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Contents
30.6.2 The W3KH Quadrifilar Helix
30.7 Satellite References
30.8 Earth-Moon-Earth (EME) Communication
30.8.1 Background
30.9 EME Propagation
30.9.1 Path Loss
30.9.2 Echo Delay and Time Spread
30.9.3 Doppler Shift and Frequency Spread
30.9.4 Atmospheric and Ionospheric Effects
30.10 Fundamental Limits
30.10.1 Background Noise
30.10.2 Antenna and Power Requirements
30.10.3 Coding and Modulation
30.11 Building an EME Station
30.11.1 Antennas
30.11.2 Feed lines, Preamplifiers
and TR Switching
30.11.3 Transmitters and Power Amplifiers
30.12 Getting Started
30.12.1 Tracking the Moon
30.12.2 System Evaluation
30.12.3 Operating Procedures
30.12.4 Finding QSO Partners
30.13 EME Notes and References
30.14 Glossary of Space Communications
Terminology
30.1 Amateur Satellite History
30.1.1 AMSAT
30.1.2 Amateur Satellites Today
30.2 Satellite Transponders
30.2.1 “Bent Pipe” Transponders
30.2.2 Linear Transponders
30.2.3 Digital Transponders
30.3 Satellite Tracking
30.3.1 Satellite Orbits
30.3.2 Satellite Tracking Software
30.3.3 Getting Started With Software
30.3.4 Orbital Elements
30.3.5 Other Tracking Considerations
30.4 Satellite Ground Station Antennas
30.4.1 Antenna Polarization
30.4.2 Omnidirectional Antennas
30.4.3 Directional Antennas
30.4.4 Antenna Mounting
30.4.5 Antenna Rotators
30.5 Satellite Ground Station Equipment
30.5.1 Receive and Transmit Converters
30.5.2 Transceivers
30.5.3 VHF/UHF RF Power Amplifiers
30.5.4 Typical Station Designs
30.6 Satellite Antenna Projects
30.6.1 An EZ-Lindenblad Antenna for
2 Meters
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Chapter 30
Space Communications
30.1 Amateur Satellite History
Many people are astonished to discover that Amateur Radio satellites are not new. In fact,
the story of Amateur Radio satellites is as old as the Space Age itself.
The Space Age is said to have begun on October 4, 1957. That was the day when the Soviet
Union shocked the world by launching Sputnik 1, the first artificial satellite. Hams throughout
the world monitored Sputnik’s telemetry beacons at 20.005 and 40.002 MHz as it orbited the
Earth. During Sputnik’s 22-day voyage, Amateur Radio was in the media spotlight since hams
were among the few civilian sources of news about the revolutionary spacecraft.
Almost four months later, the United States responded with the launch of the Explorer 1
satellite on January 31, 1958. At about that same time, a group of Amateur Radio operators
on the West Coast began considering the possibility of a ham satellite. This group later orga-
nized itself as Project OSCAR (OSCAR is an acronym meaning Orbiting Satellite Carrying
Amateur Radio) with the expressed aim of building and launching amateur satellites. (See
the sidebar “When Does a Satellite Become an OSCAR?”)
After a series of high-level exchanges with the American Radio Relay League and the
United States Air Force, Project OSCAR secured a launch opportunity. The first Amateur
Radio satellite, known as OSCAR 1, would “piggyback” with the Discoverer 36 spacecraft
being launched from Vandenberg Air Force Base in California. Both “birds” (as satellites are
called among their builders and users) successfully reached low Earth orbit on the morning
of December 12, 1961.
OSCAR 1 weighed only 10 pounds. It was built, quite literally, in the basements and ga-
rages of the Project OSCAR team. It carried a small beacon transmitter that allowed ground
stations to measure radio propagation through the ionosphere. The beacon also transmitted
telemetry indicating the internal temperature of the satellite.
OSCAR 1 was an overwhelming success. More than 570 amateurs in 28 countries for-
warded observations to the Project OSCAR data collection center. OSCAR 1 lasted only 22
days in orbit before burning up as it reentered the atmosphere, but Amateur Radio’s “low
tech” entry into the high tech world of space travel had been firmly secured. When scientific
groups asked the Air Force for advice on secondary payloads, the Air Force suggested they
study the OSCAR design. What’s more, OSCAR 1’s bargain-basement procurement approach
and management philosophy would become the hallmark of all the OSCAR satellite projects
that followed, even to this day.
Since then, amateurs have successfully built and launched dozens of satellites, each one
progressively more sophisticated than the last.
Radio amateurs point their
antennas skyward to talk via
communications satellites built
by groups of interested and
talented hams. In the first part of
this chapter, Steve Ford, WB8IMY,
provides some background and
shows what you need to get
involved with amateur satellites.
Later we’ll discuss Earth-Moon-
Earth (EME) communications,
also known as moonbounce .
The EME section, written by
Joe Taylor, K1JT, discusses the
requirements for successful
contacts via the EME path. K1JT
gratefully acknowledges input and
advice for the EME section from
Allen Katz, K2UYH, Marc Franco,
N2UO, Peter Blair, G3LTF,
Graham Daubney, F5VHX, Lionel
Edwards, VE7BQH, Ian White,
GM3SEK, Leif Asbrink, SM5BSZ
and Rex Moncur, VK7MO.
A Glossary of space communica-
tions terms may be found at the
end of this chapter.
30.1.1 AMSAT
Much of the amateur satellite progress has been spearheaded by the Radio Amateur Satel-
lite Corporation, better known as AMSAT . The original AMSAT was formed in Washington,
DC in 1969 as an organization dedicated to fostering an Amateur Radio presence in space.
The AMSAT model quickly became international with many countries creating their own
AMSAT organizations, such as AMSAT-UK in England, AMSAT-DL in Germany, BRAMSAT
in Brazil and AMSAT-LU in Argentina. All of these organizations operate independently but
may cooperate on large satellite projects and other items of interest to the worldwide Amateur
Radio satellite community. Because of the many AMSAT organizations now in existence,
Space Communications
30.1
 
