Four years ago I dismantled my amateur radio station and moved to a new location. The new location, while perfect for my lifestyle, is less than ideal for amateur radio. To begin with, it is only 10 meters above sea level and surrounded by houses and apartments. Not the location for towers and associated antennas. I have however erected a small mast with HF wire antennas and VHF, UHF & SHF Yagi antennas that allow me to enjoy the hobby of amateur radio.
In addition, I also maintain an interest in operating portable especially in the Microwave arena up to 10GHz as demonstrated elsewhere in this Blog.
Recently my thoughts have turned to my other great fascination in amateur radio, satellite communications. Having actively participated in amateur radio satellite communication from its inception in 1957 until 2011, I decided that this sphere of amateur radio might provide me with hours of enjoyment from my new location. The reasoning behind this decision, centred on the fact that this phase of amateur radio does not require a hill top location with large antennas to participate.
With the decision made, my thoughts immediately turned to the inevitable question – what do I need? In theory all I needed to do was reassemble all of my previous equipment and start communicating, unfortunately it would not be that easy. When I dismantled my previous station, I sold much of my equipment and therefore, in reality, I would have to start again!
It is said that a picture is worth a thousand words and so I decided to develop the following diagram to clearly identify the basic components for a practical satellite communication system that would allow me to enjoy this facet of amateur radio and to provide scope to utilize a variety of modes and frequencies. In devising this plan, I don’t wish to give the impression that I have very “deep pockets” for this project, I don’t! This plan was devised to produce a modest satellite station that would be capable of providing hours of enjoyment at minimal cost.
This plan and subsequent project, has been developed around the following six components:
1. The computer and software requirements
2. The transmitting and receiving radio system
3. The radio control system
4. The feed lines, pre-amplifiers and de-sense filter system
5. The antenna rotation control system
6. The antenna system and polarity switching system
While developing this plan I did think about revisiting my portable amateur radio satellite station that is based on a pair of YAESU FT817 ND transceivers and a tri-pod antenna system. However, while that may be the subject of a future article, for this project, I wanted to develop a communication system that would permit me to sit back and enjoy space communication from the comfort of my “all weather” home environment.
The following “block diagram” identifies the basic components of the Project.
From the information outlined in the plan, it should be obvious to the reader, that this project embraces multiple facets of amateur radio including elements of computer control.
Let the Project begin
Like all good projects, there is a need to start with a solid foundation. The foundation for this project centres on the need to develop a clear understanding of the characteristics of satellites and how to communicate with them. Once this is understood, then all the other elements of the project fall into place.
The following is a list of 12 questions that I believe must be answered each time satellite communication is attempted. If the answers are NOT IMMEDIATELY available then all hope of a successful QSO will be lost!
The required answers needed for each satellite communication are:
1. Where will satellites first appear at my location (azimuth in degrees – AOS)?
2. Where will the satellites leave my area (azimuth in degrees – LOS)?
3. How high do the satellites get as they cross my part of the sky (elevation in degrees)?
4. When will the satellites arrive in my area (time of day – either local or GMT)?
5. When will the satellites leave my area (time of day – local or GMT)?
6. What frequencies will I receive the satellites on (downlink frequency)?
7. Will the receive frequency change (Doppler Shift frequency +/-)?
8. What frequency will I transmit to the satellites on (Uplink frequency)?
9. Will the transmit frequency change (Doppler shift frequency +/-)?
10. What mode of operation will the satellites be using (SSB, CW, FM, PSK31, SSTV to name but a few)?
11. Will I have to turn the satellites on (activation tone (frequency in hertz))?
12. Will I need to use an access tone when communicating with the satellites (frequency in hertz)?
The answers to these questions, while quite simple in nature, must be available immediately and simultaneously when a satellite makes its appearance. It should also be remembered that each satellite requires a different set of answers each time it enters any given location. There is no time to look up the answers or calculate an answer, because in many cases the satellite you wish to communicate with will have disappeared from your part of the sky if all the answers are not known before hand.
In the early days (OSCAR1) of amateur radio satellite communication, every set of answers had to be calculated ahead of time using a slide rule or calculator and the satellites path across the sky plotted on a great circle map. This was very tedious and though it was not difficult, it was only part of the story. Once the satellite appeared then the operator had to ensure that the following conditions were met.
