[SixClub] Six is open in Oklahoma

[Chris Boone]

Now that I have time to completely  answer this, here goes:

FM vs SSB is apples vs oranges......SSB (and AM, etc) is called weak signal
mode because of the nature of it....
FM has a "threshold" inherently in it....this threshold is what gives FM the
quiet sound for local and semi local communications (especially vs AM;
ignition and other "static" noises that are amplitude in nature don't bother
FM)....welllllllll, in theory they shouldn't but they CAN in actual use..BUT
still FM is better than AM for local "noise free" comm.....the threshold of
the FM receiver is set by what signal level the limiters start to
work...limiter stages in a FM receiver clip/limit, etc the amplitude pulses
and hence "quiets" the signal....though on LOWBAND (30-70MHz) you really
need a GOOD noise blanker (ask GE, Motorola, etc about their NB on their
lowband gear ;)
Below the threshold, the FM signal can degrade rapidly and hence hard to
demod.....while AM and SSB are listenable right down to the noise
floor....FM needs a certain amount of difference above the noise floor to be
readable...

As for bandwidth, VOICE FM is 16-20kHz in normal mode....(the formula is 2x
(freq of modulation + max deviation)....so with normal 300-3000 Hz freqs,
NBFM (narrow band FM) is 2 x (3+5) or 16 kHz bandwidth....and that is not a
definite absolute.....actual bandwidth of a FM signal can go farther than
that....
What determines how much bandwidth you need and how much deviation is the
source of audio and a critical number..the Modulation Index...
The Index is usually kept above 1 for voice and is higher for music, wide
bandwidth, etc.....In broadcast FM, the deviation is 75 kHz and originally
had bandwidth past 20 kHz....so the MI was around 3 (75/25)....but FM Stereo
put filters inline to limit audio to 15 kHz (because the stereo pilot signal
that signals your FM radio the station is transmitting in stereo is 19
kHz)....BTW as a side node, FM STEREO is 23db noisier than FM MONO at fringe
coverage because the L-R subcarrier needed for stereo is (Tada!) a AM
signal! (actually a doublesideband suppressed carrier signal at 38kHz or
twice the pilot tone freq.....phase locked to one source)...In FM VOICE, you
have 5/3 or a MI of 1.3333 .....the higher the MI, the quicker the limiters
will operate BUT there is a tradeoff....for wideband audio, you need
wideband FM signal...as the BW goes up, the sensitivity of the receiver goes
down (a typical FM stereo rcvr will quiet at 2-5uV or more..where the
typical voice FM rcvr will quiet at 0.3uV or so....20-40db better! But SSB
still is best at WEAK levels......)

In a side by side comparison, a FM carrier of 100w and a SSB signal of 100w
PEP should carry the same distance....regardless of MODE, RF is RF...and
thus signal path loss is the same.....how the receiver can process the
signal is the key....in a lot of cases, you will get farther range on 6SSB
then 6FM....but that is taking it to the max limit of reception.....QUALITY
signal reception will be about the same range with MAYBE FM winning in some
cases....

NOW, 6 is called the Magic Band....the magic of it?? Look at HOW WIDE it
is....4 MEGAhertz....that  almost MORE bandwidth than 160 to 10mtrs added
together!!! Thus the MUF (maximum usable freq) can vary throughout the 6mtr
band where the SSB section at 50.1-50.4 say is open but 52.525 is dead as a
doornail....also the band conditions have to be 10-20db stronger for the FM
signals to be heard vs the SSB section........Magic band INDEED! :)

OK, before I get into Physics 101 and a complete semester of RF, etc, I hope
the above explained it....

BTW FM is not used on lower bands (except for 10m) on regular basis because
of the phase distortion in a skywave signal during skip conditions...Yes,
you CAN run FM all the way down to 160m....BUT its bandwidth is limited
under Part 97....(much more narrow than NBFM normally used....)to use FM
below 29.0, the deviation cannot be more than 3 kHz when the radio is
modulated by a 3 kHz signal....(The rules say the MI cannot be more than
ONE).. but given the bandwidth needs and the phase distortion on skywave
signals, FM is not that "great" on HF......but it can be done....hence SSB
wins on HF.....~30MHz is the lowest freq that FM can be used with less
distortion, etc (and the original FM broadcast band before WWII was
42-50MHz....after WWII, it was moved to the current 88-108MHz...30-50MHz was
the 1st commercial twoway band to really use FM; the old 1.6-2MHz band used
by cops, etc before FM caught on was AM....and the adage for lowband FM was
100watts, 100ft equals 100miles...that is still true today (though it
actually takes more than 100ft ;)....

FM is a fun mode on 6......but weak signal?? Nope....thats where SSB and AM
(yep) still shine......(and the FM signal is still 2-3x the bandwidth of the
6khz wide AM...FM sidebands tend to be like the Energizer Bunny...they go on
and on and on and ).....but the most concentrated power density is within
the 16-20kHz of the signal  when voice is concerned..

(and many years back, NBFM was NOT allowed below 52.5MHz....this was when
voice FM was 15kHz deviation...thus the bandwidth was much wider...hence,
the 1st freq that was legally usable??                       

 52.525.....Now you know the rest......... of the story....(tip of the hat
and RIP to a fellow ex-ABC Radio employee)


Ok, my dinner is getting cold.....I'm through typing  :) 

Chris
WB5ITT
(see ya on 52.525 or a 6m repeater and that's another story for later!!)

> -----Original Message-----
> From: sixclub-bounces at mailman.qth.net [mailto:sixclub-
> bounces at mailman.qth.net] On Behalf Of Donald Dubuque W4DJD
> Sent: Wednesday, June 02, 2010 10:12 AM
> To: World Wide Six Meter Club
> Subject: Re: [SixClub] Six is open in Oklahoma
> 
> 2.  Does anyone even bother with the FM calling frequency or nearby
> frequencies?  I know it isn't the sexy SSB section, but right now thats
all
> I have to work with.  (i.e. 52.525mhz  +/-)
> 
> FM your to narrow in freq and your distance is reduced greatly.
> 
> 
> Don Dubuque-W4DJD