noise levels at the national radio astronomy observatory

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  • 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

    1958 35

  • 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 pr

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