What is a single frequency repeater (SFR)?
A DMR digital radio system uses a time based method to effectively handle two simultaneous conversations within a single period of time.
Each conversation takes place within a time slot (TS) and is sent consecutively - TS1, TS2, TS1, TS2 etc.
Each radio channel is assigned to use a single time slot only.
A single frequency repeater boosts a signal by using the second timeslot to rebroadcast a conversation in the alternate timeslot, whilst ‘labelling’ it as though it's from the original timeslot.
A receiving radio will either receive the original signal from the originating radio or the signal from the SFR or both signals. In the latter situation, the radio will determine the best quality signal of the two and use this.
A conventional repeater requires two frequencies to operate - an input and an output frequency. In the UK this requires a technically assigned ofcom licence.
A single frequency repeater does not always need this type of licence and only needs a single frequency (allowing use in locations where dual frequency assignments are not available).
The drawback is that an SFR prevents two simultaneous conversations (TS1 and TS2) on the same frequency. However, you can deploy multiple SFR units quickly and easily. In many situations only a single timeslot is being used and there is no negative effect for the users. SFR systems are lower power than conventional repeaters, but this can also be an advantage if you just need a small boost to achieve the required coverage. Each scenario will be different.
In general, if you need a low cost, self installed, solution to boost a single channel over a short distance, or a temporary solution, choose an SFR.
If you want to boost two radio channels over a wider area, with an ‘installed’ solution, choose a conventional repeater.
Why are SFR systems cheaper than conventional repeaters?
A conventional repeater is more expensive because it contains a sensitive receiver and high powered transmitter, using different, but close, frequencies that are in use simultaneously. This requires significant and costly filtering and two ‘radios’ inside. The installation is also more complex, since you will either need two aerials or a duplexer (tuned filter) to minimise interference. Conventional repeaters are generally installed high up with longer aerial cable runs, to obtain the maximum coverage and range. This requires additional hardware and installation expertise. They are also capable of high power, although you may not require this capability.
Conversely, an SFR uses a single DMR radio with a special configuration. It only requires a single aerial and less filtering, since it uses time to separate the receive and transmit signals, reducing the need for filters. The units can be desktop or wall mounted and are essentially ‘plug n play’ (although your radios may require reprogramming). The lower power also helps to reduce the cost.