Simulated rain tests were carried out to determine the noise temperature contribution of liquid water adhering to the aperture cover material on both a standard DSN X-band feedhorn and on an S/X-band common aperture feedhorn. It was found that for the particular common aperture feedhorn tested, system noise temperature increases were much greater when the plastic horn cover material was old and weathered than when it was new. The age and condition of the aperture cover material is believed to be a major factor in the amount of degradation experienced by a telecommunications system during rain events

Clauss, R. C.

Franco, M. M.

Slobin, S. D.

English

NASA Technical Reports Server

N83 14332
TDA Progress Report 42-71 July - September 1982
X-Band Noise Temperature Effects of Rain on
DSN Antenna Feedhorns
S. D. Slobin and M. M. Franco
Radio Frequency and Microwave Subsystems Section
R. C. Clauss
TDA Technology Development
Simulated rain tests were carried out to determine the noise temperature contribution
of liquid water adhering to the aperture cover material on both a standard DSN X-band
feedhorn and on an S/X-band common aperture feedhorn. It was found that for the par-
ticular common aperture feedhorn tested, system noise temperature increases were much
greater when the plastic horn cover material was old and weathered than when it was
new. The age and condition of the aperture cover material is believed to be a ma/or factor
in the amount of degradation experienced by a telecommunications system during rain
events.
I. Introduction
At microwave frequencies above about 5 GHz, rain is con-
sidered to be the major contributor to large signal-to-noise
ratio (SNR) degradations in telecommunications systems. In
most temperate locales,- rain falls a total of about 5 to 7% of
the time (as measured by minute-sized sample periods). This
amounts to 400 to 600 hours per year (Ref. 1). For a location
whose average total yearly rainfall is 500 mm (Pasadena,
Calif.) this corresponds to a mean rain rate of about 1 mm/hr,
or a very light rain. Link degradation comes from both space-
craft signal attenuation and increased system noise tempera-
ture due to emission from the liquid water particles them-
selves. As an example, at X-band, 30-deg elevation angle, a rain
rate of 5 mm/hour (a moderate rain) will give an attenuation
of 0.4 dB, a system noise temperature increase of 25 K, and a
3-dB SNR degradation for a 30 K baseline system noise tem-
perature. The same rain at 32 GHz will give 9-dB attenuation,
220 K noise temperature increase, and an 18-dB SNR degrada-
tion. Fortunately, 5 mm/hour rain is exceeded only 0.2% of
the year (18 hours) at Canberra, the rainiest of the DSN track-
ing antenna locations. Again, fortunately, using the Canberra
example, fully half the total yearly rainfall (approximately
580 mm) is deposited at a rate less than about 3 mm/hour (a
rate exceeded only 0.5% of the year, 45 hours). For X-band,
the atmospheric effects of rain are expected to be small more
than 99% of the time.
It has been found, however, during tracking of spacecraft
that significant telecommunications link degradation occurs
during periods of even light rain. This degradation can last as
66
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ORIGINAL PAGE tS
OF POOR QUAtrTY
long as an entire pass (approximately eight hours) as has been
observed during a Voyager 1 Jupiter near-encounter pass over
DSS 43 (Canberra) in 1979. The rain rate during this pass was
measured by an on-site rain gauge to be quite low (approxi-
mately 1 mm/hour) but continuous for the entire day. All
antenna components were continually wet and this condition
contributed to significant and continuous link degradation.
The antenna components which were wetted, in particular,
were the plastic sheet covering the antenna feedhorn and the
metallic surfaces of the antenna reflector system, the parabo-
loid, hyperboloid, and reflex feed elements. The "attachment"
of rain to these various surfaces is of two types: "wetting" and
"non-wetting." As a common example of a "wetted" surface,
one may consider a common schoolroom blackboard that has
been washed off in order to remove chalk dust. The surface
