Radio frequencies are grouped into ranges called channels. The most interesting channel for assitive listening applications is channel 70 because it is free to use (unlicensed). It has a frequency range from 863MHz to 864.99MHz. Due to intermodulation between frequencies, only 4 frequencies within the channel can be used simultaneously. Where more than 4 frequencies are required, a license for channel 38 must be purchased.
|450 – 469.999 MHz||Shared with lots of talkback radio systems, (only high end radio mic systems can access this band)|
|Channel 21-34||470 – 581.99 MHz||Shared with television, limited availability, used for fixed audio links as well as microphones|
|Channel 35||582 – 589.99 MHz||Shared with television, limited availability, used for temporary audio links as well as microphones|
|Channel 36||590 – 597.99 MHZ||Shared with radar services|
|Channel 37||598 – 605.99 MHZ||Shared with television, limited availability, used for temporary audio links as well as microphones|
|Channel 38||606 – 613.99 MHz||Radio microphones (Also shared with radio astronomy)|
|Channels 39-68||614 – 853.99 MHz||Shared with television also used for talkback systems as well as microphones|
|Channel 69||854 – 862.99 MHz||Radio Microphones and other point to point audio links (Unusable from 2012)|
|Channel 70||863 – 864.99 MHz||License Exempt RadioMicrophones|
865 – 959.99MHZ
|Only high end radio mic systems can access this band|
When multiple frequencies when used together, eg if you have more than one radiomicrophone, the frequencies can sometimes form additional frequencies that are outside the original ones used and cause problems – it’s best to avoid this and therefore pick your frequencies carefully. This is called intermodualtion, or intermod . It’s worth noting that you can mix UHF and VHF systems without having them interfering with each other as there is a big enough gap between the UHF and VHF frequencies.
An example of 2 Sets of Usable Frequencies in Channel 70 free from intermodulation
|Mic 1||863.250 MHz|
|Mic 2||863.750 MHz|
|Mic 3||864.625 MHz|
|Mic 4||864.990 MHz|
|Mic 1||863.100 MHz|
|Mic 2||863.900 MHz|
|Mic 3||864.500 MHz|
|Mic 4||864.900 MHz|
Please note some manufacturers including our partner, Okayo,refer to the set frequencies within a band as channels.
First you need to select the channel or band that the system will operate in 863MHz to 864.99 MHz is channel 70. Once you have your system, each microphone receiver pair will need to be set to a specific channel frequency. For the above band (channel 70).
There are 16 frequency channels to choose from. Remember only 4 of which can be used at anyone time and these need to be carefully selected to avoid intermodulation.
863 - 864.99 MHz, channel 70
In the band 863MHz to 864.99MHz(Channel 70) there are 12 combinations of 4 channels that can be used.