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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
-W10
.2'
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
. 8'
3C 7
1b)
O. 9
73 ±
O. 0
12( M
ar. 1
1 -
Apr
. 5)
7. 4
1b)
13. 0
3
c) 1
. 460
± 0
. 026
(Apr
. 5 -
Apr
. 14)
)8.
68
405
451
43'
3C 3
534.
396
± 0
. 041
130
31,6
7
234
67'
65'
3C35
36.
901
± 0
. 052
184
37. 9
0
/9.7
7/9
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