NATIONAL RADIO ASTRONOMY OBSERVATORYGreen Bank, West Virginia

Electronics Division Internal Report No, 37

STRIP OF SKY AND LOW FREQUENCY SYSTEMSON THE 300-FOOT TELESCOPE

Dewey Ross and Yervant Terzian

OCTOBER 1964

NUMBER OF COPIES: 75

THE STRIP OF SKY AND LOW FREQUENCY SYSTEMSON THE 300-FOOT TELESCOPE

Dewey Ross and Yervant Terzian

Introduction

The purpose of this report is to furnish those people involved in the "Strip of Sky",

"Normal Galaxy Observations" and "Observations of H II Regions" with a record of equip-

ment used and the specification of that equipment. In this report the individual systems

and feeds are discussed as well as the parameters of the beams for each system. Also

the difficulties for the first attempt of installing a multifeed system on the 300-foot tele-

scope are discussed.

II. Installation

Installation of the Strip of Sky equipment, which consisted of three 1410 MHz units

and one 750 MHz unit, was begun on December 3 0, 19 63 and completed January 2, 19 64.

The Low Frequency Systems, one 234 MHz unit and one 405 MHz unit, were installed on

February 4, 19 64. The low frequencies were selected at 405 and 234 MHz from a survey

done by S. von Hoerner (Internal Report, NRAO, 1963) on the level of noise and inter-

ference between 2 16 and 420 MHz. The two selected frequency regions had the minimum

percentage of interference.

III. Mechanical Mounting of Feeds

The mounting was made with the 750 MHz unit and one 1410 MHz unit located in

the center box, while one 14 10 MHz unit was mounted NW of the center and one 14 10 MHz

unit was mounted SE of the center.

The center systems utilize the original frontend box which has been used on all

systems mounted in the past at the 300-foot telescope.

The East and West 14 10 MHz feeds are attached to the center box by means of a

2-inch aluminum angle. Such mounting enables one to focus all Strip of Sky systems at

the same time.

2

The above-mentioned mounting angles are so made to allow the East and West

1410 MHz feeds to be positioned 2. 63, 5.24, 7.88 and 10. 52 inches North or South of

the center. The East 1410 MHz system is variable between 18.30 and 26.28 inches

East of the center, while the West 1410 MHz system is variable between 18.40 and

26.28 inches West of the center. The measurements correspond to beam positions of

5 9 10, 15 and 20 minutes of arc North and South and from 35 to 50 minutes of arc East

and West.

The East 1410 MHz system beam position was set for 37. 6 minutes of arc East

and 5 minutes of arc North of center. However, after positions were run it was found

to be 37. 6 minutes of arc West and 6 minutes of arc North of center.

The Strip of Sky feed positions are shown in Figures 1, 2, and 3.

The two Low Frequency feeds (234 and 405 MHz) are mounted SW and NE of the

center feed.

The 234 MHz feed is 21. 1 inches East and 28.3 inches North of the center feed.

The measured beam position is 55.0 minutes of arc South and 51.3 minutes of arc West

of center.

The 405 MHz feed is 24. 5 inches West and 24.2 inches South of the center feed.

The measured beam position is 50.0 minutes of arc North and 50.0 minutes of arc East

of center.

The helical low frequency feeds were originally constructed for 450 and 250 MHz.

Due to the fact that helical feeds are broad-banded, the available low frequency feeds

were adopted for the present observations. These feeds were not exactly focused be-

cause no space was available for their mechanical motion on the 300-foot focus. These

two effects had a net result of lowering the beam efficiencies.

Table 2 gives the ratio of Tcal/Eb

for all systems derived in the way described

in section VII. Tcal is the antenna temperature of the thermal calibration and E is the

beam efficiency. The thermal calibrations for the two low frequency systems were

measured three times and the results gave 19.00 ± 0.92 degrees for the 405 MHz system

and 18.95 ± 0.42 degrees for the 234 MHz system. The beam efficiency at 405 MHz was

sixty percent and at 234 MHz fifty percent. Table 1 shows the mean thermal calibrations

for the three 1410 and the 750 MHz systems. These latter thermal- calibrations have an

3

error of about degrees. Due to the uncertainty of T the measured values werecalnot used directly. To obtain the brightness temperatures of any observed point the ratio

of T /Eb was computed for each system by observations of standard sources as it is

described in section VII.

Table 1 also includes the bandwidths, radiometer temperatures and the measured

detector law (a) for each system.

IV. Mechanical. Mountin. of Frontend Assemblies

The center systems (750 and 1410 MHz) are mounted in the original frontend box

which has been used for all frontends previously mounted on the 300-foot telescope.

The East 1410 MHz system is mounted SE of the center box and extends beyond

the "catwalk" approximately 6 inches. It is supported by two pieces of 3-inch aluminum

channel which are attached to a 2-inch aluminum angle, mounted on the frontend box, by

means of four 5/8 inch bolts.

The West 14 10 MHz system is mounted NW of the center box in the same manner

as the East box.

The low frequency box, which houses both the 234 MHz and the 405 MHz systems

is mounted on the "catwalk", near the South feed support leg.

V. Feeds

234 MHz -- A helical feed, which had a VSWR of 1. 02:1 at the center frequency

and a VSWR of 1.3:1, 2 MHz on each side of center frequency. The feed was tuned by

means of a coaxial stub stretcher which was made from RG-9. A drawing is shown in

Figure 4.

405 MHz A helical feed, which had a VSWR of 1.02:1 at the center frequency

and a VSWR of 1.3:1, 2 MHz on each side of center frequency. The feed was tuned by

means of a coaxial stub stretcher which was made from RG-9. A drawing of the feed is

shown in Figure 4.rif-D MHz and 1410 MHz center: The Jasik Type .27 L,SN 3. At 750 MHz the VSWR

is 1. 15:1 and less than 1:2 from 690 MHz to 775 MHz.

At 14 10 MHz the VSWR is 1.08:1 at the center frequency and less than 1.2:1 from

1365 MHz to 1465 MHz.

4

14 10 MHz East and 1410 MHz West -- Both feeds were built by the NRAO

Electronics Division machine shop and have a VSWR of 1. 15:1 at the center frequency

and less than L 2:1 between 1390-1425 MHz. A drawing of these feeds is shown in

Figure 5.

VI. Electronics

234 MHz -- The 234 MHz system is an RF switched system and uses an argon

source fed through a directional coupler and attenuator for calibration. Two calibration

levels are made available by the use of coaxial relays.

The mixer is an Empire Devices single-ended mixer and the preamplifier is an

LEL IF 31BS which was tuned to a 4 MHz bandpass. The noise temperature of the sys-

tem including feed cable is 1000 °K.

A block diagram of the system is shown in Figure 6.

405 MHz -- The 405 MHz system is an RF switched system and uses an argon

source fed through a directional coupler and attenuator for calibration signal. Two cali-

bration levels are made available by the use of coaxial relays.

An LEL Model UBC-3 mixer-preamplifier, tuned for 4 MHz bandwidth, was used.

The system noise temperature, including feed cable is 875 °K.

A block diagram of the system is shown in Figure 6.

750 MHz -- The 750 MHz system is an RF switched system. Two systems were

used. The initial system utilized the low noise mixer with a ceramic tube pre-amplifier.

(This unit is covered in detail in Electronics Division Internal Report No. 8, "Ceramic

Tube - Low Noise Front Ends" by Joe Carter.)

The system noise temperature is 470 °IC and the bandwidth is 6 MHz.

On March 12, 19 64 this system was removed and a Micro State Electronics tunnel

diode amplifier and a LEL Model UCC-3 mixer-preamplifier were installed.

The system temperature is 450 °K and has a bandwidth of 8 MHz.

The calibration system is the "poor man's calibration system" which is covered

in Electronics Division Internal Report No. 26 entitled "750 Mc-1400 Mc Receivers at the

300-Foot Telescope" by D. Ross. A block diagram is shown in Figure 7.

5

1410 MHz center -- The 1410 MHz center system is a RF switched system. The

MPC L-band parametric amplifier (covered in Electronics Division Internal Report No.

25 entitled nMicrowave Physics Corporation L-Band Parametric Amplifier' by B. Hansson

and B. Pasternak) and a LEL Model LAC-3 mixer-preamplifier were used.

The system temperature including feed cable is 190 °K and the bandwidth is 8 MHz.

The calibration system is the npoor man's calibration system". A block dia-

gram is shown in Figure 8.

1410 MHz East -- The 1410 MHz East system is a RF switched system. The AIL

Model 2877 parametric amplifier (covered in Electronics Division Internal Report No. 9

entitled "Parametric Amplifier - Airborne Instruments Laboratory" by D. Ross) and a

LEL Model LAC-3 mixer-preamplifier were used.

The system noise temperature including the feed cable is 138 °I< and the bandwidth

is 8 MHz.

The calibration system is the "poor man's calibration system". A block diagram

is shown in Figure 9.

1410 West -- The 1410 MHz West system is a RF switched system. The AIL

Model 1930A parametric amplifier (covered in Electronics Division Internal Report

No. 9) and a LEL Model LAC-3 mixer-preamplifier were used.

The system temperature including the feed cable is 210 °K and the bandwidth is

8 MHz.

The calibration system is the "poor man's calibration system". A block diagram

is shown in Figure 10. Figure 11 shows the standard receivers of the multifeed system.

VII. Beam Parameters and Standard Sources

During the course of the observations, the shapes of all the beams were deter-

mined, either by making observations of a strong point source day after day at a slightly

different declination, or by moving the telescope north and then south successively

while a strong source was in transit.

The central beams were found to be fairly symmetrical, but each of the 1410 E

and 1410 W beams had one pronounced sidelobe. This was expected due to the fact that

the feeds of these two systems were off-center. The low frequency beams were slightly

6

symmetrical since the feeds were also off-center. No sizable sidelobe was present in

these cases. The exact orientation of all the beams relative to the central ones is given

in Table 1. The half-power beamwidths (HPBW) of the main beams in declination and

right ascension were measured from the contour maps of the beams and are given in

Table 2. Figure 12 shows the three 14 10 MHz beams.

Daily observations of two or more standard sources were observed with each sys-

tem. A thermal calibration was taken before and after each observation. The standard

sources were normalized to the thermal calibrations, and a least square linear fit was put

through the points. (These were performed with the cooperation of M. DeJong.) The

source 3C 71 was adopted as a standard source for the 1410 C and 750 MHz systems.

The source 3C 196 was used for the 1410 E and 1410 W MHz systems, and 3C 353 was

used for the low frequency systems. The flux densities of these sources were obtained

from Conway, Kellerman, and Long 19 63, M. N. 125, 2 6 1). Due to changes of the thermal

calibration at 7 50 MHz, three values of the normalized deflections of 3C 7 1 were obtained.

Table 2 includes the values of the normalized deflections of the standard sources D /Dst cal'

together with the standard deviation of the mean am . The adopted flux densities and the

calculated ratios of the calibration temperature Tai

to the beam efficiency E b are also

given in Table 2.

In order to calculate the flux density of an extended source we need to know its

brightness temperature distribution. The brightness temperature T of any observed

point s is

D Ts

cal

ED

cal b

In the case where standard sources are used, the ratio Tcal/Eb

can be computed

directly with the knowledge of the flux densities and normalized deflections of these

sources. The HPBW of the beam must also be known. The flux density of a source

SA is given by

2kSX= T

bsource

(1)

(2)

calS

X stX2 ( D

s E

b

2k D

cal

11.133 Oce 0

6(5)

7

where k is the Boltzmann constant and X is the wavelength. In terms of the antenna

temperature Ta and the beam efficiency E13 9 we can write for the flux density

2k 1 Td.3x x2 E

source

Considering a standard point source, its maximum antenna temperature Ta, st

is the normalized peak deflection of the source Dt /D ,times the experimentally mea-

s calsured temperature of the thermal calibration T ear Hence, the flux density of a standard

point source can be written as

2k T

cal I st dC2sX st A.2 E

bD

calmain beam

The main beam solid angle can be represented by a Gaussian function, and can be written

as 1. 133 ea

0

6' where Oa and 0

6 are the HPBW in right ascension and declination.

Therefore, we can write

(3)

(4)

The half-power beamwidths of the main beams were measured directly from the

contours of the beams. In the case of the two off-center 1410 MHz beams, a correction

was made to the half-power beamwidths of the main beams in order to include the contri-

bution of the sidelobes in deriving the flux density of extended sources. By integrating

the sidelobes and the main beams separately, the contribution of the sidelobe for the

1410 E system was found to be 4. 6 percent, and that of the 1410 W system 3. 6 percent.

Having calculated T cal/Eb

from equation (5) we can then obtain the brightness tempera-

ture Tb of any observed point with the knowledge of its normalized deflection from equa-

tion (1).

- 8 -

VIII. Summar and Remarks

Installation of the Strip of Sky systems was begun on December 30, 1963 and

all systems were made operational on January 2, 1964.

A great deal of observing time was lost during the month of January due to insta-

bility of the equipment.