1. The antenna was always pointing at the satellite (correct azimuth, elevation and signal polarity based on accurate time)
2. The transmitter and receiver were on the correct frequency (different frequencies / different Doppler shift that is changing from horizon to horizon)
3. Correct mode on receive and transmit (eg., USB on TX & RX or inverting LSB on TX & USB on RX)
4. Correct gate tone and access tone if used (these are usually different. The gate tone comes first and it must be reset every 10minutes while the access tone is required for each communication exchange)
This meant that the operator had lots of things to think about and complete – hardly a simple exercise! (An octopus comes to mind!)
Thankfully this has changed with today’s satellite communication process. It is now a relatively simple process due to the availability of computers and smart software.
It is this last sentence that identifies the basic foundation system for all modern amateur radio space communication stations. Without the use of a computer and smart software this project would be doomed to failure. It doesn’t matter how good the antenna, feed lines and transceivers are, if the answers to the 12 questions discussed above are not known or incorrect, satellite communication won’t happen.
It should therefore be evident to the reader that the computer system and its software, have an important role to play in a satellite communication station.
The computer and smart software
The choice of a computer is a very personal thing and it really does not matter what computer is used except it must have the ability to provide a number of services. Some of the services are displayed on the computer screen, some are in the form of control codes that are sent from the computer ports, some services interact with the internet gathering data, while others require the operator to input data from the keyboard.
All of these services require the use of smart software if they are to be successfully applied.
A search of the Internet will reveal a number of smart software packages designed to provide answers to the questions outlined above. In addition, many of these software packages have been designed to work with a variety of computer operating systems that can be found on smart phones, tablets of all kinds and desktop or laptop computers. In fact the amateur radio operator is spoilt for choice.
Before choosing one of these smart software packages, a decision must be made about the type of computer and operating system that is to be used?
In reality, the sheer volume of INTEL/Microsoft based computers in the marketplace tends to place them at the head of the queue for most people. Further because of their popularity, the prices of these systems can be relatively low. In addition there are many second hand systems that are available at very low prices that will provide years of faithful service.
Over the years I have tried using Microsoft, Apple and Linux computer systems, using a variety of smart software packages; however I ultimately returned to a Microsoft system with the program SatPC32 as it satisfies all my needs. The SatPC32 smart software is an excellent program that provides the answers to all the questions outlined above. Erich Eichmann DK1TB wrote the program, that has gone through a number of version updates over the years, with the current version being 12.8c. DK1TB donated the program to AMSAT to help progress the use of satellites for the benefit of amateur radio throughout the world. This program can be downloaded from AMSAT for a small fee. The moneys collected are used to supplement a variety of AMSAT projects.
No matter what computer is used, it must provide as a minimum, two (2) ports to talk to the physical world. One of these ports is used to interface with the antenna rotation system (azimuth and elevation) and the other is used to control the radio transmitter and receiver (Doppler shift, frequency of receiver and transmitter, mode SSB, FM etc.,). These ports must function in real time to ensure accurate satellite tracking. The computer must also provide an Internet connection to gain access to the Keplerian elements for the satellites to be worked. Keplerian elements or “Keps” as they are generally called, provide accurate orbital information for every satellite being tracked.
The SatPC32 program has an excellent manual included with the software package, that covers all aspects of the software setup and while it may appear a little daunting at first, once the software is setup, the manual is rarely consulted.
The satellite radio transmitter and receiver
The selection of the satellite radio is central to the successful implementation of this project. There are a number of single band and or dual band transceivers that are suitable and it is not my intention to look at all of the possible makes and models. However the single band and or dual band transceivers selected need to have a number of specific attributes. The major amateur radio manufactures such as ICOM, YAESU and KENWOOD have produced suitable single band & dual band transceivers over the years and many can be purchased on the second hand market at quite modest prices.