character changes as the water fills the microscopic pores and
scratches of the board. The board becomes covered with a thin
sheet of liquid water. After a short while, the water evapo-
rates, leaving a dry, non-shiny surface. A "non-wetted" sur-
face is one such as a newly waxed automobile hood, which
when rained upon becomes covered with beads or drops of
water ranging in size from 1 mm diameter to as much as
25 mm diameter, approximately. It was found that the wetted
surface causes larger noise temperature effects than does the
non-wetted surface. This noise temperature increase can cause
serious problems during low signal level tracking periods, for
example, Voyager at Uranus and Neptune. It has thus become
necessary to ascertain the effects of rain on antenna compo-
nents and to propose methods of overcoming the resulting
degradation.
A previous study at the DSN Madrid tracking station
(Ref. 2) has shown a remarkable agreement between experi-
mental and theoretical results of noise temperature measure-
ments of sheets of water held in a container over the aperture
of both S-band and X-band horns. Significant levels of noise
temperature (100 K at X-band) and attenuation were mea-
sured for water layers as thin as 0.1 mm. Those tests did not
involve wetting of the feedhorn cover; the results presented in
this report are obtained from wetting of actual DSN antenna
feedhorns.
II. Experiment Description
An experiment was devised to test the effects of simulated
rain on the X-band feedhorns used on the DSN antennas. A
standard X-band feedhorn, an S/X-band common aperture
feedhorn, and an ambient load were connected to a maser as
shown in Fig. 1. The system noise temperature (Top) for each
horn path was calculated by comparing the power level with
that from the ambient load path. A Y-factor measurement
technique is used in which
vt ^\ power from ambient loadi (ratio)
power from horn
_ op, ambient load
f
op, horn
T + T
ambient maser
0)
where
op
ambient
op, horn
system operating noise temperature, K
ambient load physical temperature, K
maser noise temperature, K
The maser noise temperature is assumed to be 5 K for the pur-
poses of these calculations. The actual determination of T0
by Eq. (1) is insensitive to the value of Tmaser. Top includes
the cosmic background noise temperature (2.7 K), the atmo-
sphere (2.5 K, clear sky), horn and waveguide (approximately
11 K in this test), and maser (approximately 5 K). The maser
noise temperature itself can be determined from a separate test
in which a calibrated waveguide horn is attached directly to
the maser. Power levels are recorded looking at both the clear
sky and an aperture load. A Y-factor is determined:
Y = power level on aperture loadpower level on clear sky
op, load
op, clear sky
T + Tmaser p
T + T . + T + Tmaser cosmic aim horn
(2)
where
cosmic
= maser noise temperature to be determined
= cosmic background noise temperature (2.7 K)
T = physical temperature of aperture load and horn
Tafm = atmospheric noise temperature (2.5 K)
T. - noise temperature of calibrated horn (1.5 K)
The maser noise temperature is then found from
T - Y(T . +T + T. )
__ p v cosmic atm nom' (3)
The system operating noise temperature (Tg ) when look-
ing at the large S/X horn is determined from Eq. (1) for a
specific example using the following input values.
ambient = 290.2 K (17°C)
Y= 14.09 (11.49dB)
Then,
op, S/X = 20.95 K.
This value is typical of the clear sky noise temperature and
includes approximately 9K contributed by 127 cm of wave-
guide, orthomode transducer, waveguide bend, and curved
waveguide switch port.
The method of carrying out the rain tests was to measure
the degradation of each horn relative to the other horn, which
then serves to eliminate the effect of small changes in the
atmospheric noise temperature contribution.
A plastic spray bottle with a positive displacement (0.8 cm3)
pump was filled with distilled water and used to provide the
"rain" for these tests. Both horns were pointed vertically
(toward the zenith), which, however, is not a normal tracking
position. This position allowed a maximum of water to remain
on the horn, although there was no pooling of liquid in the
center of the aperture cover, due to the domed shape caused
