How does it work?

P-25 is a communication system essentially created and utilised by public service agencies in North America, the system allows communication between several agencies such as Fire Fighters and Paramedics and Police Officers. Digital communication systems are available to Ham radio Operators, D-star and Fusion to name but two. Like all other digital modes, the normal analogue signal which we produce when we speak, is sampled, in the case of G.S.M. mobile phone technology 8000 times per second each sample produces an eight bit word.

We have a similar digital system used here in the United Kingdom for our Public Service personnel; it's called TETRA, or Terrestrial Trunk Radio. Many of us started out in Ham radio by listening to the Emergency services although in those days the Emergency services used Amplitude Modulation and Frequency Modulation to communicate between mobile and base stations and there was no encryption in those days. I learned a lot about radio by converting receivers to the Emergency service frequency allocations.

Short wave listening or utility monitoring gave you a great sense and awareness where other services transmitted, utility listening added to this awareness and soon you became knowledgeable about out-of-band transmissions and how to keep harmonics from your transmitters to a minimum. A requirement by law for the active Ham Radio Operator.

Digital communication has the advantage over analogue technology because it allows many operators to use the same channel by splitting up time into bite size portions; these bite size chunks of data are converted into digital streaming packets of data and can be sent around the World via the Internet. By using this method of superimposing speech on to a radio signal, radio bandwidth is used more efficiently. Digital systems have the advantage over analogue in many other ways, once the speech is converted into digital signals they can be mixed up in a particular way that's how encryption is produced, making the transmission secure. Frequency hopping is also employed to make the process of recovering the digital signal more difficult to intercept.

Ham Radio Operators can now send and receive these digital signals from all over the World because devices are placed before and after transmissions to allow these digital streams to be sent over the World-Wide Web.

My Ham Radio days started with building simple crystal set receivers and tuned radio frequency, T.R.F. equipment to listen to Hams on top band, 160 metres. I soon developed an interest in the V.H.F. and U.H.F. spectrum two metres and seventy centimetre bands where I soon discovered transmissions from Aircraft and Ships.

Ham radio is a rich and varied subject and many find themselves experimenting with all of the modes and bands. I have no doubt that P-25 will be on the experimental radio hams experimental agenda.

John Allsopp G4YDM

Article Source:

Article Source: