[Radio Wrangler]

Squelch is difficult. The radios are required to have their squelch open for any signal of -93dBm or stronger, and they should give at least 6dB SINAD at that level. You'd suppose that the spec was set intelligently to give enough sensitivity and therefore range, so you'd want the receiver to be a bit better to give some margin for manufacturing tolerances and environmental conditions. But no, most pilots reckon that's a bit on the deaf and noisy side and want to be able to have a squelch level several dB lower and have several dB better SINAD. When choosing a new radio, it has to meet the Minimum Operational Performance Standard spec and if the aircraft is certified, the radio has to have the right approvals certificate and have a certificate for fitment to that model of plane. It has to match the appearance of other gear in their avionics stack. Display and button illumination has to track the other equipment as the single panel dimmer knob is turned, then they start comparing numbers for sensitivity, weight, power output and power consumption. This is a very technically-literate market. Once they've got the chosen radio installed they want it to just work. Squelch isn't an easily accessible panel control. ATC do not want people turning it up and becoming deaf to radio traffic. Pilots really really do not want squelch that keeps opening and blasting their eardrums with QRM and QRN. They do not want squelch that chatters with traffic from aircraft so far away that they are of no concern.

With such narrow channels and low deviation, the 'FM advantage' is not much and capture effect is not much either, so the performance of noise squelch systems isn't wonderful, and there is still that spec on squelch being open at an absolute level of input power. Now chuck in the issue that some engines are well known for less effective screening of their ignition than others...

Yep, squelch is difficult.

Not that long ago, A gentlemen's agreement got torn up and governments started licensing new FM broadcasters all the way up to 107.9 MHz.They saw it as a new revenue stream. Aviation radios saw it as a new source of stress. Required standards were updated and planes were given a deadline to comply with them. This meant either replacing all radios or fitting anti-broadcast filters in the antenna feeders and then doing certification tests on the installation. It was seen as easier to fit a new radio which was certified already to the new standard.

We are now in the run up to another deadline and all aviation radios have to be replaced by models which are narrower band and can tune in 8.33 kHz steps.

There is considerable opposition to repeated, forced replacement of equipment, voiced through commercial airline bodies and private pilot associations. America has decided that it doesn't need to change to 8.33kHz radios.

The change-overs have been made manageable by new standards maintaining compatibility with the old. there are still some '25kHz' channels left in the band plan because not all have been split in three. But aircraft outside the US are still required to fit new 8.33kHz radios and those radios have to know which channels are '25kHz' ones and to switch in wider IF filters.

This looks like an easement for ground stations and not aircraft, but the wide channels are needed for retaining area coverage via multiple, offset frequency, transmitters.

FM in such narrow bandwidths would lose the advantages of wideband FM, and not offer much difference. The offset transmitter trick would be lost and this is a major issue. Above all the politics of another forced replacement of equipment and an incompatible one at that would be horrendous.

David