When To Use Military Radio Receivers
Many military radio receivers are available for the public to buy, as long as they aren't for restricted channels or contain classified technology. Usually, this is army surplus or new consumer models for unrestricted modern frequencies. Let's use a particular receiver as an example: The r-390 military radio receiver can pick up radio in the 0.5 to 32 megahertz bandwidth. This is a portion of the electromagnetic spectrum used by the military, just pushing out of the high frequency spectrum and into the VHF low range. It has a "mechanical digital dial", which means that although the dial is physical, it is not simple attached to a potentiometer or tuning capacitor that directly affects signal reception. Instead, the position of the dial is digitally encoded and passed to the digital circuitry within the receiver that does the real decoding. Like many military radios, it's rack-mountable, so it can be mounted along side other equipment securely in a vehicle or station.
Modern military radios are also required to work in the 138 to 144 MHz range. This is a narrow band FM system spaced at 5 kHz intervals. The military also use VHF (very high frequency) and UHF (ultra high frequency) signals for various purposes. These kinds of receivers are more expensive and as such less popular with civilian enthusiasts. One particularly interesting kind of military radio receiver is point-to-point radio. In a combat situation, it is critical that communications are not tapped, but with a traditional radio transmitter, signals radiate from the antenna and can be picked up by any listener. Often the messages are encrypted, such that they can only be understood by those in possession of a "key" - a code that is used to decrypt the signal. This has disadvantages, though. Firstly, people get captured. If someone in possession of the key is captured, the encryption mechanism is no longer usable with that key. Changing keys regularly will limit the damage this does, but there are endless amounts of problems involved in distributing the keys to people that need them. If all of the keys are distributed before-hand, a captured comrade will still cause the whole system to be compromised. So, in addition to encryption, point-to-point radio is used. This is usually a very high frequency signal, since this kind of signal is more easily targeted and propagates poorly through obstacles. This characteristic makes it "line of sight" transmission. Normally a bad thing, since buildings and other structures put large areas into a radio shadow, but when the idea is to limit reception it is perfect. The transmission is further protected by blocking all paths that do not point at the target. That is, the signal is blocked apart from a single hole that faces the receiver. In this way, no third party can receive the signal unless it lies within the very tight cone of transmission. Interestingly, these kinds of transmission are so high a frequency that they are actually into the microwave portion of the electromagnetic spectrum. This kind of technology is still quite prohibitively expensive for anyone but large organizations and the military, so amateur users are rare. |
