FM Immunity questions

[StopStart]

This little subject has been causing a fair degree of amusement to us military folk of late given that we can we no longer do ILS or VOR approaches (or SIDs and STARs) as our nav kit is non-compliant (NDBs all round then!).
What I've been wondering (and hoping one of you tech-minded folk could explain) is why an increase in the upper limit of FM sound broadcast range from 105.0 to 107.9 MHz means that all aeronautical nav/comms frequencies in the band 108.0 - 137.0 might be affected and require filtering. Why should an increase in broadcasting over 2.9 MHz affect the next 29 MHz?
Along the same lines, ish, as we're going to be doing a fair NDBs over the coming months, what is the status of these navaids? I know they're all maintained as published but were there not moves to get rid of the things recently?
Finally then, I assume I am right in thinking that all you civilian folk all now have fully compliant kit?

[BEagle]

SS - the broadcasters have had access to the upper sections of Band II for a number of years now; however, the change is the nature of the agreement between Radio Authorities and Aviation Authorities. I understand that harmonics and intermodulational products are the little devils which will cause the difficulty rather than the fundamental radio frequency on which 'The Sound of WootyB' intends to transmit! Modern kit filters and eliminates the problem; HMFC's dear old receivers don't. It wasn't that long ago that some 4-jets still had to have special frequency-specific ILS crystals fitted to allow approaches at other than their normal bases; the Hamilton Victor prang was, I understand, partly the result of 'not having the right crystal fitted'!!
I haven't yet even turned a VOR/ILS receiver on this year.....I'm glad that the cloudbase was only 500 ft here yesterday as otherwise we would have had to divert to Scotland!

[This message has been edited by BEagle (edited 06 January 2001).]

[Stan Evil]

FM immunity - a subject dear to my heart. The silly thing is that the decision to muck about with the frequency spectrum was made over 8 years ago but, like many things in life, everyone ignored it and hoped it would go away. The shocked realisation that it wasn't going to hit the GA civilian community a few months ago and, since then, there have been frantic efforts to get the kit sorted out (at a cost of around £2000 per box with typical Bendix/King kit). Public transport a/c need 2 FM immune VOR/ILS boxes, the rest need just one.

That said, there are no interference problems expected in the UK at the moment and only a few in Europe. I'm surprised that the military have gone overboard on this one as they usually drive a coach and horses through the civilian nav kit requirements. I spent many a happy year crossing Europe with just a TACAN and a 50 KHz spaced VHF and lying to anyone who asked me to go to 123.42.

As far as NDB is concerned, remember that NDBs are just omni-directional transmitters - a bit of wet string would do for an aerial. The problems come from the propagation effects of MF signals and the aircraft equipment - see AP 3456 for details. A typical DOC (protected range) for an NDB is only 20 nms. The aircraft equipment may suffer from dip which causes the needle to fall about 10° to the low wing in a turn. This all goes to make an NDB approach challenging. The only good thing is that, these days, most places have a DME so you don't need to play about with your stopwatch.

A final thought - how many military guys are going to call up for an NDB approach and then follow the ILS indications anyway????