Document text
General Disclaimer
One or more of the Following Statements may affect this Document
• This document has been reproduced from the best copy furnished by the
organizational source. It is being released in the interest of making available as
much information as possible.
• This document may contain data, which exceeds the sheet parameters. It was
furnished in this condition by the organizational source and is the best copy
available.
• This document may contain tone-on-tone or color graphs, charts and/or pictures,
which have been reproduced in black and white.
• This document is paginated as submitted by the original source.
• Portions of this document are not fully legible due to the historical nature of some
of the material. However, it is the best reproduction available from the original
submission.
Produced by the NASA Center for Aerospace Information (CASI)
FACIUTY FORM 602
«••••••••••
• •••••••••a
• •••••••••a
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
aaaaaaaa a*a*
*a*»*a*a*a*a*a*»*a***
• • a • a a*a a a*a*
a a a a a a a a*a a*
MSC INTERNAL NOTE MSC-EE-R-68-I6
ANALYSIS «:.F FM DEGRADATION BY MULTIPATH
(ACClSsiONNUMBERf
^
(PAGES)
(NASA <;r or TMX OR^AD NUMBER)
(THRU)
---/
(CODE)
-Jjl
(CATEGORY)
J
'■P
^ :
• i
, ^
C3
Ut
c,-> ■*
la-* **»
■ 5,
cc a
S1&
m
MANNED SPACECRAFT CENTER
HOUSTON. TEXAS
*a*a*a*a* • • • •
a a^a aaaaaaaaa
aaaaaaaaraa
MSC INTERNAL NOTE MSC-EE-R -68-lb
Project Apollo
AH\LYSIS OF m DEGRADATION BY MULTIPATH
Prepared by
A/
arles V. Spencer
Approved by
^ Joe Fowler, Head
RF Communications Section
Approved by
E. L. Chicoine, Chief
Electromagnetic Systems Branch
Approved by
L. E. Packham, Ass't. Chief for
Electronic Systems
Approved by
Ralph S. Sawyer, Ch^ef
Instr. & Elec. Sys. Div.
Instrumentation and Electronic Systems Division
Ifetional Aeronautics and Space Administration
Manned Spacecraft Center
April 1968
FM IJiCGRALATION Fi MULTIBVIH
This report Invest igaces the effects of multipath in an enclosed
chamber, such as the orbital worVshop. At VHF frequencies, (^00 MHz in
this test), it was believed that multipath, through sideband cancellation,
could cause distortion of a transmitted signal in the orbital workshop.
The RF Communications Section of the Electromagnetic Systems Branch
(instrumentation and Electronic Systems Division) has conducted the
following tests in order to determine what degradation, if any, there is
of signal due to multipath at these frequencies.
Laboratory results are presented here which show spectrum distortion
due to multipath and the rer olting transmitted signal (utilizing square
wave modulation). The test was conducted in the RF Communications Labo-
ratory in Building l 4 . Existing facilities were made available and used
throughout .
When a transmitted signal is received via two or more paths (multi-
path) at a receiver, the multiple of signals may add or subtract depending
on their relative phases. The relative phases of the signals depend on
their relative path lengths between the transmitter and the receiver.
A preliminary test, simulating actual conditions expected in the
orbital workshop, was conducted by the RF Communications Section in
Building l 4 . An enclosed cylindrical tunnel (Laser Experimental Tunnel
of IESD) 13 feet in diameter and 280 feet in length was utilized in
order to simulate the enclosed chamber. There were several random objects,
extruding and intruding, within the enclosure such as one mi^t expect
to exist in the workshop.
2
•Die necessary equipment in this test included a Standard Signal
Generator for transmitting the desired FM signai^ a Spectrum Analyzer for
receiving and displaying the transmitted signal, and a camera for recording
the various spectrums. Portable euitennas were also used at both the
signal generator and the spectrum analyzer. Figure 1 shows the equipment
set up. The test consisted of photographing the various spectral displays
generated by multipath as the antennas were positioned at numerous sites
within the chamber. It was concluded after a considerable number of trials
that there was no place within the tunnel that multipath would completely
cancel out the transmitted signal. This condition could in no manner be
forced on the system during the test.
In order to reproduce these spectrums in the laboratory, the trans-
mitted signal was hard lined through two separate paths between the trans-
mitter and receiver. Ihe lab test configuration is shown in Figure 2.
An adjustable line was utilized in order to produce phase variations in
the two paths. Throughout the test, the transmitter was frequency modulated
with a square wave at a repetition rate of 4.2 KHz. A constant deviation
of 500 KHz was maintained, !Ihe test procedure was as fo3J.ows;
a. Initially, the carrier was applied only. Attenuators and an
adjustable line were adjusted so that the carrier was nulled (zero amplitude)
on the spectrum analyzer. It was evident from this procedure that the
attenuation over the two paths was equal, and one path was an integral
number of wavelengths plus I80® longer than the other.
b. ‘Hie carrier was modulated (utilizing a square wave) and the
spectrum adjusted for the waveforms illustrated ir, Figures 3 and 4.
c. spectrum v;as made symetrical about the center frequency by
adjusting the line. Photographs were recorded of the sj>ectral display and the
output signal (see Figure 3D and 4 d).
d. Several multipath conditions, such that portions of the spectrum
sidebands were attenuated, were displayed on the aneulyzer. Ihe conditions
were chosen so as to represent the possible phase shifts that might be
encountered at the receiver. 'Ihese spectral displays and the resulting
output signals were photographed and are illustrated in Figures 3A. through
3G and Figures kA through 4G respectively.
As indicated, this part of the test was performed in order to
reproduce a periodic sampling of multipath conditions which were recorded
in the preliminary test of this report. Figure 3D is the resulting spectrum
when the spectral sidebands are equal in magnitude that is, there is no
phase difference in the two signals at the receiver. A comparison of this
received signal (Figure 4 d) with the received signals for the various
multipath conditions when sidebands are unequal in magnitude show a
minimum of distortion. There was no loss of signal whatsoever.
It is concluded from these tests that VHF frequencies are useable in
the orbital workshop regardless of multipath. In summary, it may be
stated that at any single point within the enclosed chamber there exists
an infinite number of transmitted signals which may add or subtract as
described herein. Since this is true, it becomes practically impossible
as proven in the test, to force cancellations of the transmitted signal due
to multipath conditions.
Antenna r . , t ^ Antenna
FIGURE 1
Attenuator Adjustable Line
Figure 3A Figure
I
m
y
t)0
•r-»
Pu,
oo
cu
•rH
i r 'rtl TI^ r«Ti rf
Figure 3C
Figure 3D Figure
Figure 3E Figure hE
Lgure 3F Figure ^F
Figure 3G Figure i.G