Fuji,
Perhaps I did not make myself clear.
The agreement in 1979 regarding the expansion of the FM band and the increase in radiated power did not have immediate effect. The expansion was planned to start in the late 1990s.
That is why between 1979 and the late 1990's there was no problem with non-FM immune equipment - the problem was not yet there.
The 20 year lead time was to allow technology that was to replace ILS etc to be developed. That technology did not happen for various reasons and we ended up in the position of not having suitable technology but having already promissed to give away the spectrum.
The solution was simply fitting filters to existing equipment.
You will find that technology developed in the 1990's is FM immune because the designers had to meet the requierd specs which included FM immunity.
Remember that radio stations are licensed. The authorities can base FM immunity requriements at any particular time on what FM stations are licensed and the conditions of those licenses.
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Englishal,
Ask your friendly avionics engineer for info.
A chat with the following company who have an approved filter will also help you;
http://www.chelton.co.uk/
Regards,
DFC
If people had paid attention to what was going on and simply put £1 per flight hour aside since 1979, the average spam can could have dual GNS430 or similar with the cost of instalation paid for and some money left for updates!
Ok so not many here remember back that far but the aircraft they fly do!