Repeaters and Internet Linking
There exists ways to extend and expand the outreach of a PMR446 channel. Repeaters and internet-linked gateways are used for this purpose.
Repeaters are radios that are used to extend the range of a radio system by retransmitting the signal from other radios at an improved coverage.
Repeaters on PMR446 are simplex otherwise known as "parrot" repeaters.
In commerical and amateur radio, repeaters are not simplex instead they are known as half-duplex repeaters, this is where the repeater will listen for signals on one frequency and then retransmit it on another.
This requires the radios to be tuned to the output frequency of the repeater when listening and transmit on a seperate frequency where the repeater is listening on (Input frequency). The repeater then retransmits in real time the signal from the input, thus boosting the range and coverage of the transmission.
Because a close transmisson on a similar frequency causes radios to desensitise, the output and input frequencies will be sevearl hundred khz or even mhz away from one and other as well as emplying large cavity filters to prevent this effect.
If there were no filters or the input and output of the repeaters were too close, the second a transmission from the input is detected and rebroadcast, the signal from the repeaters own output would overcome the weaker input and stop the transmission entirely.
Since PMR446 only spans a mere 200Khz of spectrum, a semi-duplex type system is unsuitable and unattaniable. Instead what is known as a parrot repeater system is employed.
A parrot system, needs only one channel to operate on, and as the name suggests, the radio simply rebroadcasts any transmission heard on the frequency after said transmission is over (Think of a parrot, repeating what the owner has said).
Apart from the obvious disadvantage of hearing your own transmission repeated back to you, it is effective on PMR446 and is the most common way of range-extending especially when the repeater is installed at high ground.
A gateway is where a radio channel is linked to the internet by connecting its audio output and microphone input to allow a link to and from internet connected devices as well as other gateways across the world. As the name suggests it is a "gateway" to and from the internet/radio.A radio can be connected by audio leads or using a serial interface. Typically the audio from the radio will be connected to as a microphone input to the computer and the audio from the computer gateway is connected as a microphone input to the radio.
Software is then used to connect the radio to a server the most popular being the Free radio network allowing other users to join and talk, similar to a virtual channel and their own voices will be rebroadcast over the air on to the gateway itself.
This also means that any transmission picked up from the gateway will be rebroadcast on to the virtual channel and heard by the internet listeners. Multiple gateways can also be set up on the same server in different locations allowing one transmission to be rebroadcast on all other gateways extending the range and outreach.
Since gateways are not only limited to PMR446, it's possible to have a server in which a gateway has been set up on a CB radio channel (27 Mhz) or a PMR service in another country such as GMRS in the US or Canada, all cross-linked and interconnected together by the gateway server.
Gateway exampleLet us take the example of creating a gateway using a Baofeng UV-5R set to channel 6. The 2.5 mm audio speaker jack is connected to the computers microphone jack, this allows any audio from the radio on the channel to be picked up by the computer software and rebroadcast on to the gateway server.
Then in order to transmit traffic from the gateway server back on to the channel, the 3.5 mm microphone input on the baofeng is connected to the speaker output of the computer, in order for the radio to detect the presense of the voice transmission, VOX (Voice activation ) is enabled on the baofeng.