The dichronic subreflector for the Rosman 2 dish antenna is described. Tests were conducted to make focusing adjustments, to measure the system parameters indicative of a focused condition for the Cassegrain feed, and to determine losses for the focal point feeds due to the presence of the dichroic

Kaiser, J. A.

English

NASA Technical Reports Server

TECHNOLOGY ACCOMPLISHMENTS, 1970 163
DICHROIC SUBREFLECTOR FOR ROSMAN II
Julius A. Kaiser
This paper reports on the dichroic subreflector of the Rosman II
antenna. Figure 1 is a picture of the Rosman II dish, which has both
focal-point and Cassegrain feeds. The rear of the ground screen on which the
focal-point feeds are mounted can be seen between the square feed box and
the solid metal subreflector, which was used with the Cassegrain feed
operating at 4 and 6 GHz. In order to use the focal-point feeds in this
system prior to installing the dichroic subreflector, it was necessary to
remove the subreflector and to replace it for Cassegrain feed use. The
turnaround time required for this was several hours and required that riggers
be available whenever the changeover had to be made. The risks to
personnel and equipment involved in removing and replacing the sub-
reflector, and the personnel complications associated with changes in shifts
made this operation somewhat undesirable.
A dichroic subreflector functions as shown in Figure 2. It is virtually
transparent to a baseband of frequencies and the focal-point feeds look
through it as though it were not there. At the Cassegrain feed frequencies,
on the other hand, it is highly reflective, which permits normal Cassegrain
operation. Simultaneous operation with both focal-point and Cassegrain
feeds is thus provided without the need of removing the subreflector.
Also shown in Figure 2 is a picture of the 3.35-m dichroic subreflector,
developed at GSFC, for the Rosman 1I antenna system. It is a sandwich
construction, 1.9-cm thick, and consists of two laminated epoxy glass skins
1-mm thick separated by a phenolic honeycomb core. Visible on the front
surface is an array of circular copper discs. On the opposite surface is a
similar array. These two surfaces were designed to give maximum
reflectivity in the 4-GHz portion of the C band. Since the arrays are
resonant, some deterioration in performance is to be expected in the 6-GHz
portion of the band.
The dichroic subreflector was installed in the Rosman II dish during the
latter part of last November and is still in operation. Some solar heating of
https://ntrs.nasa.gov/search.jsp?R=19720018145 2020-03-17T03:53:00+00:00Z
164 GODDARD SPACE FLIGHT CENTER
the diehroic was encountered in the dish. This was eliminated by painting
the surfaces with white epoxy paint so that if one were to see the dichroic
now it would be very similar in appearance to the solid metal subreflector
which it replaced. Tests made to date have been directed at (1)making
focusing adjustments and measuring those system parameters which would
be most indicative of a focused condition for the Cassegzain feed and, to a
lesser extent, (2)determining losses for the focal point feeds due to the
presence of the dichroic.
The C-band test results were derived from measurements of radiation
patterns, noise temperatures and carrier-to-noise ratios at 4.1 GI-Iz, and the
relative transmitter power at 6.3 GHz that was required to produce a 1-MHz
tone in the ATS 3 satellite. This latter measurement, when compared to
transmitter power level required in operations with the original solid metal
subreflector, provides a measure of loss at the transmitter frequency
attributable directly to the dichroic subreflector. The measured loss at
6.3 GHz was 1.3 dB at the best focus position.
The increase in noise temperature caused by the use of the dichroic
over that caused by the use of the solid metal subreflector arises from
resistive losses in the dichroic. The measured increase in noise temperature
for the dichroic over that for the original metal subreflector at 4.1 GI-lz was
approximately 8 K, corresponding to a resistive loss of about 0.1 dB.
There is also a reactive or scatter loss at 4.1 GHz which must be added
to the resistive loss to account for the total loss in gain when the dichroic
is used. This scatter loss does not, in general, contribute to noise temperature
but does represent energy not collimated usefully by the antenna. With the
ATS 3 used as a source, the carrier-to-noise ratio was measured. When this
parameter is related to the noise temperature of the system, a gain figure
can be established. If the gain figure obtained using the dichroic is compared
with that obtained using the solid metal subreflector, the total gain loss due
to the dichroic can be deduced. At the best focus position, this gain loss was
0.3 dB at 4.1 GHz. It was felt at the time those measurements were made
that even better performance could have been obtained with more time
available for adjustments.
For the focal-point feeds, abbreviated measurements of loss due to the
dichroic were made at 137,400, and 1700 MHz and compared to that with
TECHNOLOGY ACCOMPLISHMENTS, 1970 165 
no subreflector at all. Gain degradation at 137 and 400 MHz was virtually 
unmeasurable. At 1700 MHz, the measured value was under 1 dB. 
We have heard that the focal point feeds have been used more 
extensively since the installation of the dichroic. The deterioration in 
performance associated with use of the dichroic is believed to be more than 
offset by the increase in operational availability of the system. 
K X A L  Y U I N f  ttto 
ETALLlC SUBREFLECTOR 
I Figure 1-Rosman I1 antenna. 
166 GODDARD SPACE FLIGHT CENTER 
Figurc 2-Dichroic function. 


Dichroic subreflector for Rosman 2

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server