All stations should be modulating as close to 75khz as possible without exceeding it, regardless of their license.
87.6, 87.8 and 88.0 can all perfectly co-exist when the laws and standard practices are followed.
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Okay, now I know where youâre getting things confused & where you pulled the 75kHz from.
All FM stations (commercial, community, narrowcast), have an allocated 200kHz bandwidth channel.
To modulate the transmitter at 100% all of them are allowed to deviate plus & minus 75kHz from the carrier frequency which gives an actual channel bandwidth of 150kHz, & a 25kHz guard space on the top & bottom of each FM channel for a total channel bandwidth of 200kHz.
If a narrowcast station was only allowed to have a channel bandwidth of maximum 75kHz, they would only be deviating plus & minus 37.5kHz from the carrier frequency & the transmitter would only modulate to 50% max & would have no guard space in the channel.
When you tune to an FM station, that frequency is the centre frequency of the channel, so for example 87.6 channel is 200kHz wide & goes from 87.5MHz to 87.7MHz, 87.8 channel is 200kHz wide & goes from 87.7MHz to 87.9MHz, so yes they butt up against each other, but if a station is transmitting to itâs licence or 75kHz deviation, thereâll be a blank 50kHz guard band in the middle of the 2 channels, 25kHz top of 87.6 & 25kHz bottom of 87.8.
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And any good modern radio (with a hard 200khz bandwidth filter) shouldnât have an issue isolating each station. Filtering on older analog radios isnât as good.
I have noticed some occasional issues decoding RDS on stations 200khz apart, but thatâs understandable given the RDS carrier sits furthest from the centre carrier (closest to the adjacent station).
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oh wow! Ok I understand now!
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Yes this is correct but not the whole story. Although the carrier can deviate no more than +/- 75 KHz from the centre frequency, this results in some signals being generated outside this range as well as inside. This applies for all FM stations.
There is a rule called Carsonâs Bandwidth Rule which says that around 98% of the signal power falls within a bandwidth of twice the (maximum carrier deviation + maximum encoded frequency) (ref).
For mono, the maximum frequency encoded is the highest sound frequency of 15KHz, leading to most of the power with a 180 KHz bandwidth (presumably +/- 90 KHz of the centre frequency).
For stereo, the signal is encoded at 23-53 KHz + the 19 KHz pilot tone, bringing the total to 256 KHz (+/- 128 KHz).
With RDS encoded at 57 KHz +/- a bit, and supplementary audio channels at higher encoded frequencies, the numbers go up after that.
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Yes, & weâre getting really technical now so most here wonât have any idea what any of what it means, but even with everything youâve said, which is correct, the signal (channel) when deviating no more than 75kHz has to, or will fit inside a âChannel Maskâ where the power level of any of those far out frequencies drops so low so fast, that they are insignificant & wonât interfere with any adjacent channel.
Donât forget if youâre going to bring up Carsonâs Rule, that theoretically the bandwidth of an FM channel is infinitely wide if itâs not limited.
To get that FM channel bandwidth to 200kHz & to not interfere with an adjacent channel, every sub carrier in the baseband is limited in some way to achieve the 75kHz deviation required & fitting the FM modulated signal within the spectral channel mask.
As weâre talking narrowcast or FM LPONâs here, that spectral levels mask, coupled with the enforceable required measured signal power level at 2km or 10km from the transmission site which is so low that any distance further a receiver will have trouble or impossibility to successfully decode the FM signal, means that using an adjacent frequency in an area next to or close by another wonât result in or shouldnât result in any noticeable interference, & youâll find, ACMA wonât allocate, 2 LPON licences within the same 2km or 10km area on adjacent frequencies, minimum distance between two 1 watt LPONâs with 200kHz separation is 5km apart & 15km apart for two 10 watt LPONâs 200kHz apart.
For two 1 watt LPONâs on the same frequency they need to be 10km or 30km apart for two 10 watt LPONâs, given if everyone is doing the right thing & transmitting at their correct power, not over modulating/deviating & operating from their correct location, there shouldnât be any interference to any other LPON station.