Table 30.1
Currently Active Amateur Radio Satellites
Satellite
When Does a Satellite
Become an OSCAR?
While worldwide AMSAT organiza-
tions are largely responsible for the
design and construction of the mod-
ern day Amateur Radio satellites, the
original “OSCAR” designation is still
being applied to many satellites car-
rying Amateur Radio. However, most
Amateur Radio satellites are not usu-
ally assigned their sequential OSCAR
numbers until after they successfully
achieve orbit and become operational.
Even then, an OSCAR number is only
assigned after its sponsor formally
requests one.
For example, let’s make up a satel-
lite and call it ROVER. The ROVER
spacecraft won’t receive an OSCAR
designation until (1) it reaches orbit and
(2) its sponsor submits a request. Now
let’s presume that ROVER makes it into
orbit and the OSCAR request is made
and granted. ROVER is now tagged as
OSCAR 99 and its full name becomes
ROVER-OSCAR 99. You’ll ind, how-
ever, that many hams will abbreviate
the nomenclature. Some will simply call
the satellite ROVER, or OSCAR 99.
They may even abbreviate its full name
to just RO-99.
If a satellite subsequently fails in
orbit, or it re-enters the Earth’s atmo-
sphere, its OSCAR number is usually
retired, never to be issued again.
Uplink (MHz)
Downlink (MHz)
Mode
AAUSAT-II
—
437.425 (packet)
Telemetry
AMSAT-OSCAR 7
29.502
Beacon
145.975
Beacon
435.100
Beacon
145.850 - 145.950
29.400 - 29.500
SSB/CW, non-inverting
432.125 - 432.175
145.975 - 145.925
SSB/CW, inverting
AMSAT-OSCAR 27
145.850
436.795
FM repeater
AMSAT-OSCAR 51
145.920
435.300
FM repeater
(67 Hz CTCSS required)
145.880
435.150
USB/FM voice
145.880
2401.200
FM voice
145.860
435.150
9600 bps digital
145.860
2401.200
9600 bps digital
1268.700
2401.200
FM voice
1268.700
435.150
FM voice
ARISS (ISS)
144.490
145.800
Crew contact, FM
(Region 2/3)
145.200
145.800
Crew contact, FM
(Region 1)
145.990
145.800
Packet BBS
145.825
145.825
APRS digipeater
—
144.490
SSTV downlink
437.800
145.800
FM repeater
Compass-One
—
437.275 (CW)
Telemetry
—
437.405 (packet)
Telemetry (incl images)
CubeSat-OSCAR 57
—
436.8475 (CW)
Telemetry
—
437.4900 (packet)
Telemetry
CubeSat-OSCAR 58
—
437.4650 (CW)
Telemetry
—
437.3450 (packet)
Telemetry
CubeSat-OSCAR 66
—
437.485 (packet)
Telemetry (incl images)
Cute1.7 + APDII
1267.600
437.475
9600 baud packet
Dutch-OSCAR 64
—
145.870 (packet)
Telemetry
435.530 - 435.570
145.880 - 145.920
SSB/CW, inverting
Fuji-OSCAR 29
145.900 - 146.000
435.800 - 435.900
SSB/CW, inverting
Gurwin-OSCAR 32
145.930
435.225
9600 baud APRS packet
145.850
435.225
9600 baud APRS packet
145.890
435.225
9600 baud packet
the US AMSAT organization is frequently
designated AMSAT-NA.
Since the very first OSCAR satellites were
launched in the early 1960s, AMSAT’s inter-
national volunteers have pioneered a wide
variety of new communications technologies.
These breakthroughs have included some of
