I saw a release recently about a new family of receiver protection diodes. Receiver protection is a pretty important issue in radio transceivers of all kinds. Think about it…the receiver front-end is built to amplify very small signals – fractions of a microvolt – and needs to be protected from the output of the transmitter. Adding any kind of classic protection device like a resistor in series with the input and clamping diodes would degrade the noise figure of the receiver, so that’s a bad idea. There are easy ways to do this. One is to use separate antennas for transmitting and receiving. Another easy way is to use separate frequency bands for transmitting and receiving, and brute-force filtering on each port.
But having two antennas or using a brute-force filter is a problem in a lot of radio systems. Filters get pretty big when the transmitter is delivering a lot of power. And sometimes the radio needs to listen on the same frequency as the transmitter, as in radar systems. It is generally preferable to have the same antenna serve for transmitting and receiving…it saves space, and the radio benefits from the antenna’s gain and directivity on both the receiving and transmitting sides. Switching the antenna between the transmitter output and the receiver input quickly with minimal loss is a long-standing problem in radio design.
One design where a successful T/R switch (as the transmit-receive switching function is called) became a game-changer was at the MIT Radiation Lab during World War 2. The story of this lab makes fascinating reading – it was detailed in a book called “The Invention that Changed the World” by Robert Buderi. The Rad Lab work had nothing to do with developing the atomic bomb…instead, the engineers there invented millimeter-wave radar. In the book, the author describes how the T/R switch problem became critical – it was impractical to carry two high-gain antennas on each aircraft.
The problem was solved by Dr. James L. Lawson, known to hams as W2PV. Lawson applied his ham radio experience and lateral-thinking skills to develop a solution that used a spark-gap diode and a combination of strategically-placed quarter-wave and half-wave transmission lines to switch the antenna and protect the receiver. The photo of the vintage 1940 RF switching diodes is from Volume 14 of the 28-volume Radiation Laboratory series of books on radar system design. Lawson was a major figure in the Rad Lab effort. In Buderi’s book, he quoted one engineer as saying that “If the lab’s salary had been paid commensurate with each employee’s contribution, Lawson would have taken home at least half the Lab’s budget.” Lawson went on to become a VP at the GE Corporate R&D lab in New York, and passed away in 1982.
When the first radar using a single antenna was tested successfully on the roof of the Rad Lab building in Cambridge, MA, the management of the radar project received a telegram stating “Have succeeded in looking with one eye.” Everyone knew what that meant. The work at the Radiation Lab won the war. Now that’s what I call a “game-changer.”
While this is a wonderful history, the practical application today stems from TDD systems such as WiFi in various bands. Since Tx and Rx are on the very same band, often protection is offered in terms of protocols such as CTS but the receiver itself operates only using higher signal levels (about +26dBm Tx and -85dBm Rx on the worst case). Higher the signal level of Rx, higher modulations can be used.
Although this article mentions it in passing, the FDD method (Tx and Rx on separate frequency bands - and 45 MHz is typical) offers a better solution and opted by most wireless service providers today.
The magnetron was indeed a game changer. Where would we be without the microwave oven?
Receiver protection is very important though. I designed ultasound front ends and got a good appreciation for that.
At the start of World war II ( 1939 to us ) the British, and the Germans had working single dish radar systems. With 'quite good' receiver protection, might not have been as the MIT team did, who undoubtedly patented the invention, but I do think that compared to say the magnetron, which made small powerful CM radars possible, the TR switch was not that big a step change.
UPDATE: I was talking about this entry with a friend the other day and he informed me I got the quote wrong - and the actual story is even more colorful. The success in using a single antenna was communicated to England over a commercial shortwave broadcast as "Today we saw Mary Baker Eddy with one eye". This meant they had produced a radar image of the dome of the Christian Science church (founded by Ms. Eddy) across the river in Boston.
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