This guest post is by Michael C and entry in our non-fiction writing contest .
(This is part one of a four-part series - intro, history, AM, Carrier)
There are less than 4 million active ham radio operators in the USA – about 1% of the population. I use my ham radio mainly to contact a friend about ten miles away. I don’t get involved with the contests or DXpeditions - just wanted a better radio then the CB radio. Ham radio is about contact between two people at a distance – sometimes great distance like around the world. I am going to try to explain Ham radio above the “refer to a book” level for those considering ham radio as a means of communication. A slight history of Radio is in order to bring those who don’t know much about “radio” up to speed.
A man by the name of Hertz discovered electromagnetic waves could move through the air and effect a similar shaped “receiver”. The first real radio was made by someone named Marconi. Radio was born but not before a man by the name of Tesla figured out that tuning different frequencies were possible. Amateur radio operators were some of the first people to take advantage of the “wide open airwaves” with the first radios built.
The radios were cheap to create – a crystal set only required finding a good crystal, a coil of wire and an earphone. The Edison light lead the way for the radio tube which allowed the “amplification” of any signal. The incoming signal from the antenna is amplified and then the audio signal is amplified. A small tube receiver can amplify signals hundreds of times – allowing weak/far away radio to be heard. So, for a few bucks more you could have a pretty good radio that played through a speaker. The transistor was capable of amplifying the signal thousands of times and saving some battery power.
Even though Morse code is the easiest signal to transmit, it is not vocal and I will confine my article to verbal transmission. Audio signals are the “real time” type of communication needing constant update. AM, SSB & FM transmission modes were developed for audio. It would be criminal to not mention ARRL (Amateur Radio Relay League) an organization that has supported ham radio operators almost since the beginning. For those who have never operated a radio – just press the key on the microphone and talk, then release the key and listen for a response.
The first and easiest radio mode is the AM (Amplitude Modulation) radio. CB radio is AM radio transmission based. The concept is simple – put a sound wave inside (or outside) of a “carrier” wave. The sound wave is audio frequency, for the early microphones about 300 to 2000 Hertz. The carrier wave is a mirrored sine wave that cycles at the “air wave” frequency. The sound wave is simply “fitted” into a carrier wave shape.
Although the sound wave “edges” the carrier, if there were no sound – the carrier would still be created and transmitted. This was called an empty carrier or in CB slang a “dead key”. This is used to “drown” out other CB’ers. I mention mirrored sine wave – the “electromagnetic” field created (by the antenna) has a north pole and a south pole. Both of these poles (amplified signal) must be beamed out by the antenna. The frequency of “AM Radio” is around 525 to 1710 (1.71 MHz) Kilo cycles and if we care about ham radio frequencies then the first ham frequency begins at 2 MHz (2000 Kilo cycles) and extends to 4 MHz. AM dominates the HF (High Frequency) spectrum up to about 29.0 MHz where FM is finally available. It is the age-old problem for FM – not enough band width to support the mode.
This contest will end on December 16 2012 – prizes include:
- First Place winner will receive a Go Berkey Kit valued at $150.
- Second Place: $100 Cash.
- Third Place: $50 Cash.