T. Waterman, Director of the National ScienceFoundation, were based on the primary importance tothe United States of good fundamental research inradio astronomy. Protection of two kinds was requested.First, the existence of the radiation from neutral hydro-gen at a frequency of 1420 mc provides the only pres-ently known line radiation in radio astronomy. Thisradiation may be observed directly, or absorption ofradio waves of the same frequency can be studied. Theatoms responsible for the emission or absorption maybe in motion relative to the observers on the earth, andso the frequency observed may differ by a Doppler shiftfrom 1420 mc. There is, therefore, a very good case forclearing that part of the frequency spectrum near1420 mc from all transmissions. The FCC was asked toconsider clearing the frequency range from 1400-1427mc over the whole of the United States.The second kind of protection which was requested
was specific to the observatory at Green Bank. Since itis intended that the National Radio Astronomy Ob-
servatory should be a center for research open to quali-fied scientists from all institutions, an attempt shouldbe made to accord it particular protection.
Accordingly, the following requests were made to theFCC:
1) Flag all new applications which are made by oper-ators within 50 miles of Green Bank, except mobileoperations. Consult with a group of scientists des-ignated by AUI at Green Bank on any interferenceproblem which might arise from the new applica-tions and work out each problem on a case-by-casebasis. This same group should review interferenceproblems created by existing uses, and reach solu-tions if possible.
2) Exchange where possible tv channels 53 and 54with other uhf tv channels so that as far as possibleno tv transmitter operates on channels 53 and 54within 150 miles of Green Bank. (The frequencyconcerned here is 700-712 mc, the half-frequencyof the hydrogen line.)
3) Adopt the same procedure as in 2) for tv channel14. (The frequency concerned here is 472-476 mc,the one-third frequency of the hydrogen line.)
4) Require any tv station in channels 14, 53, and 54which remain within 150 miles of Green Bank toattenuate their harmonics to 80 db below carrierlevel.
These requests follow the pattern of extending theprotection of the hydrogen line frequency, by askingin 2), 3), and 4) for a restriction of radiation of the sub-harmonics of the line from stations near to Green Bank,and also suggest in 1) a means by which the effects ofinterference from licensed operators near Green Bankcan be reduced to a minimum.The measures which have so far been taken for pro-
tecting the Green Bank site will, it is hoped, result in themaintenance, and possibly the improvement, of an al-ready good site for radio astronomy in the UnitedStates of America.
ACKNOWLEDGMENTWe gratefully acknowledge the work of all who have
assisted in making the measurements of radio interfer-ence at Green Bank, and particularly Jansky & Bailey,Inc., Dr. Locke and E. Argyll, the Westinghouse Elec-tric Corp., and the Monongahela Power Co.