It's a commonly used system at ATC units with several sectors. It's called "cross-coupling". When traffic dies down the sectors are "bandboxed", or put together with one controller instead of several. However, the frequencies of different sectors are frequently published in charts so to avoid confusion, and the need to have to tell adjacent sectors of frequency changes, the frequencies are kept the same and the ground equipment is switched to re-transmit all received signals on to the "bandboxed", or "cross-coupled" channels. So, all traffic on all frequencies hears everyone else but it sounds like one channel to the controller. (Hope I've made that clear!). It means that the "usual" frequencies may be used all the time.
It also works on a permanent basis at aerodromes where ground vehicles operate on UHF but need to contact ATC on VHF. The signals received on UHF are re-transmitted on the VHF channel so everyone can hear them... and ATC doesn't have to monitor two frequencies.
As for spikes on the radar - not too sure what you mean, but interference can cause radial lines to appear. Sometimes a whole sector of several degrees will be effected. I saw it many times at Heathrow and West Drayton - once due to an American Warship off the UK coast whose radar was on a similar frequency to ours!
Last edited by HEATHROW DIRECTOR; 3rd March 2005 at 18:05.