Do ham radio operators make good engineers or do good engineers make ham radio operators? Either way, the profession and the hobby go hand-in-hand in an investigation as to why my father's toys were destroying each other. "Son, it's time to pay for some of that college education!"
My father spends a considerable amount of time with his ham radio
hobby. Over the years he has created an elaborate station composed of a
room full of radios and a backyard full of antennas. Together the
equipment can operate most frequencies from DC to light.
Perhaps dad slacked off on his operating schedule this summer, because
recently it seemed like the radios were becoming jealous of each other
and competing for dad's attention. When the radios failed to gain his
attention by conventional means (whatever conventional for a radio is),
they began seeking revenge on each other and began destroying each
other's components. What is going on here?
A few crops from K8TL's Antenna Farm. The satellite antenna is on the
left. The Moonbounce antenna is in the center. The HF antenna is on
One of dad's favorite communications modes is satellite communications where a signal is relayed through a satellite from one side of the country to the other. He got this bug in 1983 when the space shuttle Columbia carried aloft Owen Garriott W5LFL. Owen used some of his spare time in space to try to communicate with earth-bound hams. We failed to contact the shuttle but had the bug and dad proceeded to build a bigger and better station.
The east wall holds the VHF, packet radio and satellite equipment.
Staying with the "space" theme, dad recently expanded his interests to an operating mode affectionately known as "Moonbounce." Technically, it is called Earth-Moon-Earth or EME. In the 1960s, NASA had a monopoly on this type of communications. Today's electronics have placed this extraterrestrial communications ability into the hands of your neighborhood amateur radio operator. To "Moonbounce", a high-powered radio signal is transmitted directly at the moon. The moon acts as a passive reflector and a portion of the radio signal is reflected back to planet Earth. Any stations that can see the moon can talk to each other.
The Northwest corner holds the Moonbounce equipment. The power amplifier is shown on the right.
In order to reach a low-orbit satellite or space shuttle at a mere 400 miles, 25-50 W of transmitter power is sufficient. That is in the neighborhood of a cellphone base station of approximately 50 W. Your pocket cellphone is less than 2 W. To reach the moon at a more impressive 238,857 miles, a transmitting power of 350 W to 1500 W is used. On top of that, you will need an antenna gain of 100 times or more (21 db). The closer you are to the 350 W, the more antenna you will need. On the receiving side, you will need a good GaAs FET Low Noise Amplifier (LNA) that provides a gain of 50 times (15 db). The lower your partner's transmitting power the more gain you will need.
Prior to the ubiquitous Internet email we all use, ham radio operators pioneered some of the concepts by using their orbiting satellites to store and forward messages. It became convenient (and fun) to create an automatic station that could lock onto the satellite and transfer the mail. This feature could be left running if the equipment is not in use for another activity. Since dad's ham station has multiple radios and antennas, he was able to leave the satellite email running while he operated another portion of the station such as HF or Moonbounce.
Shortly after the Moonbounce station upgraded from 350 W to 1500 W, strange things began to happen. The satellite email system stopped working. It seemed like the receiver could no longer copy the reflection of the transmitted signal. Tuning around the band revealed that the receiver worked fine. That left the transmitter as the culprit. Trouble shooting and repair revealed that a number of components including the power amplifier MOSFETs were dead. This appeared to be a cascade effect from a toasted tuned circuit on the antenna side of the power amplifier. Transmitting into the shorted tuned circuit caused the failure of the MOSFETs. But what caused the tuned circuit to fail?
While leafing through the log, dad noticed that the last successful transmission from the satellite station occurred prior to the 1.5 kw upgrade to the moon station. On a hunch, the satellite station was replaced with a watt meter and a dummy load in place of the receiver. With the antennas parallel, I keyed the EME transmitter. With about 750-W output, I measured a bit under 200 W coming down the satellite antenna coax. The satellite antenna was receiving a massive signal from the EME transmitter. Though careful thought had been given to protecting the receiving portion of the system, it wasn't originally obvious to protect the transmitter.
When the satellite antenna and the Moonbounce antenna cross paths while in action, you are now putting high power into a sensitive receiver from 20 ft away. The transmitters finals want to send, not receive. The receiver wants a weak signal, not a strong one. Mismatch these conditions and the life expectancy of your electrical components (LNA and PA) becomes quite short.
So if either radio was operating and the antennas were pointed together, various combinations of transmitting and receiving could cause the pre-amp to be overloaded and fail. Or, either transmitter power amplifier could "receive" a high power signal and fail.
A little attention to antenna aiming solved the mystery of the failing components and caused the toys to begin playing nice with each other (and with dad).
(KE8FP@arrl.net) is a software engineer building premium automotive suspension systems for Beijing West Industries. Steve frequently appears in embedded systems contests by Circuit Cellar and most recently, by Renesas. Steve has an Extra Class Amateur Radio license and he repays his college "loan" by building Amateur Radio embedded systems for his father K8TL.