by internal waveguide and horn pressurization. A normal
average tracking elevation angle might be 45 deg, in which case
some water is expected to run off the horn in worst-case con-
ditions. The spray bottle delivered 0.8 cm3 per spray, generally
in a large number of very tiny drops. Upon striking a surface,
many of these drops would coalesce to form 3-mm to 10-mm-
size drops on the surface, depending on whether the surface
was wetting or non-wetting.
The first test series was carried out on the large S/X feed-
horn with weathered Kapton (manufactured by Du Pont)
aperture cover material. This material was approximately four
years old at the time of the tests and had been exposed every
day to sunlight, rain, and blowing dust and sand (2 years at
Goldstone and 2 years at JPL). Water was sprayed on the
Kapton first on the center and then at increasing distances
from the center to create as uniform a coverage as possible.
After a given number of sprays, the water droplets on the
Kapton were smeared by hand. Then an additional 250 cm
(approximately) of water was poured all over the horn cover.
This amount of water caused runoff and represented a maxi-
mum water retention case for vertical pointing. It is expected
that more nominal pointing angles (45-deg elevation) would
not retain as much water. Power levels were measured at all
points in the experiment. These tests were repeated on new
shiny Kapton material placed over and touching the old
material. Clean, dry, new Kapton did not measurably increase
the noise temperature of the system. Table 1 gives the results
of these tests. It is seen that the weathered Kapton material
exhibits much higher noise temperature effects than does the
new material.
The film thickness of 100 sprays (80 cm3) of water smeared
evenly on an area 116.8 cm in diameter is 0.074 mm; 312 cm3;
of water make a film thickness of 0.29 mm. Smearing water
on a shiny surface does not distribute it evenly but merely
causes it to coalesce into larger drops or areas of water.
In addition to tests on the large S/X common aperture
horn, the small (18.4-cm-diameter) standard DSN feedhorn
was tested in a similar manner. The aperture material was
Kapton, in shiny, good condition. Table 2 presents results of
the small horn tests. Five sprays (4 cm3) on the small horn
deposited a substantial amount of water (the horn was very
wet); the noise temperature increase when smeared was mod-
erate (37 K). The ratio of horn areas is about 40 to 1; thus
4 cm3 on the small horn is equivalent to about 160 cm3 on
the large horn. The effects of water on Teflon (Du Pont)
material was measured on the small horn. A piece of Teflon
sheet (0.13 mm thick, 30 X 70 cm) was placed over the
horn. The clean dry Teflon did not measurably increase the
noise temperature of the system. System performance is
substantially improved by the use of Teflon as opposed to
the new shiny Kapton material for small amounts of water
that would be equivalent to conditions experienced during
light rainfall.
Another test compared the large and small horns when
equal water densities were applied to the aperture covers.
The diameter of the large horn is 116.8 cm, and of the small
horn is 18.4 cm. The area ratio is 40.3. Numerous single-
spray measurements were made on the small horn. Each
single-spray covered most of the aperture of the small horn
with water drops. Forty-spray measurements were made on
the large horn with both the weathered aperture material and
the new Kapton material. Table 3 presents the results of these
68
tests. These results are more or less consistent with" thejejults.
shown in Table 1 for the large horn. The one-spray .value in-
Table 2 for the small horn is about 70% higher than for this
test. This may be due to a difference in water particle dis-
tribution in the two cases. The fact that the large horn/new
Kapton increase of 3.74 K (Table 3) is double that of the
small horn/new Kapton value (1.80K), for the same water
particle density, may be attributable to the fact that the
phase center of the large horn is located 171.5 cm from
the aperture, whereas the phase center of the small horn is
only 3.6 cm from the aperture. Perhaps differences in aper-
ture electric field strength create the measured difference.