The key requirements for the single band and or dual band transceivers are:
1. Frequency coverage (145 – 146 MHz) and (435 – 436 MHz)
2. Duplex operation (simultaneous transmit and receive eg., TX on 435Mhz and RX on 145Mhz) This can also be achieved with two single band transceivers coupled together, however dual band transceivers are certainly more convenient
3. Computer control of frequency, mode (USB, LSB, CW & FM), access tone and band switching
It should be stated, that depending on the satellite and its orbital height, the satellite can be transmitting and receiving over a distance of greater than 4500 Km. This will mean that the receivers must have reasonable sensitivity and have a low noise figure. However deficiencies in receiver sensitivity and noise figure can be enhanced by the use of masthead preamplifiers (discussed later) that can also compensate for feed line losses.
Controlling the satellite radio
As stated previously, for the SatPC32 smart software to be successfully used, the computer, on which it is running, needs to have external ports to control the radio and antenna systems.
The software makes provision for the control of individual brands of transceivers and discrete receivers and transmitters however in every case there is the need to install a hardware interface between the computer port and the radio hardware. A search of the Internet will reveal hardware interfaces for ICOM, YAESU, KENWOOD and many others. These interfaces must be bi-directional to ensure complete handshaking occurs between the radio and the computer software. I have used a variety of these interfaces and all have performed flawlessly. These types of interfaces are available commercially or they can be constructed using a plethora of circuits that can also be found by searching the Internet.
My current interface is a copy of an ICOM CT-17 interface (I am currently using an ICOM transceiver). The circuit is very simple and easy to construct and is mounted into a metal box. My current transceiver is an ICOM IC-910H and this interface allows all the features of the transceiver to be controlled. I also have the ICOM twins, IC-275 and IC-475 transceivers being controlled using this type of interface without a problem. I have also built the YAESU equivalent of this circuit, to control two (2) FT-817ND transceivers for portable satellite operation (as mentioned earlier) using SatPC32 without problems.
This circuit provides an RS-232 interface for the computer. My computer however, provides only USB ports and therefore it is necessary to have an RS-232 to USB interface to complete the connection. I use a TARGUS brand RS-232 to USB interface (PA088E). These units are compatible with all Microsoft operating systems and totally compatible with SatPC32.
This radio interface is the critical area for satellite operators. Failure in this interface effects all areas of SatPC32, with the exception of the antenna control, so time spent in ensuring the radio control interface is working well is critical.
Amateur radio satellite communication is a duplex communication mode, meaning that when communicating through a satellite, the operators can hear their own signal when they are transmitting. Failure in this area will see some operators simply continue calling the satellite even when they can’t hear their own signal and in many cases they transmit on top of other QSO’s, because they are not correctly transceiving. Every satellite operator has done this so while it can be frustrating for all concerned, it is a common problem, when first starting to communicate through satellites and the technique is soon learned.
SatPC32 has been designed to assist with this problem by providing a separate menu in the toolbar called “CAT”. When this menu is selected the operator is presented with netting controls that allow the operator to change the frequency of the transmitter/receiver in very small increments so that correct netting can be achieved. Success in this area is only possible if the radio interface is working correctly.
The most efficient way to check that the transceiver/transmitter and receiver are transceiving correctly, requires the operator to tune to an unused frequency within the band pass of the satellite. Then raise or lower the frequency of the transmitter using the CAT menu controls, while transmitting a string of “dots” in CW. Listen to the received frequency until a clean CW note can be heard. Once this has been achieved, save the offset using the menu settings.
This operation has to be undertaken for each satellite. Once set, there should be no reason to change these settings for future QSO’s using the satellites configured. Any slight discrepancies in transmit and receiver frequency can be adjusted using the “RIT” on the receiver. For further information on this topic, especially in the area of Doppler shift, please consult the SatPC32 software manual.
The following image is a picture of the SatPC32 actually running. The image shows a number of amateur radio satellites and their given locations at a particular time as they orbit the earth. The circles around each satellite represent the “footprint” or coverage area of that particular satellite. It should be noted that there is quite a variation in the diameter of the circles which is a result of each satellite having a different orbit height. The larger the circle the higher the satellite orbit.
The letters of the alphabet in the lower right of the image identify the satellites that are shown in the “count down table” at the top right of the image. The table shows satellites AOS/LOS times and maximum elevation that the satellite will achieve relative to the earth station. In this instance it is my location (Lat/Lon shown on the bottom left of the image) in Brisbane.
This software covers every aspect of satellite communication, all of which has been well documented in the supplied user manual.
Antenna feed-lines and pre-amplifiers.
