Quick, Inexpensive and Effective: A Simple Satellite
Mobile QRP Station for the Beginner
by Douglas Quagliana, KA2UPW
This paper first appeared in the Proceedings of the AMSAT-NA 14th Space Symposium (November 8-10 1996). AMSAT is a registered tradmark.
This article describes how the author's AO-27 satellite station evolved from its simple beginnings. The author has used this station to make contacts with fifteen states and over twenty grid squares while operating satellite QRP mobile. This article was written for the Amateur Radio operator who has little or no experience using satellites and is interested in creating and operating a simple inexpensive satellite station for AO-27.
AO-27, also known as EYESAT, was one of several amateur radio low-earth orbit satellites that hitched rides as secondary payloads on an Ariane V-59 rocket launched in September 1993. AO-27 is a small microsat class satellite that performs commercial functions and also acts as a part- time mode J FM repeater within the amateur radio bands. At present (August 1996), the satellite is configured to turn on its FM transponder for a fixed amount of time starting several minutes after it emerges from the Earth's shadow. AO-27 has a very sensitive receiver that will detect even a few watts from an HT.
The Satellite Station
My original satellite station was a Tempo S1 two meter HT, a 5/8 wavelength magmount, a homebrew quagi and a handheld scanner. I have improved upon the original quagi and replaced the scanner with a preamp, a downconverter and a Uniden HR2600 10 meter transceiver. My whole station fits neatly into the trunk of the car, and easily sets up in less than five minutes. (See Photo 1) When necessary, everything except the quagi boom and 5/8 wavelength antenna can collapse down into a backpack or carrying case. The entire station runs off of batteries, which allows me to operate from just about anywhere. Since the very beginning I have tried keep everything as simple as possible, consistent with successful operations and good operating practice.
AO-27 : The QRP satellite
While AO-27's uplink receiver is very sensitive, the downlink is usually at the 600 milliwatt level. This means that a good low noise preamp with 15 to 20 dB gain, or at least a five element beam, is needed. I made my first AO-27 contact with a homemade five element quagi that I built from an article in the December 1987 QST. The first quagi was just thrown together. The boom was a wooden dowel. The reflector and driven element were #12 insulated solid copper wire supported by wooden dowel spreaders which were held in place with hot-melt glue. The directors were one-eighth inch welding rods secured to the boom with rubber bands. The quagi was pointed manually and fed a few feet of RG-8 connected to my Radio Shack Pro-38 scanner. It worked. For an uplink signal, I used my HT and a 5/8-wave antenna magmounted on my car. The antenna was is a commercial version, but the magmount was homemade. The HT put out about one and a half watts. Using the quagi, scanner, two meter ht, and 5/8-wave, I worked six states in my first month of QRP mobile satellite operations.
Downlink Improvements : The Receiver
After the first few contacts, I found that the scanner didn't receive very well except during the highest elevation passes. I also observed that there were times when the downlink would doppler between two channels, and neither one was copyable. This had more to do with the fact that the scanner tuned in 15 kHz steps at 435 MHz than the doppler. My first solution was to use a 435 MHz downconverter with the scanner set at 29 MHz, where it tuned in 5 kHz steps. This reduced, but didn't eliminate completely, the problem of the downlink being between channels on the scanner. However, it also added a new bigger problem : there were now three times as many channels, and the scanner could only scan in one direction. If I scanned too far I had to go all the way to the end and start over. If not for this problem, I probably would have stayed with the downconverter and scanner with 5 kHz tuning. The current solution uses my Uniden HR2600 10m HF rig with the 435 MHz downconverter, effectively turning it into a 435 MHz all mode receiver. This provided several advantages over the handheld scanner. The combined preamp/downconverter/HR2600 receiver has finer tuning (100 Hz) and copies the downlink at lower elevations better than the scanner. The only disadvantage of the HR2600 was the higher current that it requires. This was easily overcome with a 7 amp-hour "brick" gel cell.
Downlink Improvements : The Quagi
Soon after I finished the first quagi, I started thinking about improvements. While it fit in the trunk of my car, it seemed to occupy more volume than it should. Since I wanted it to be portable, it had to be collapsible, light weight and easy to hold and point. In addition, I didn't want something which was overly complicated or required tools for assembly and disassembly. With these goals in mind, I redesigned the quagi.
To solve the pointing and holding problem, I took an angled handle from an old garden tool and attached it to one end of the boom. It allows my hand to grip the handle at a natural angle. I also used a smaller diameter dowel as the boom to make the whole antenna lighter. These two improvements made the antenna aiming much easier and reduced the arm strain. Next, I connected four short wooden dowels to each of the loops with some creatively cut plastic tubing. The other ends of the dowels are plugged into wooden spools attached to the boom. This allows the loops to be removed from the boom if necessary. The directors are held in place with cord locks from the local camping supply store. Two cord locks, one above the boom and one below, effectively hold the directors in place while providing an extremely quick assembly and disassembly. (See Photo 2) The preamp was tied to the boom and configured to run off of a 9-volt battery. (See Photo 3) The director is soldered to an N-type connector for convenience of connecting it to the preamp, but it should work just as well with the director connected to the coax shield and braid.