III. Noise Temperature of S/X Common
Aperture Feedhorn
The noise temperature contribution of the S/X common
aperture feedhorn was determined by comparing the sys-
tem noise temperature of this horn with that of a standard
X-band reference horn (not the DSN X-band horn). This
reference horn was carefully calibrated by the National
Bureau of Standards and was found to contribute a noise
temperature of 1.5 K. Using the DSN horn as a baseline, the
S/X and reference horns were alternately connected to the
same waveguide switch port and common waveguide run.
;. =The -results indicate that the S/X common aperture horn (with
^,^1.2.7-cm round aluminum waveguide/rotary joint, 10.2 cm
quarter-wave polarizer, and 0.13-mm-thick Kapton) is 1.14K
hotter than the NBS reference horn with 0.025 mm Kapton.
IV. Conclusion
The series of tests described here indicates the seriousness
of the noise temperature effects due to water on the aperture
covers of DSN feedhorns. Noise temperature increases in
excess of 30 K begin to degrade the telecommunications link;
increases of 100K or more generally result in loss of space-
craft signal.
It has been shown that weathered Kapton aperture cover
material exhibits greater rain-induced degradation than does
new, unweathered material. New Teflon exhibits less degra-
dation than new Kapton for small amounts of water. The
aging and weathering properties of Teflon are not presently
known. A careful review of data taken during these prelim-
inary tests shows that additional work is needed to deter-
mine methods of reducing the rain-induced degradation
using passive means. These tests indicate that large improve-
ments in spacecraft communications can be made in the
DSN during periods of rain.
References
1. Potter, P. D., et al., "A Study of Weather-Dependent Data Links for Deep Space
Applications," Technical Report 32-1392, Jet Propulsion Laboratory, Pasadena,
Calif., Oct. 15,1969.
2. Urech, J. M., "Preliminary Report on DSN System Performance Under Local Weather
Effects," DSN Progress Report 42-52, Jet Propulsion Laboratory Pasadena, Calif.,
Aug. 15, 1979.
69
ORIGINAL PAGE 13'
QJF POOR QUALITY
Table 1. Results of simulated rain tests on S/X common aperture feedhom with old and new
Kapton aperture cover material
Test
X-band noise temperature increase, K
Old Kapton
4. Water poured on
(0.074 mm thick)
141.16 K (312 cm3 total,
0.29 mm thick)
188.11 K (910 cm3 total,
0.85 mm thick)
New Kapton
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
1.
2.
3.
1.
2.
3.
Clean, dry
One spray in center (0.8 cc)
2 sprays total
3 sprays total
4 sprays total
5 sprays total
6 sprays total
7 sprays total
8 sprays total
9 sprays total
10 sprays total, 38 cm square
Smeared by hand, 46 cm square
Clean, dry
30 sprays total (24 cm3)
30 sprays total smeared 90 cm
diameter
Clean, dry
100 sprays total (80 cm3)
100 sprays total smeared
0.00
1.31
2.07
3.55
5.00
6.98
7.67
8.76
9.68
10.20
11.04
12.22
0.00
25.78
28.16
0.00
54.20
43.93
0.00
0.33
0.75
0.89
1.10
1.52
1.81
1.95
2.17
2.75
3.49
4.26
0.00
4.34
4.96
0.00
_
9.59
112.81 K (397 cm3 total,
0.37 mm thick)
70
Table 2. Results of simulated rain tests on standard DSN
feedhom (18.4 cm diameter) with and without Teflon sheet
on top of Kapton aperture material
X-band noise temperature
increase, K
1.
2.
3.
4.
5.
6.
7.
lest
Clean, dry
1 spray total (0.8 cm3)
2 sprays total
3 sprays total
4 sprays total
5 sprays total
5 sprays smeared
Horn with
Teflon
0
0.43
3.35
9.59
7.33
11.39
-
Horn without
Teflon
0
3.04
6.73
8.49
13.17
14.50
36.73
Table 3. Equal drop-density comparison of large and small
feedhorn noise temperature increase
X-band noise temperature
increase, K
Small horn
New shiny Kapton
1 spray average
Large horn
Weathered Kapton aperture material
40 sprays
Large horn
New shiny Kapton
40 sprays
1.80
35.09
3.74
71
ORIGINAL PAGE 18
Of POOR QUALfTY
S/X-BAND COMMON APERTURE
FEEDHORN, 116.8cm DIAMETER
APERTURE LOAD
NBS-CAUBRATED
REFERENCE
FEEDHORN
AMBIENT LOAD
FOUR-PORT
WAVEGUIDE
SWITCH
DSN X-BAND
FEEDHORN,
18.4cm DIAMETER
MASER POWERMETER
Fig. 1. Experimental setup for testing rain effects on X-band feedhorns
72


X-band noise temperature effects of rain on DSN antenna feedhorns

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