This is an area that can make or break the enjoyment of satellite communications. It is very important to select the best feed-lines that your financial budget will allow. Amateur Radio Satellites have quite low power transmitters (1w or less) in comparison to their commercial counterparts and therefore the satellite signal arriving at an antenna on the surface of the earth is very small indeed. As mentioned earlier, the signal from the satellite may have travelled in excess of 4500Km (the actual distance is dependent on the orbital height of the satellite). In the case of OSCAR AO-07 (with an orbital height of almost 1100Km) when it first appears over the horizon is approximately 4448Km away and while many of the satellite orbits pass overhead which brings them as close 350Km (ISS a typical example) a great many orbits only position the satellites a few degrees above the horizon, resulting in the satellite remaining about 3000Km away.
Based on this situation and considering the frequencies in use are VHF and UHF, it should be obvious to the reader that using high loss feed-lines will result in little or no signal being fed to the receiver especially when the receiver is located well away from the antenna.
My solution to this dilemma was to use LDF-450 Heliax on both 2m and 70cm as my feed-line requirements were for a run of 20m.
This solution, while quite satisfactory, still produced measurable losses and to compensate I added mast head pre-amplifiers to compensate for the losses and to improve the overall noise figure of the system. The pre-amplifiers provide a maximum of 10db gain that covers all the losses between the antenna and the receiver input. These pre-amplifiers are positioned at the top of the mast just below the AZ/EL rotator. A word of warning! Don’t use a pre-amplifier at the receiver/transceiver end of the feed-line as it amplifies the noise picked up by the feed-line as well as the satellite signal, destroying the signal to noise ratio.
When satellite communication requires the operator to transmit on 145 MHz and receive on 435 MHz (Mode J) the third harmonic of the transmit frequency can overload the receiver on 435 MHz masking the duplex down link signal from the satellite and destroying any possibility of duplex operation. To overcome this situation there is a need to insert a filter in the receiver feed-line to suppress this unwanted signal. There are many types of filters that can be used but the one I have used for many years is based on a diplexer. A version of the filter is described on the AMSAT-NA site.
Any brand of diplexer can be used (I have used a Diamond MX 72N with great success). The antenna connection of the diplexer is connected directly to the 435 MHz antenna and the 435 MHz output connection of the diplexer is connected to the 435 MHz pre-amplifier antenna input. The 145 MHz connection of the diplexer can be left un-terminated however I have found in practice that terminating the 145 MHz connection with a 1watt 50ohm load removes all vestige of the de-sense problem. The diplexer, used in this configuration, acts as a high pass filter providing approximately 70db rejection of the 145 MHz up link signal.
As mentioned previously, the SatPC32 software has been designed to control the azimuth and elevation rotator systems to ensure that the antennas accurately follow the path of the satellite across the sky. This process requires a hardware interface between the computer port and the rotation system. Since the first amateur radio satellite was launched in the early sixties, amateur radio operators have used a variety of rotators to effect azimuth and elevation control of their antenna. During that period a variety of circuits were published to control automotive windscreen wiper motors and the like to perform the task. Many of the circuits used 4bit and 8bit semiconductor microprocessor with varying success.
In the late 1970’s the KENPRO Company released its KR-5400 and KR-5600 series of rotators with a proprietary KR-10 microprocessor controlled interface. The satellite community worldwide soon adopted this solution and it became the standard against which all other rotator manufactures were judged. While the proprietary interface worked well with the early DOS based programs such as GRAFTRAK, with the release of the Microsoft Windows based operating systems, it was soon replaced by a variety of industry based systems.
When the YAESU Company bought out KENPRO the YAESU Company continued to produce the rotation systems releasing their own hardware interface that is still commercially available today. There are also quite a number of hardware interfaces that are also manufactured for these rotation systems by other companies around the world. One such company, FoxDelta is based in India and this small company has won wide acclaim, throughout the amateur radio fraternity, for its range of kits and assembled projects including a hardware interface “ST2 LCD Satellite Tracker and Rotator Controller” for the KR/G series of rotators.
The SatPC32 software is designed to accommodate this and a variety of other controllers and rotation systems. Most but not all of these interfaces require an RS-232 port to interface with the computer system. Unfortunately, as mentioned previously, most computers have moved away from this style of interface providing only USB. The solution is to use an RS-232 to USB interface to make the conversion.