the very first satellite voice transponders as
well as highly advanced digital “store-and-
forward” messaging transponder techniques.
All of these accomplishments have been
achieved through close cooperation with
international space agencies that often have
provided launch opportunities at significantly
reduced costs in return for AMSAT’s tech-
nical assistance in developing new ways to
launch paying customers.
AMSAT’s major source of operating rev-
enue is obtained by offering memberships in
the various international AMSAT organiza-
tions. Membership is open to radio amateurs
and to others interested in the amateur explo-
ration of space. Modest donations are also
sought for tracking software and other satel-
lite related publications. In addition, specific
spacecraft development funds are established
from time to time to help fund major AMSAT
spacecraft projects through donations. In ad-
dition to money, AMSAT makes creative use
of leftover materials donated from aerospace
industries worldwide.
145.930
435.225
9600 baud packet
1269.700
435.225
9600 baud packet
1269.800
435.225
9600 baud packet
1269.900
435.225
9600 baud packet
Navy-OSCAR 44
145.827
145.827
1200 baud APRS packet
Saudi-OSCAR 50
145.850
436.795
FM Repeater
(67 Hz CTCSS required)
VUSat-OSCAR 52
—
145.860
CW Beacon
—
145.936
Carrier Beacon
435.220 - 435.280
145.870 - 145.930
SSB/CW, inverting
across entire hemispheres. The last such satel-
lite was OSCAR 40, which became silent in
2004. Unfortunately, it is becoming difficult
for non-profit amateur satellite organizations
to build such complex, powerful birds while
relying solely on member dues and donations,
not to mention thousands of hours of volunteer
labor. To make matters worse, launch opportu-
nities for large satellites have become increas-
ingly scarce and prohibitively expensive.
The German AMSAT organization,
AMSAT-DL, is building a high-orbit satellite
that is presently known as Phase 3 Express.
AMSAT-NA has been working on their ver-
sion of a DX bird known as Eagle. Neither sat-
ellite is complete at the time of this writing.
For the foreseeable future, Amateur Radio
satellite activity will likely be confined to
spacecraft in low Earth orbits.
AMSAT-NA has just one paid employee.
The rest of the organization, from the presi-
dent on down to the workers designing and
building space hardware, all donate their time
and talents to the organization.
30.1.2 Amateur Satellites
Today
When this edition of the Handbook went
to press, there were 15 active Amateur Ra-
dio satellites in orbit (see Table 30.1 ), not
counting the Amateur Radio presence aboard
the International Space Station. All of these
are low Earth orbiting (LEO) spacecraft that
function either as communication relays or
research platforms.
Hams have been longing for a high-orbiting
“DX” satellite that would relay conversations
30.2
Chapter 30
 
