Insight: FM Vs Poor conditions

Although many feel that discussion of modes is really a bit gear-head stuff, and due to licensing restrictions, many only get at best AM/FM and FSK Voice (aka digital voice systems use), I’ll start by outlining really where the proverbial heaven and ■■■■ of FM (be it delta modulation or true FM) can be found.

On an objective note, NBFM is kind of popular mostly because its the most commonly permitted under most civil radio licensing modes across the world - in line, so you find (as it maximises world sales and meets many demands) most equipment is NBFM enabled or focused. Often, barring some out of market and ‘export’ sets, access to other analogue modes is mostly Rx only in limited ranges where commonly used - VHF aviation coverage Rx will be a very common example, a much rarer example would be a split ‘air band’ mode which covers NBAM for VHF and SSB in an HF segment for Rx equally, but this is much more general coverage receiver territory.

So operationally, you’re most likely using NBFM and not out of choice.

For very similar reasons why many people feel WBFM broadcasts sound ‘better’, it’s commonly felt NBFM is ‘better’ than say NBAM. If there is one factor that does actually separate the two, it’s one of the reasons WBFM took off, it’s resistance (in terms of reception) to electrostatic atmospheric conditions (lighting and high level electrostatic build up which doesn’t always hit discharge levels). It’s also believed that FM maybe better under stratospheric ‘lift’ (VHF/UHF/SHF ranges) where instead of EM charged layer bouncing, signals find a better path through tunnels formed by low pressure air between high pressure regions. But unless you’re a bit of an FM mode DX chaser, you’ll probably never notice the effect beyond when say on VHF/UHF/SHF and normally distant mostly phantom-like ‘buried in the noise’ SIG’s seem to lift out of the noise floor and become possible to discriminate easily or much more easily.

But what can be a factor is by virtue of radio design and and how FM modulation actually works, there’s a tendency for NBFM to sound more ‘punchy’ than NBAM and it’s that, along with a lot less variation on commonly used fairly standard response mics where you get a fairly distinctive sense of what many call ‘comms quality’ where it sounds more like modern telephone quality in tone when there’s an adequate received SIG strength or a condition called ‘fully quietened’ where there is no sign of the harsh white noise you hear on an receiver with an open squelch on inactive/empty frequencies. Harsh being relative, as DSP’s take off the harsh edge on some radios, but if your experience goes back to pre-DSP affected gear, you’ll recall how harsh it could be keeping the squelch open for extended periods looking for really weak transmission sources.

Now where FM as a whole (all variants) start to notably fall apart is when the SIG level drops below the fully quietened threshold, where you start to hear interspersed white noise creep in (the SNR is worsening), and whilst some people are pretty good at discriminating FM modulated SIG’s even when 90%+ noise (I’m one of those, but it’s a combo of dogged determination and having a very good ear for the subtle in sounds which made really good audio restoration one of my pro work areas) is the case, for the most part, once the SNR has worsened to the point where any perceptible residues of voice audio are phantom like and questionable in existence, your well past the usable comms range effectively by some distance. This is where AM was preferred at one time and some still prefer it as AM modulated audio just goes quieter and quieter until you start to hit perceptible limits at the phantom like stage, but what’s actually there is bit easier to discrimate.

So really, FM is limited in range use not literally by SIG strength alone, but the balance where the SNR worsens until you can’t heard the modulated audio is lost in the white noise.

If you are dogged and determined enough, you can see 30+ miles sustained FM mode comms at 400mW Tx output into a quarter wave vertical equiv on a handheld transceiver at 10/11m where you’re in the ■■■ end of HF characteristics and not yet benefiting from the VHF characteristics you see at 30Mhz upwards.

Clearly, for propagation reasons I’ll leave you to research, that kind of range vs Tx power ratio is going to be near-impossible with high band VHF and UHF on handhelds, unless there’s a low level atmospheric lift and even then it’ll be a struggle, but it shows that FM can be a long-range (relative to environment and scenario) mode when combined with frequency use that’ll make the best of it. Alongside of DV through DMR/C4FM, I still use FM on VHF/UHF myself (although I favour SSB over FM on 6m/50Mhz and 10m/28mhz), so I’m definitely not writing it off - the fact it’s there as a fallback alone gives it continued value to me and enables A/D crosslinking where permitted, so even the differing technologies of mode bridge can be crossed and made.

So the heaven and ■■■■ of FM is the extremes of really clear punch audio at great fully quietened high SNR situations and ■■■■ on the ears and sanity when you’re trying to resolve that which is realistically a lost cause in the noise.

And its as you get into poor to very poor SNR, combined with shorter actual transmission periods per second of voice transmission and subsequently lower duty cycle reaping battery endurance benefits, is where DV modes gain real advantage as then you’re talking about clear FM characteristics audio wise but it gets to be more resilient over worsening SNR conditions because what’s sent is sent as FSK TG, which is far more resolvable even at the white noise infested ■■■ end poor SNR state than analogue FM could ever hope for.

So that in closing, in terms of mode and effectively how well comms grade transmission is maintained, is where FM and DV are markedly differing in usability.

I’m not by any means saying dump NBFM, just be aware if it’s limitations and whilst you can live with them, use FM by all means. But do consider that in a world where the powers that govern our radio use are out to maximise occupancy and contention on often quite small allocated frequency ranges (civil usage wise) that you’re eventually going to see NBFM radio service licensing gradually get pulled away in favour of multiplexed digital modes which can accommodate many simultaneously sent transmissions within the same 10-12.5 KHz sized ‘channel’ bandwidth - most dual slot DV uses two 6.25Khz slots or sub channels within one existing 12.5Khz channel space/bandwidth.

So if you’re a hardened FM fan/user, make the best of it as the more hard work you make it for reasons why an FM mode usage allocation should be pulled, the longer it’ll be still good to use.

Whilst I’m an SSB and TG user to the core, I respect FM and when it’s a good option or maximises who can be taking part in some activity, it’s used here, ditto any other mode as the situation best suits.