These images show the KENPRO KR 5600 rotator controller. The external interface can be connected to the DIN socket on the rear of the unit.
There is no right interface or rotator. The best interface is the one that supports the rotation system being used and once setup, will allow the operator one less thing to
adjust when communicating using satellites.
In my case, I have used the KENPRO KR5600 that I purchased in the early 1990’s and a home brew interface.
The frequency used by satellites varies, however there is a series of standards that have prevailed from the beginning of the amateur radio satellite program. These standards allow the amateur radio operator to confidently build or purchase antenna systems capable of communicating with satellites operating now and in the future.
These standards refer to satellite modes that are the combination of uplink frequency, downlink frequency, and transmission mode. The following is a list of common satellite modes used by satellites in current use:
A – This mode requires a 2 meter SSB/CW transmitter and a 10 meter SSB/CW receiver and supports CW and voice.
B – This mode requires a 70 cm SSB/CW transmitter and a 2 meter SSB/CW receiver and supports CW and voice.
Some satellites also support RTTY and SSTV in this mode.
K – This mode requires a 15 meter SSB/CW transmitter and a 10 meter SSB/CW receiver and supports CW and voice.
This mode is a unique satellite mode in that communication can be achieved with a standard HF transceiver.
JA – This mode stands for J Analog and requires a 2 meter SSB/CW transmitter and a 70 cm SSB/CW receiver and
supports CW and voice.
JD – This mode stands for J Digital and requires a 2 meter FM transmitter and a 70 cm SSB/CW receiver and
S – This mode requires a 70 cm SSB/CW transmitter and a 2.4 GHz SSB/CW receiver and supports CW and voice.
Of these six (6) standards, only three (3) are in common usage. These are mode “B, JA and JD” and therefore initial antenna needs are for 2m and 70cm. However it should be remembered that mode S has been used in the past with outstanding success and will be used again in the future.
Many antenna types have been used to communicate with satellites since they were first launched in the early 1960’s but, with the benefit of hindsight, the antenna type that has stood the test of time is the Yagi antenna. The most successful adaptation of this antenna is the crossed Yagi. This antenna can act as a circular polarized antenna that can be configured to accommodate a clockwise (CW) or counter-clockwise (CCW) signal capture from a satellite as it rotates about its “Z” axis. To capitalize on this feature, the antennas can be fitted with a relay switching system that facilitates CW and CCW polarization switching by the operator.
This is a very critical part of the project. The type of antenna and its placement will determine the successful or failure of the project. Unfortunately there are many articles on the Internet that describe antennas for satellite communication that are really impractical for Australian conditions. In Australia the number of satellite operators is small in comparison to the USA, Europe and many other parts of the world. This means that most of the satellite contacts occur when most operators are thousands of kilometres apart compared to Europe and the USA for example where operators are only a few hundred kilometres apart. This means that in these countries most operators can have multiple, short duration, contacts with stations that are quite close together in distance. Hence the antenna requirements are very modest indeed.
When deciding on the types of antennas for use in Australia for amateur radio satellite communication it is necessary to consider details that have little relevance in other fields of amateur radio. The decision on what type of antenna to use is determined by the “link budget” required when transmitting to the satellite and the “link budget” required to receive the satellite. These are NOT the same as they are on different frequencies and are distance dependent (The higher the frequency the greater the losses).
Link budget and the satellite antenna
Since the beginning of the space race in 1957 there has been considerable attention paid to the path between the satellite and a ground station. While this path is line of sight, it is made up of a number of components that ultimately attenuate the radio signal. Many of these components are quite small while others can eliminate the signal all together. Many of the components that attenuate the signal are frequency dependent and hence the attenuation experienced on 145MHz is small in comparison to that experienced on 435MHz. To this is added the attenuation of the signal over distance which is smaller on 145MHz than on 435MHz.
