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Message: <blockquote data-quote="Fisty McB" data-source="post: 1159162" data-attributes="member: 389824">IIRC the timing of the DAB Single Frequency Networks in the UK is limited to 74km - that means that if a distant DAB TX is more than 74km more than what is received from a more local TX from the same frequency, then the two (or more) transmitters will destructively interfere with each other (i.e. the most likely outcome is that the more distant TX will raise the noise floor). However if all transmitters are received within the 74km time frame, then the transmitters should constructively interfere with each other - this should mean that they actually reinforce reception, but it'll come down to the way signals are received in and out of phase at the same time, which in a moving vehicle will never be constant and not something most people need to worry about - it just means that most of the time reception is improved.So for example, if I am receiving a DAB signal from 3 separate transmitters in an SFN that are 12km, 37km & 108km, then 108km-12km = 96km will mean the most distant transmitter can cause destructive interference. However if the TX from 12km goes off air, then 108km-37km = 71km which (just!) fits in the timing frame for constructive interference, and so the two transmitters here will combine to reinforce reception.Those that are responsible for planning coverage will want to minimise destructive interference as much as possible.If you look at the ERP levels for DAB TXs in the UK, then aside from three (I think) SDN transmitters that are run at 25kW maximum ERP, all are at 10kW or less with the very high sites like Holme Moss running at lower powers than this to help ensure that the coverage areas where they could cause destructive interference is minimised. Overall, the three national DAB networks use a transmitter plan sort of halfway between the multi-frequency high-power FM networks, and those used by mobile phone networks whom use SFNs themselves for their 4G & 5G networks.A similar principle works for SFNs for DTT, although this isn't really used in the UK aside from some small & very localised examples (especially where available frequencies are at a premium) - the transmission configurations for each multiplex don't have a large enough guard interval to allow for anything much bigger, so Multi Frequency Networks (MFNs) are used otherwise.</blockquote>

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