AO-40 as servious referred to is also known Oscar 40, and represents the latest international effort of ham radio operators worldwide to put a satellite in orbit to provide worldwide amateur radio communications. As originally launched it was to provide uplinks on up to 9 amateur bands and downlinks on 6 bands. With so many uplinks and downlinks, it was supposed to provide relatively easy worldwide communications under a variety of conditions. Its predecessor AO-13 provided worldwide communications, but required fairly large and expensive antenna arrays and their associated preamps, positioners, and so on to in order to work it. Unfortunately, its orbit decayed and AO-13 burned up in the atmosphere in December of 1996. Another eagerly awaited feature was an orbit of 16 hours, which meant that the satellite would appear in the same place in the sky every 48 hours.
The Launch, "The Malfunction" and Recovery
It was launched in November 2000 aboard an Arianne 5 rocket booster from French Guiana as a standby payload, after waiting years for a launch opportunity. The launch went well, and during the month after launch all seemed well until the 400 newton thrust motor used to boost the satellite into its final orbit malfunctioned. Nothing was heard from the satellite, with the exception of some very weak telemetry after the malfunction, and even then it was not really known if the signals were indeed from AO-40. After waiting for an automatic reset of the satellite's onboard computer, or RHU, hams using moonbounce class stations were able to copy telemetry from the S-Band beacon, and were able to send commands to the satellite to control its rapid spin and get the antennas pointing toward the earth again. This was a slow process, but after several months of patient and careful evaluation and operation of the remaining control systems, the AMSAT control team was able to maneuver the satellite into a usable elliptical orbit about 725 miles high at perigee, and about 40,000 miles at appogee. Its orbit, while not a convenient 16 hours as originally planned (it is about 19 hours) is easily tracked with software, and is projected to not decay for at least 20 years, unlike AO-13, whose orbit decayed within 8 years of launch. During the recovery from the "Malfunction", it was discovered that many of the satellite's transponders were damaged, including the downlink trasponders on 2 meters, 70cm, and 3cm, leaving only the 13cm transponder on 2401 Mhz, and the 1.5 cm trasponder on 24 Ghz functioning.
How to Communicate through AO-40
While the "Malfunction" took away many of AO-40's options, it is far from being an orbiting white elephant. Here is an overview of how to do it. This information is taken from my personal experiences, as well as the wealth of information available on the net provided by AMSAT and many others. It is not intended to be a comprehensive guide, just an overview
Technician Class or higher Ham License
Basic Knowledge of Electronics and and Computers
Ability to scrounge and modify old TVRO and computer hardware.
Transmitter must be capable of transmitting on SSB or CW on 435 Mhz or 1269 Mhz. 2 meter and 13 CM uplinks are working, but are not for general use. With an adequate antenna, 25 to 50 watts should be enough power.
Your receiving system needs to be capable of receiving 2401 MHZ. Although almost all ham receivers or transceivers lack receive capability on 2401 Mhz, do not despair, it is not necessary or even desirable to have a 2401 Mhz receiver inside your radio, and there is plenty of inexpensive equipment that can be easily modified to do the job. Here is the scoop: Frequencies in the 2400 to 2800 Mhz range have been used in the past for transmitting wireless Cable TV, otherwise known as MMDS. Because of the difficulties of sending a 2.4 Ghz signal down a piece of feedline, the 2.4 Ghz signal is downconverted into a more manageable frequency, usually 122 Mhz, which can be easily fed a long way down even a cheap piece of coaxial cable with a device called a downconverter, which is built into the antenna assembly. Often it takes little more than a crystal change to change the downconverted frequency to 145 Mhz or so, perfect for feeding into a 2 meter ham transceiver. Googling or a trip to eBay should yield you a suitable downconverter for less than $100 USD. As of this writing in January, 2003, the AIDC 3731 seems to be the most suitable for conversion. Down East Microwave also makes a downconverter with a bit better performance, but it costs several hundred dollars. The most common setup for AO-40 is to use a transceiver such as a Yaesu FT-847 or FT-736, or a similar rig such as the Kenwood TS-2000, or Icom IC-910H along with the downconverter. A few Yaesu FT-736's have a 13 cm module built in to the radio, allowing you uplink on L-Band without an external transverter.
The antenna system consists of three main parts: Computer Software which predicts where AO-40 will appear in the sky at any given time, the antennas themselves, and a means to turn the antennas to where the computer software predicts the satellite will appear in the sky.
