An excellent answer by a Ppruner from a few years ago (MM 43)
If there is a local VHF FM station on 97.3MHz this could cause problems to some radios having a Local Oscillator Frequency 0f 10.7MHz. If memory serves its called Second Channel interference...97.3Mhz might just be a local commercial station...No, you happen to be wrong. 118.70MHz would (if the Intermediate Frequency of the receiver is 10.7MHz) have a Local Oscillator frequency of 118.7 + 10.7 = 129.4MHz or 118.7 - 10.7 = 108.0MHz.
So, what you are really talking about is Image Frequency interference which can enter the receiver if there is insufficient front end filtering, i.e. poor rejection of out of band signals. The interfering signals will be at +/- twice the IF frequency from 118.7MHz, i.e. 97.3MHz or 140.1MHz. 97.3MHz has already been identified as the FM broadcast station LBC transmitting from Croydon.
Any decent Aeronautical VHF receiver will have high attenuation notch filters centered on the 88 ~ 108 FM broadcast band, and problems from that band should be extremely minimal.
The answer to the "breakthrough" question is that it is most likely tropospheric ducting caused by temperature inversions in the troposphere which can happen when a large high pressure system has been hanging around for a while. In this case the transmission breakthrough probably came from an aircraft using the same frequency with an ATC somewhere in Europe, e.g. Orly Approach.
An ICAO paper has been written on this subject and can be found at -
http://www.icao.int/anb/panels/acp/w...0/wgd10_07.pdf
mm43