30.2 Satellite Transponders
In its most basic form, a transponder is the
satellite communications system that receives
signals from the Earth, alters their frequen-
cies, amplifies them, and sends them back to
Earth. The word originates from transmitter
and responder . The transponder is at the heart
of every satellite’s ability to relay signals.
There are three transponder types currently
in use.
Fig 30.1 — A bent-pipe
transponder gets its
name from the way it
functions. It takes the
received uplink signal
and instantly relays it
on the downlink back
to its source. Bent-pipe
transponders are most
often found on satellites
that function primarily
as FM repeaters.
30.2.1 “Bent Pipe”
Transponders
A bent-pipe transponder is the simplest
transponder design in terms of function. It
receives a signal at one frequency and simul-
taneously retransmits it at another. The meta-
phor is that of a U-shaped pipe that captures
an object coming toward it and redirects the
object back to its source ( Fig 30.1 ).
A terrestrial FM repeater is a typical ex-
ample of a bent-pipe transponder. Earthbound
repeaters monitor one frequency and retrans-
mit on another, usually (though not always)
within the same band. Thanks to their elevated
antenna systems, sensitive receivers and con-
siderable output power, terrestrial repeaters
can extend the coverage of mobile or hand-
held FM transceivers over hundreds or even
thousands of square miles.
Bent-pipe satellite transponders also func-
tion as FM repeaters (although they could just
as easily relay other modulation modes such
as SSB). Satellite repeaters lack the output
power of terrestrial repeaters because power
generation in space is a difficult proposition.
But what satellite repeaters lack in output
power they more than make up for in antenna
elevation. Even something as meager as a 1 ⁄ 4 l
whip antenna can become a transmitting and
receiving powerhouse when it is hundreds of
miles above the planet.
The principle advantage of a bent-pipe
transponder operating in the FM mode is that
it is readily compatible with common Ama-
teur Radio FM transceivers. Satellites such as
AMSAT-OSCAR 51 and AMRAD-OSCAR
27 can be easily worked with the same trans-
ceivers you’d otherwise use to chat through a
local FM repeater. It isn’t uncommon to hear
mobile and even handheld portable stations
communicating through these satellites.
The principle disadvantage of a bent-pipe
transponder is that it can relay only one signal
at a time. Multiple signals on the satellite
uplink frequency interfere with each other,
usually resulting in an unintelligible cacoph-
ony on the downlink. Thanks to the “capture
effect” common to FM receivers, only the
strongest signal will come through clearly.
With only 10 or 15 minutes available during
typical low-orbit passes, an inconsiderate op-
erator running high power can use the capture
Fig 30.2 — Linear transponders repeat all signals received in an entire slice of
spectrum. This allows the satellite to relay many conversations simultaneously.
effect to monopolize the satellite, effectively
shutting out all other stations.
The solution to the bent-pipe problem is to
have a satellite that can relay more than one
signal at a time. That requires a completely
different sort of transponder.
signals linearly, and then retransmits every-
thing back to Earth. Rather than relaying only
one signal, a satellite equipped with a linear
transponder can relay many signals at once
( Fig 30.2 ).
From an operator point of view, linear
transponders have an enormous advantage.
With single-channel bent-pipe transponders,
the user is obligated to finish a conversation
quickly and clear the frequency so that some-
one else can make a contact. Not so with
linear transponders. With the ample spectrum
available through a linear transponder, there
is more than enough “room” for everyone to
30.2.2 Linear Transponders
Unlike a bent-pipe transponder that can
relay only one signal at a time, a linear tran-
sponder receives signals in a narrow slice
of the RF spectrum (the passband ), shifts
the frequency of the passband, amplifies all
Space Communications
30.3
 