For your uplink antenna, you will need to make or buy an antenna that provides gain, to amplify your signal in the desired direction. This usually means a long yagi or helix antenna if you are uplinking on 435 Mhz (70 cm). Using 25 watts as the typical output of a common UHF transceiver, you will need an antenna that will provide at least 12-15 dB of gain. A long 40 element cross-yagi such as a KLM 40 element yagi from an old AO-13 station will be more than adequate for most needs. A shorter antenna will work, but you will be more restricted to favorable parts of the satellite's pass. It is possible to have too much uplink power, a device called LEILA will sound an alarm and cut your transmission short if you use excessive uplink power, in order to conserve power on the satellite. On 1269 Mhz, (23 cm) a Helix, Yagi, or Dish can be used.
For the Downlink antenna, which receives on 2401 Mhz, a dish is the only real way to go, although I have heard a few people trying to make Helices work, usually with pretty poor results. The larger the dish, the fatter your downlink signal will be. The good news is that there are plenty of surplus microwave dishes floating around, and you can usually scrounge one up for next to nothing. There are mainly 3 classes of dishes that can be used. First are the little DirecTV and DSS dishes, that are less than 60 cm (20 inches) in diameter. These dishes can provide an adequate signal during favorable conditions, are easy to mount and move, and are ideal for portable stations.
Next class up are dishes up to about 1.2 meters. One source for dishes of this class are old Primestar dishes, named for a defunct Satellite TV service. They can be found today as lawn ornaments, hamfest and flea market trash, and can often be had for next to nothing. Another source of dishes of this class are microwave data link dishes, also becoming surplus as fiber optics takes over. These dishes are usually 30 to 48 inches in diameter, and are made of fiberglass. A dish of this class makes an excellent choice for a beginner's setup. They are small and light enough to be positioned manually if necessary, or can be handled by a conventional ham type rotor, and are large enough to provide good signals over the majority of the pass.
At the top of the class for potential AO-40 antennas are BUDs, short for Big Ugly Dish, which is slang for surplus C-Band satellite TVRO dishes. These dishes range in size from about 1.5 to 3.5 meters in diameter, and were very popular from the late 70's to the early 90's in outlying rural areas and small towns that lacked regular TV service, and many of these dishes have been replaced by the much smaller DirecTV and DSS dishes. They can be found as lawn ornaments in a nearby rural community (especially a community tucked into a mountain valley) or your local junkyard. A BUD is not without problems and challenges, however. Because of their size and weight, a pretty beefy mounting system is needed, and a large dish's support structure and positioning hardware must not only be able to cope with the weight, but the wind as well. A BUD will have up to 10 M2 of surface area! It will take some creative engineering to modify or create a positioning system to precisely steer the big dish. The rewards for the effort will be a downlink signal that is 10 dB stronger than a 1 meter class dish will provide, making copy of the signals easy on the ears.
With any dish, you will need a feed for the dish. A feed provides a signal pick up or transmit point at the focal point of the dish, like the light bulb in a flashlight. G3RUH and K5OE have excellent websites devoted to feed design. There are 3 major types of feeds, a linear feed, which is often built into or part of the downconverter itself, a patch feed, or a helical feed. All are fairly easy to build, so don't be intimidated by the prospect. Mount your downconverter as close to the feed as practical, since the 2.4 Ghz signal attentuates quickly in coax.
Tracking software is used to calulate the position of the satellite at any given time, using a data set for each satellite called Keplerian Elements
, which describe the orbit of the satellite. In the Apollo
days, this software needed a whole roomful of equipment to provide text data to calculate the orbits of celestial bodies. Today software that provides this text data and a nifty graphical display to boot can be carried on your palm pilot
, or that hopelessly obsolete PC that is headed for the landfill. For DOS and Windows, Instant Trak, NOVA for Windows, and WISP provide all the necessary data with a nice graphical interface, and Instant Trak can be run off a floppy on even an old 286 laptop with a bad hard drive
, so you don't really need to tie up your "good" computer. Most programs also have a provision for automaticly controlling the rotor box on popular rotors such as the Yaesu G5400/G5500 rotors. Programs for satellite tracking are also available for Linux
as well, such as Predict (which also has a DOS version).
