[Yaesu] FM Receive on FT-757GX

[Glen Zook]

Although you probably have something else wrong with
the audio circuitry in your rig, FM is such that it
doesn't depend on how strong the signal is to
determine how loud the sound comes out of the speaker
(this is unlike AM which is affected by the signal
strength).  The "deviation", how far the modulation
frequency goes from the center or carrier frequency,
determines how loud the signal sounds once you have
reached the "full quieting" level.  Thus, a station 5
miles away will sound just like one that is 100 miles
away so long as the "full quieting" point has been
reached.  If "full quieting" is not reached, then
there will be noise present in the received signal
which will affect the intelligibility of the received
signal.  Also, too many newer amateurs forget that all
signals that come from a repeater are the same signal
strength no matter where the original transmitter is
located.  What you are hearing is the retransmitted
signal from the repeater and that has nothing at all
to do with the signal strength of the originating
transmitter except for the audio quality.

Amateur repeaters are now almost universally set for
the commercial standard of +/- 5 KHz deviation.  If
the deviation is much greater, the filters in the
receiver (especially in the repeater with narrower
filters than most amateur band only mobiles) will
cause the signal to "chop out" since the deviation
takes the signal beyond the passband of the filter. 
If the deviation is significantly less than +/- 5 KHz,
then the audio will sound weak.

Use of +/- 5 KHz deviation is allowable on those
portions of the amateur bands above 29.0 MHz which
allow phone type of operation.  Below 29.0, FM is
still allowed, but, the deviation cannot exceed +/- 3
KHz which gives a bandwidth the same as a "normal" AM
signal.  This is often called NBFM (narrow band
frequency modulation).  Although many rigs now operate
below 29.0 MHz have FM as a mode, the vast majority of
them are set up for +/- 5 KHz deviation and are thus
illegal to use for FM below 29.0 MHz.  Also, the
closest that you can actually operate to 29.0 MHz
using +/- 5 KHz deviation is 29.005 MHz since the
width of the signal is such that the modulation will
take it down to 29.000 MHz.  In fact, as a practical
matter, I would never operate using FM with +/- 5 KHz
deviation below 29.010 MHz to allow for any additional
modulation products that might be present.

Glen, K9STH


--- Joshua Heidbrink <joshua@hamlinelectric.com>
wrote:

I have an FT-757GX that seems to have a problem
receiving FM.  I can hear other operators if I turn
the volume all the way up, even if the other operator
is just across town.

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