When selecting satellite tracking antennas, the antenna must provide sufficient gain to receive a signal from a low power, battery operated, transmitter on-board a satellite in space. A LEO (low earth orbit) satellite can vary in height above the earth’s surface from approximately 390km to 1100km. The satellites footprint (the area that will allow communication to and from the satellite) can have a diameter of over 9000 km. This means that the antennas used must be capable of hearing a signal from the satellite that may be 4000 to 4500 km away at the horizon if maximum communication time with the satellite is to be achieved. As an example of this, if the OSCAR FO-29 satellite passes north over Brisbane in its orbit around the world, it is possible for a satellite station in Brisbane to speak to a satellite station in Tokyo. To achieve a QSO of this type, the stations at each end need to have satellite antennas that are capable of transmitting and receiving signals over a distance of greater than 4500 km. This cannot be achieved with low gain antennas.
In previous years, satellites such as AO-10, AO-13 and AO-40 were placed in elliptical orbits that achieved heights of 43,000km to 45,000 km at apogee (maximum distance from earth) requiring still higher gain antennas to allow communication all over the world.
It is not my intention in this project to explore all of the components that affect the attenuation rates of the signals to and from a satellite. I do however wish to point out why it is necessary to use antennas in Australia that provide more gain than many of those used elsewhere in the world.
The calculation of the link budget to determine the amount of gain required by the antennas used in Australia for amateur radio satellite communication on 145MHz and 432MHz is outside the scope of this article.
The antenna system
As outlined above, the initial antenna system needs to provide transmitting and receiving capabilities on the satellite frequencies in 2m and 70cm bands. Coupled with this is the need to establish antenna systems that can provide sufficient antenna gain to permit satellite communication to cover a distance of 4500km on each of the satellite bands identified to ensure maximum benefit from all the satellites that are available in this part of the world.
There are a number of antenna manufacturers/suppliers that offer satellite antennas, however there are only a few that provide polarization switching facilities and therefore many operators elect to make their own antennas.
In my case I elected to build my own antennas. I decided to use a crossed YAGI design on each band. I also decided to add the facility of polarity switching thereby ensuring maximum efficiency from the antenna.
The antenna I selected for 145MHz is a 6 x 6 element circular polarized YAGI with one set of elements displaced ¼ wave ahead of the other. This antenna has a gain of 10dBd.
The 432MHz antenna is 16 x 16 element, circular polarized YAGI with one set of elements placed ¼ wave ahead of the other. This antenna has a gain of 15dBd.
The following image shows the mounting method I used to gain element offset on 435 MHz. The same method was used on the 145 MHz Yagi.
The forward plastic box on each of the antennas contains a small 12 volt relay that is used to change the sense on the antennas from CW to CCW circularity. The theory for this operation can be found in Mak SV1BSX excellent series of articles on polarity switching.
Both these antennas have taken advantage of the link budget requirements and provide an adequate margin in signal strength from all satellites at 4500km.
While I acknowledge home brewing of antennas is not for everyone, a search of the Internet reveals quite a number of excellent antenna designs. I think however, one of the most informative of these sites is that of Martin DK7ZB. I have built a number of his antenna designs in the past and they perform very well indeed. As mentioned earlier, another site that provides a wealth of information relating to the requirements of circular polarization is that of Mak SV1BSX. This site explores the important and critical dimensions associated with the circularity switching of crossed YAGI antennas.
The following are pictures of my antennas. The cross boom supporting the antennas is made from fiberglass. Having a non-conductive support boom prevents coupling of antenna fields from each antenna, therefore preventing distortion of the antenna radiation pattern. The antennas are mounted in the form of a “X” that helps prevent noise pickup radiated from the ground that can be quite noticeable when beaming at the horizon. The grey box mounted on the mast houses the pre-amplifiers for each band and also a de-sense filter. It should be noted also that the relay switching cables and feed lines are routed from the rear of the antennas to prevent antenna pattern distortion.
This image shows the 435MHz crossed YAGI under test in the garden.
What a terrific project this has been for me! I have gained a great deal of pleasure in exploring the many pathways that can be followed when setting up an amateur radio station to focus on communicating with satellites. The results have allowed me to catch up with some old friends that I have worked on satellites over the last forty nine plus years.
Looking back over my logs of satellite communication, I find I have worked a total of 190 countries using amateur radio satellites with greater than 95,000 contacts and my interest in space communication is still as keen as ever!
I hope that this small expose’ may provoke a spark of interest in space communication to those that take the time to read this Blog.