Locating your Antennas
Select a location that has a good view of the southern horizon (assuming you live in the Northern Hemisphere, for your antennas (for those "down under", replace South with North). If your property's southern exposure is tall woods over most of the arc from east to west you will have a very difficult time hearing S Band signals through the trees. To determine if trees will be a problem, obtain a copy of the tracking software, and plot the path of the satellite across the sky against your treeline. Do this before making a major investment in equipment! If trees are a major problem, try to locate the antennas to provide at least a partial window of coverage. Other options are to mount the antennas on a tower, break out the chainsaw and cut down the offending trees, or go mobile. Try to avoid excessively long coax runs, since signal attentuation becomes a major problem at UHF frequencies and up. If you must have a long coax run for your uplink, use a good quality coax such as LMR-400, 9913, or Hardline. Hardline can be expensive, but one option for cutting costs is to use "tag ends" of CATV Hardline. You may have to improvise your connectors, since CATV hardline connectors are less common than short usable pieces of hardline. Since your downlink is probably going to be on 2 meters, common coax such as RG-8 or RG-214 is adequate on runs up to 75 meters, assuming your downconverter has adequate gain.
Downconverter care and feeding
Most downconverters are designed for outdoor use, and are gasketed against the elements. If you open up the downconverter to modify it, take care not to damage these gaskets. Most downconverters have a provision to power them using the coax as the power cable, but some ham transceivers have built in powering capabilities. It is not particularly necessary to use top quality coax for the downconverted signal, but care must be taken to waterproof your connections. Another caution that must be taken is to never transmit into the downconverter! They are not designed to survive being transmitted into at typical power levels. If you use your transceiver for uses other than AO-40, take care not to accidentally transmit into the downconverter, either by using an RF Fuse, installing an attentuator, or just plain vigilance.
Finding the Beacon
Once your station is assembled, you need to find the Middle Beacon. Start by opening your satellite tracking program. and acquire current keplerian elements for the program. Point the antennas at the coordinate shown in the tracking program and scan for the beacon. Tbe beacon transmits on 2401.323 Mhz, A good idea is to adjust the downconverter's frequency is so that the Middle Beacon is heard on the 2 meter receiver at 145.323 Mhz. Depending on the downconverter, you may have to scan up and down the band several hundred Khz to find the beacon. It will sound sort of like a buzzsaw, and may vary up and down in strength. Try to select a time when the satellite is in a clear area of the sky and the squint angle is less than 15 degrees. Once the beacon is found, make a note of the frequency it was received on. If you wish, adjust the frequency trimmer on the downconverter to adjust the beacon's received frequency to the value I suggested above. Try to make this adjustment when the satellite is near apogee, or else doppler shift will cause your readings to be erroneous.
Finding your own signal and talking
Next, you need to find your own downlink. AO-40 uses an inverting transponder, which means that the higher the uplink signal is in frequency within the passband, the lower the frequency of the downlink signal will be. To find your own signal, tune to the beacon, and set your uplink (on UHF) to 435.667 Mhz. If you are using single sideband set your transmitter on lower sideband and your receiver on upper sideband. On most modern satellite receivers, you will want to set the SAT control to Reverse, and tune the tune the downlink to about 30 Khz below the beacon. Now turn the SAT control to receive while transmitting your callsign or series of dots. After tuning around a few Khz, you should hear your own downlink, delayed by about 1/3 of a second if the satellite is near apogee. Set the Sat control on Reverse again and tune to the area about 20 to 40 KC above the beacon. Look for a quiet spot to call CQ, and listen for any responses or your downlink. Doppler shift will cause the received frequency of your downlink to drift over time, and it can get confusing in a conversation. To minimize the confusion, it is best to transmit on a set frequency, and make corrections for doppler by adjusting your receiver's tuning. Over a period of several hours, the doppler effect can change the frequency by as much as 10 Khz or more. Responding to a call is much the same, make sure your uplink and downlink frequencies match before answering a call. Computer controls can make it easier, but it is not absolutely necessary.
Finally, as the AO-40 moves across the sky, it will become necessary to reposition your antennas from time to time, so keep an eye on your tracking program to keep your antennas pointed at the satellite.
73, and see Y'all up on the the bird.
Bruce N3LSY (aka N3Bruce)
Update January 31,2004:
Early on January 27, 2004 Telemetry from AO-40 indicated that primary battery voltage dropped from 26.2 to about 13 volts, indicating that one of 2 primary batteries had failed shorted, and before telemetry went silent, multiple sensor failures had occured. Attempts so far to restore communication have failed, although there is some hope that the auxillary battery could be recharged enough to bring the satellite back on line. Stay tuned for further updates.