Of course the reality is, not everyone does what theyâre supposed to do, even in commercial radio transmissions, so the possibility of interference to or from another station/channel is a reasonable chance.
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Nothing to do with the Australian TV series from the 1980s âCarsonâs Lawâ!
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Fair play as far as Iâm concerned; more critical thinking can only be a good thing. And weâre all nerds here.
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It may be that all Geelong area LPONs in Wyndham, Geelong(87.6), Leopold(87.6), Queenscliff, Portarlington, Torquay, Anglesea (87.6), Lorne, Apollo Bay & Colac are carrying the 3GL program & with RDS.
Most of these are on 88.0MHz.
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Just like the scenario a few weeks back with 4AAA and the 600khz wide channel interfering with Rebel and Newsradio 98.5 northern rivers. if everyone stayed in their own lane, there wouldnt be issues
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Great discussion on FM bandwidth.
As per the carlson rule,
RF Bandwidth of FM =
2 x
(Modulation in KHz +
Frequency response in KHz).
Modulation is essentially how loud the audio is (i.e. volume).
Frequency Response is the maximum audio frequency that is transmitted. For example, the human ear can hear frequencies at high as 22 khz, however this degrads with age).
Hereâs some bandwidth calculations for some common FM radio servicesâŚ
Mono FM
2 x (75khz + 15khz) = 180 khz
Stereo FM
2 x (75khz + 53khz) = 256 khz
Stereo FM with RDS
2 x (75khz + 57 khz) = 264 khz
The above are maximum figures and are somewhat an over simplification of the formulaâŚ
Interestingly, about 25 years ago, I recall that all FM radio licences (as per ABA) listed bandwidth as 300 khz, then sometime after this all FM radio services licences were issued with bandwidth as 200 khz.
Not sure why this was changed, but i guess one explanation might be that even if you run the full MPX Stereo RDS signal, the bulk of your radiation would still sit within the 200 khz envelope, as MPX (L-R) and RDS is sent at a lower modulation level than the main Mono (L+R).
Infact, i think i just answered my own question, as an FM radio service can stay within the 200 khz by ensuring each component of your signal sticks following deviation and modulation levels:
Mono (L+R) at 85 khz modulation, 15 khz deviation.
Stereo MPX (L-R) at 47 khz modulation, 53 khz deviation.
RDS at 43 khz modulation, 57 khz deviation.
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I assume you mean that the whole signal (L+R, L-R etc.) is modulated at 47KHz or 43KHz. Although these are theoretically possible I donât think this is actually done here in Australia.
If youâve ever heard TV on the FM band (e.g. Channel 3 or 4 audio, it would have sounded softer than a typical FM station. TV sound is modulated at +/- 50 KHz rather than 75 KHz. So 43 KHz sould sound even softer.
Iâve been to countries where FM stations sound noticeably softer than in Australia. Perhaps they could only use a lower bandwidth.
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I too often see SI units & SI unit prefixes used/typed incorrectly, although everyone knows what weâre talking about. 
For education purposes; this page might help folk to type them correctly, if one wishes too. 
k = kilo (magnitude/multiplier of 1000)
K = Kelvin (temperature)
Hz = Hertz - measurement of frequency (not HZ, not hZ, not hz)
M = Mega (magnitude/multiplier of 1,000,000)
m = milli (magnitude/multiplier of 1/1000th)
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Rete Italiaâs âNiche Radioâ Network is sowing its seeds again on LPON FM frequencies.
Noted the station this week in Albury/Wodonga on 87.6MHz & from the Wangaratta area on 87.8MHz both with a stereo carrier, but no RDS.
Other LPON outlets can be found on their webpage:
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Hott FM in Charleville now has zero mentions of frequencies on their website, and hasnât mentioned a frequency on their Facebook page since early March. In fact, their Facebook page hasnât been updated since April.
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To add to this;
- Noise FM (Alternative Rock, Pop and Dance)
- Dead Air
- River FM (As of July)
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An identity problem? 
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true definition of âyou never know what we will play nextâ
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Is the reincarnated 3GL still going?
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yes, still on-air.
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