When building this antenna, the prospective quagi builder should follow the old axiom, measuring the lengths of the loops and directors twice then cutting once. Also, the builder shouldn't be overly concerned about the exact resonant frequency, the antenna pattern or the absolute gain. The 435 MHz quagi was only used on receive with AO-27 so the dimension were not as critical as they would be if the antenna was used for transmitting.
Although it is possible to get started without one, I highly recommend the use of a preamp. I built a Down East Microwave 70-cm preamp from a kit. This kit was almost entirely surface mount but contains only a few parts. I mounted mine directly on the boom by tying it down with a strap from an old backpack. Power was supplied from a 9- volt battery attached to the case with double sided sticky tape. A small battery connector with alligator clips served a means of connecting and disconnecting the battery. The difference was dramatic. A downlink signal which was completely unreadable without the preamp became easy to copy with it. The 436.8 MHz downlink signal, once amplified by the preamp, was converted to 29.3 MHz by a Hamtronics 435 MHz downconverter that was also built from a kit. Interested builders should be aware that the kit contains numerous small surface mount parts. The version which I bought requires an enclosure and connectors. I mounted mine in a small 3"x5"x2" box and powered it from another 9-volt battery. While it does an adequate job of downconverting, my version required a preamp to allow reception of AO-27 with the HR2600.
Once the downconverter was built, I needed to test and align it. Normal alignment required a 435 MHz signal generator, which I didn't have. I suspect most beginners won't have one either. I did have the previously mentioned handheld scanner, which it turned out was a very effective VHF/UHF programmable signal generator. The scanner allowed me to easily test and align the downconveter. A quick examination of the insides of the scanner revealed a crystal and what was probably a 455 kHz ceramic resonator. This suggested that the first intermediate frequency (IF) was near 10 MHz and that the second IF was 455 khz. Knowing that the scanner did receive on 435 Mhz, it seemed reasonable that the local oscillator was near 435 Mhz also. Not knowing the exact IF frequencies, I set the scanner for 446.00 MHz, connected the downconverter and HR2600 and tuned until I found the local oscillator signal. On my scanner the frequency of the local oscillator (LO) was about 10.85 MHz below the scanner's programmed receive frequency. When the scanner was set to receive 447.65 MHz, the local oscillator became a 436.80 MHz signal generator. Other scanners use different intermediate frequencies and different mixing schemes but will probably still have a LO frequency about ten of megahertz from the received frequency.
The local oscillator signal was quite strong and easily detectable when placed near the downconverter. To distinguish it from other signals and images in the receiver bandpass, I configured the scanner to scan between two or more channels; only one of which had a LO signal near 435 MHz. The desired LO signal will be present and absent as the scanner moves among different channels, giving it a distinctive beeping sound.
The preamp can be tested in a very similar manner. I again configured the scanner such that the downconverter could receive the local oscillator signal. Next I placed the scanner at a distance from the receiver such that the signal was just barely discernible and installed the preamp with the power disconnected. The preamp was inline after the antenna but before the downconverter. After I found the local oscillator signal and applied power to the preamp, there was a noticeable increase in the received signal strength. As a check, I removed the power from the preamp and moved the scanner farther away to the point where it was undetectable. When the preamp was turned back on, the signal was clearly audible.
It also turns out that the antenna can be tested using this technique. Although the actual gain can not be determined, I verified that my antenna was directional and that it did provide some amount of gain. As before, I placed the scanner at a distance from the receiver. Then I alternately pointed the antenna at the scanner and away from the scanner. The received signal strength increased as the antenna moved towards the scanner and decreased as antenna was pointed away from the scanner.
Basic Operations : Making your first contact on AO-27
Before you can make your first contact through any satellite, there are several pieces of information you will need to know: when the satellite is above your horizon, exactly where in the sky it will be, and when it will be available for use. The first and second are determined through satellite tracking and the third requires knowledge of the transponder schedule. You should also find out what your grid square is. This isn't really necessary, but just about everyone will ask you for it.
These days the way to track satellites is to use a computer with a satellite tracking program. Several tracking programs for various computers as well as the necessary up to date Keplerian elements are available from AMSAT at http://www.amsat.org. Any of the tracking programs will give all of the details necessary to access the bird: the exact time that AO-27 will be visible above the horizon, the compass direction to point the antenna (the azimuth) and how far above the horizon to tilt it (the elevation).
Once I started working stations on AO-27, I noticed some of the nice regular characteristics of its orbit: most of the passes are near lunch time, and on these passes it will always rise towards the north and set towards the south. Whether the satellite track more to the east or west depends on the particular pass. The transponder schedule for AO-27 has the mode J transponder active for these passes all the time. Less frequently, the transponder will also be active on the evening passes when AO-27 rises from the south and moves north. Compare this to RS-10 or MIR, whose schedule and directions vary greatly from month to month. It is relatively easy to follow AO-27 in its path across the sky once the signal is found. At the time indicated by the tracking program, point the antenna at the azimuth where the satellite should rise. You may need to adjust the antenna to obtain the best signal on the downlink. Try moving it left or right, up or down, or rotating it ninety degrees clockwise or counterclockwise. The important point is to aim the antenna for the strongest signal, using the exact azimuth heading as a rough guide only. Once the downlink has been acquired, adjust the antenna in azimuth, elevation and rotation as necessary to maintain the signal. It might take a few tries to get the hang of how to do this.
