Just because spies often have to work alone, doesn't mean they don't have to communicate with each other. The secrets you uncover are useless if you can't transmit them to your superiors, and any large scale operation is destined to fail if you can't coordinate with other operatives in the field.
There's a vital piece of information you need to get from one of your fellow operatives. Only problem is, you can't be seen meeting with each other. It's up to you to find a way to communicate without alerting your enemies. Assume all phone lines and computers are being monitored, so the only way to communicate will be to improvise something.
Whatever you construct must allow you and your fellow operatives to communicate covertly across a distance of at least 100 feet, with at least one man-made barrier separating them.
THE SCIENCE OF COMMUNICATION
Communicating over great distances is a remarkable scientific feat with many interesting engineering challenges.
The longest transmission ever received originated from the spacecraft Pioneer 10 in 2003 when it was more than twice as far as Pluto is from the Sun. At that distance it takes a radio signal more than 11 hours to arrive at Earth! On board is a plaque with the engraving of humans and a diagram of our solar system in the event an alien might find it and wish to locate us. Communicating with aliens is one mission of the radio telescope at Arecibo.
Closer to home, Elephants are known to communicate by using Infrasound. With vocal chords eight times longer than humans they can produce sounds we cannot hear. What is amazing about low frequency sound is that it can transmit long distances through the atmosphere without much attenuation. You likely recognize this whenever you hear the rumbling of an approaching storm; you are able to hear the low frequency thunder but not the high frequency cracks of lightening until the storm is right above you. Elephants can make these low frequencies transmit nearly ten kilometers!
Long before electricity people communicated with the use of signals. Native Americans used drums and smoke. Napoleon actually used a Semaphore line to communicate in battle. Semaphores are an alphabetic code symbolized by the use of two flags held in different positions. By using line of sight between permanent stations signals could be transmitted thousands of miles. It was even possible to send messages across the English Channel using a similar system with light houses. The rate of data transmission is extremely limited just as it is with the telegraph, invented by Samuel Morse in 1837. The fastest anyone can send Morse Code is about 75 words per minute. Communicating with submarines is also very data limited.
Courtesy: Michael Lampert, Science Teacher, West Salem High School, Salem, OR & Advisor to the Burn Notice Science Challenge