The cause of the instability was never determined. However, on February 1, 1964,

the original switch, switch drivers, and gain modulators (switch was an AEL Model SNB-

690A, diodes in same direction; switch driver was solid-state driver described in Elec-

tronics Division Internal Report No. 27 entitled "400 cps Solid State Switch Driver" by

Hermann von Hoerner) were changed to the standard type components (switch was an AEL

Model SNB-634A, one diode r -r ed); switch driver (standard tube type) on the 750 MHz

system and the 14 10 MHz center system.

The changes eliminated the instability of the two systems mentioned above; however,

it still existed, but not as bad as before, on the East and West 1410 MHz systems.

On February 25, 1964, the East and West 1410 MHz systems were modified in the

same manner as the 750 MHz and 1410 MHz center system, thus eliminating the instability

problems on all systems.

After a series of tests in the lab (insertion loss as a function of frequency; VSWR

with the switches locked in one position, then the other; and VSWR with the switch switching

at a 400 cycle rate) these same switches, switch drivers, and gain modulators were

operated on the Little Big Horn receiver. Both lab and receiver tests have shown that

the systems used in the original installation were as good as the standard system, which

was used for replacement. Therefore, we can almost, without doubt, say that the in-

stability problems were as a result of either the coupling of switch driver outputs within

the cable or ground loops. Steps are being taken to eliminate both problems.

On February 4, 1964, the low frequency systems were installed. During the first

week of observations a great deal of time was lost due to interference, some of which was

at the signal frequencies and a great deal at the intermediate frequency (30 MHz).

The amount of interference was reduced by installing a high pass filter (200 MHz)

in the 234 MHz system and a bandpass filter in the 405 MHz system.

9

Results of the observations show that 14.60 percent of the total observing time

on 234 MHz was lost due to interference while the time lost on the 405 MHz system was

4.02 percent.

A systematic record of the types of interference was taken over a 6-day period

(Monday-Saturday) on 234 MHz and 405 MHz. The results are shown below.

Cause of Percent of Inter-Interference

Aircraft - - - - - -

Traffic (ignition)-

Lightning - - -

Unknown - - - - - -

ference Time

39

29

3

29

Direct analog records were made on a 6-channel recorder. Simultaneously,

the output was recorded digitally on punched paper tape. Also, a printed record was

made at the same time. Six outputs were recorded every 10 seconds, one for each

channel.

Tab

le 1

Par

amet

ers

of th

e R

ecei

vers

and

Pos

itio

ns o

f th

e A

nten

na B

eam

s

J)(M

Hz)

X(c

m)

Ban

d-w

idth

( MH

z)

Syst

emte

rn-

pera

ture

'X

Tcai

(deg

rees

)a

Bea

m P

osit

ions

Rel

ativ

e to

the

Cen

tral

Bea

ms

1410

-C•

21. 3

•8

190

7. 5

1. 6

9C

entr

al b

eam

(fe

ed-h

orn)

1410

-E21

. 38

138

6. 6

1. 7

437

. 5' W

6. 0

' N(f

eed-

horn

)

1410

-W2

13

821

07.

.61.

72

37. 6

' E10

.9' S

(fe

ed-h

orn)

750

40. 0

a) 6

a) 4

709.

71.

7i

Cen

tral

bea

m (

dipo

le f

eed)

b) 8

b) 4

50

405

74.1

•4

•875

19.0

1.72

50. 0

1 E(h

elic

al f

eed)

50. 0

' N

234

128.

24

1000

18.9

1.71

5 L

3' W

55 O

S (

heli

cal f

eed)

.r

Tab

le 2

Par

amet

ers

of th

e B

eam

s an

d S

tand

ard

Sou

rces

v(M

Hz)

HPB

W S

HPB

WSt

anda

rdso

urce

D st/D e

al

' u m

S 1)

-2-6

--1

10W

mH

z

T/E

cal

b(IC

)

1410

-C10

. 010

. 0'

3C 7

10.

613

±0

009

5. 4

615

. 23

1410

-E10

. 8'

10. 0

'3C

196

1. 6

52 ±

0. 0

2714

. 886

13.6

4

1410

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12.0

?3C

196

1. 4

07 ±

0. 0

1814

.886

14.2

9

a) 1

.232

± 0

. 008

a) 1

0,29

(Feb

. 2 -

Mar

. 11)

750

18. 8

'18

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3C 7

1b)

O. 9

73 ±

O. 0

12( M

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1 -

Apr

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7. 4

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3

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(Apr

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Apr

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68

405

451

43'

3C 3

534.

396

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. 041

130

31,6

7

234

67'

65'

3C35

36.

901

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184

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