 
 
 
 
 
 
 
 
 
 
communicate for as long as they please, or at
least until the satellite travels out of range.
A linear transponder can be used with any
type of signal when real time communication
is desired. From the standpoint of conserving
valuable spacecraft resources such as power
and bandwidth, however, the preferred user
modes are SSB and CW. Transponders are
specified by first giving the approximate input
frequency followed by the output frequency.
For example, a 146/435-MHz transponder has
an input passband centered near 146 MHz and
an output passband centered near 435 MHz.
The same transponder could be specified in
wavelengths, as a 2-meter/70-cm unit.
Fig 30.3 — A simpliied block diagram of a linear transponder.
TRANSPONDER MODES
To keep linear transponder specifications
as simple as possible, the satellite commu-
nity often uses so-called Mode designators.
In the early years of amateur satellite opera-
tion, these designations were rather arbitrarily
assigned and bore little resemblance to the
actual frequencies in use. Fortunately, the am-
ateur satellite community has since settled on
a newer series of transponder specifications
that are far more intuitive. For instance, in our
previous example, the 2-meter band is tagged
with the label “V” (for VHF) while the 70-cm
band is labeled “U” (for UHF). Therefore, a
transponder that listens on 2 meters and re-
peats on 70 cm is today usually called a Mode
V/U transponder. See Table 30.2 .
Transponder design is, in many respects,
similar to receiver design. Input signals are
typically on the order of 10 –13 W and the out-
put level can be up to several watts. A major
difference, of course, is that the transponder
output is at radio frequency while the receiver
output is at audio frequency. A block diagram
of a simple transponder is shown in Fig 30.3 .
For several reasons, flight-model transpon-
will encounter signals in its passband differ-
ing by perhaps 40 dB. Good HF receivers
solve the problem by filtering out all but the
desired signal before introducing significant
gain. A satellite, however, has to accommo-
date all users simultaneously. The maximum
overall gain can, therefore, be limited by the
strongest signal in the passband.
Considering the state-of-the-art in tran-
sponder design and available spacecraft
power budgets, an effective dynamic range
of about 25 dB is about the most that can
be currently obtained. In earlier satellites,
the receiver AGC was normally adjusted to
accommodate the loudest user. As a result,
stations 25 dB weaker were not able to put
a usable signal through the spacecraft even
though they might be capable of doing so
when the AGC is not activated. In the ideal
situation users would adjust uplink power so
that spacecraft AGC is never activated.
The “power-hog” problem is a serious one.
A single station running excessive power can
effectively swamp a linear transponder. Far
too often, an inexperienced operator may be-
lieve that cranking up transmit power will im-
prove downlink signal strength. Instead, that
increase in power only serves to depress the
downlink signal levels of all other stations.
In the short-lived OSCAR 40 satellite, the
designers tried an innovative approach to the
power-hog problem with the addition of
LEILA ( LEI stungs L imit A nzeige [Power
Limit Indicator]). LEILA’s computer con-
tinuously monitored the 10.7 MHz trans-
ponder IF passband. When an uplink signal
exceeded a predetermined level, the com-
puter inserted a CW message over the of-
fender’s downlink. The message indicated to
the transmitting station that the transponder
was being overloaded, and served as an in-
ducement for the offending station to reduce
power until the CW signal disappeared. If the
overloading signal continued, or exceeded an
even higher preset level, LEILA activated a
notch filter tuned to the offending station’s
frequency. LEILA turned out to be highly
effective. If you suddenly heard CW being
transmitted over your signal on the down-
link, you knew you had to reduce power im-
ders are more complex than the one shown.
In spacecraft applications a key charac-
teristic of a linear amplifier is its overall ef-
ficiency (RF output/dc input). Once we reach
power levels above a few watts the use of class
A, AB or B amplifiers cannot be tolerated.
The power and bandwidth of a transponder
must be compatible with each other and with
the mission. That is, when the transponder is
fully loaded with equal-strength signals, each
signal should provide an adequate signal-to-
noise ratio at the ground. Selecting appropri-
ate values accurately on a theoretical basis
using only link calculations is error prone.
Experience with a number of satellites, how-
ever, has provided AMSAT with a great deal
of empirical data from which it’s possible
to extrapolate accurately to different orbits,
bandwidths, power levels, frequencies and
antenna characteristics.
In general, low-altitude (300 to 1600 km)
satellites that use passive magnetic stabili-
zation and omnidirectional antennas can
provide reasonable downlink performance
with from 1 to 10 W PEP at frequencies
between 29 and 435 MHz, using a 50- to
100-kHz-wide transponder. A high-altitude
(35,000 km) spin-stabilized satellite that uses
modest (7 to 10 dBi) gain antennas should be
able to provide acceptable performance with
35 W PEP using a 300-kHz-wide transponder
downlink at 146 or 435 MHz. Transponders
are usually configured to be inverting in order
to minimize Doppler shift. This means, for
example, that a signal with a frequency at
the low end of the uplink passband shifts to
a corresponding point on the high end of the
downlink passband. In the case of an SSB
signal, the sideband modulation inverts as
well — lower sideband on the uplink becomes
upper sideband on the downlink.
Table 30.2
Satellite Uplink/Downlink Mode
Designators
Satellite Band Designations
10 m (29 MHz): H
2 m (145 MHz): V
70 cm (435 MHz): U
23 cm (1260 MHz): L
13 cm (2.4 GHz): S
5 cm (5.6 GHz): C
3 cm (10 GHz): X
Common Operating Modes (Uplink/Downlink)
V/H (2 m/10 m)
H/V (10 m/2 m)
U/V (70 cm/2 m)
V/U (2 m/70 cm)
U/S (70 cm/13 cm)
U/L (70 cm/23 cm)
L/S (23 cm/13 cm)
L/X (23 cm/3 cm)
C/X (5 cm/3 cm)
DYNAMIC RANGE
The dynamic range problem for transpon-
ders is quite different from that for HF receiv-
ers. At first glance it may seem that the situa-
tion faced by satellite transponders is simpler.
After all, an HF receiver must be designed to
handle input signals differing in strength by as
much as 100 dB, while a low-altitude satellite
30.4
Chapter 30
 
 
 

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