All satellite passes are not created equally. When I was using my original station with the scanner, I only tried to work AO-27 on those passes that would reach a maximum elevation of at least forty-five degrees. Lower elevation passes will place the satellite at a greater distance exceeding the capability of both the HT and the scanner. Passes earlier in the day will be more to the east and those later in the day will be more to the west. You can use this to your advantage. For example, along the eastern coast of the United States, the earlier passes will place most of the satellite's footprint out over open ocean. This reduces the number of satellite stations which can access the transponder, but it also reduces the interference caused by other signals in the two meter band. Certain passes over the Atlantic include both the United States and England within the footprint for a few minutes. When combined, these features make the lower elevation earlier passes a favorite among some of the operators on the eastern United States.
As the satellite travels overhead, its signal will appear to change frequency. This phenomenon is known as the Doppler shift. In order to compensate for doppler on AO-27, lower the receiver's frequency gradually as needed. Start listening five to ten kilohertz above the actual downlink frequency of 436.8 MHz. Set the two meter FM transmitter to the satellite's uplink ( 145.850 MHz ) and leave it there. The satellite will compensate for the doppler on the uplink. On FM, you want to listen for silence or a drop in the static level. This is your clue that you have the antenna pointed in the right direction and AO-27 is getting closer. Now wait until you can hear the downlink from AO-27. DON'T TRANSMIT UNTIL THE DOWNLINK FROM THE SATELLITE CAN BE HEARD! This is very important. The satellite WILL retransmit all the signals it hears. Some people transmit without hearing the downlink and only succeed in disrupting the pass for everyone else who can. You might want to listen to two or three complete passes before you even try transmitting. When the satellite hears your signal you will hear your own voice on the downlink. This is normal, but might take some operators by surprise. Headphones or earphones are highly recommended. It's very easy for the sounds from the receiver to get into the microphone and distort your uplink signal. There will be times when the signal from the satellite appears to be rapidly switching polarity and times when the signal fades for short periods of time. This is normal. Just try to work around it.
Satellite contacts on low Earth orbit birds like AO-27 are usually short and contest style, especially on weekends when many stations are trying at the same time. On weekdays there are far fewer stations.
Assorted tips and tricks for working amateur radio satellites
- I try to be ready for the pass five to ten minutes before the expected starting time. This allows adequate time for any last minute complications.
- During each pass I wrote down call signs, names and grid squares of every station that I heard that I didn't recognize. If I later worked one of those stations, I could use this information as a check that I had copied the QSO information correctly. I used a preprinted list that contained the time, azimuth and elevation along with room on the right hand side to write in callsigns, names and grids. This served as a quick log during the pass. The details were later entered into my logbook.
- I concentrated my efforts on improving the downlink. The uplink was easily obtained with just a few watts from my HT. The only problem is when it's captured (and denied to others) by stations running excessive power. As a beginner, I could not compete against the higher powered stations with large directional antennas, but I was able to make contacts when I dropped my callsign immediately after two other stations ended their conversation. I found that adding "QRP" or "mobile" to the end of my call makes me a more desirable contact.
- When working AO-27 mobile, I always brought two sets of keys and tried to find a good spot to set up my station. A great spot would have a perfectly flat horizon in all directions. Being on top of a hill was not necessary a good thing, especially if there was a radio tower on the hill with me. Strong signals from the tower could desense the receiver. I usually set up the components of my station on the trunk of my car in roughly the same positions. This minimized the confusion when looking for a particular knob, the microphone, log sheet, or whatever. Permanent stations don't have this problem.
- I didn't try to start off with a great station. I started simple. I got it to work first and then made incremental improvements. This way I got on the birds sooner than if I had waited and tried to build a really terrific station. I also learned how well the various parts of the station worked, and this allowed me to know when a modification had improved or degraded the performance of the whole system.
- I noticed that there is sometimes a slight difference between the predicted azimuth for a pass and the actual azimuth which gives the best signal. In these cases, I just pointed the antenna for the best reception and adjusted it periodically.
Getting started on AO-27 is easy. Anyone can do it, and most amateurs probably already own everything they need for the uplink! In addition, several characteristics of AO- 27's transponder and orbit make it ideally suited to the beginner. I hope this paper has been informative and instructive.
That's all folks! And of course, thanks to my wife who assisted in the preparation of this work. If you have questions or comments, feel free to contact me at firstname.lastname@example.org.
See you on the birds.
- Photo 1: The KA2UPW Satellite QRP Mobile station (43k)
- Photo 2: Closeup picture of the 435 quagi directors and cord locks (32k)
- Photo 3: The 435 MHz DEM preamp mounted directly on the boom (53k)
Last updated: September 1997