IARU Region 1 VHF Managers Handbook - UBA

8 Technical Recommendations of IARU Region 1
8.1 An introduction
In order to facilitate the operations of Amateur Radio Stations the IARU at its triennial conferences adopts
Recommendations. Most of those are so-called "operational recommendations" ( such as band-plans and
contest-rules ). But during the past 30 years several "technical recommendations" have been adopted as
well.
This note deals with the content and background of those technical recommendations.
8.2 FREQUENCY MODULATED TELEPHONY
8.2.1 The basic FM standard (Recommendation FM.1.)
One of the longest standing IARU Region 1 technical recommendations, adopted at the Region 1
Conference in Brussels (1969) deals with the basic parameters of Narrow Band FM Telephony. It states
:"For FM within Region 1 a maximum modulation index of 1 and an audio band restricted to 3 kHz shall be
used"
Between 1963 and 1969 the gradual introduction of VFO controlled SSB, replacing AM( x-tal controlled),
was taking place on VHF. At the same time, however, the problems with LF break-through in radio-, TV-
and audio equipment became embarrassing. Using FM and PM appeared to be the right solution in this
case. Of course the recently acquired habit of VFO control and co-channel working was used with FM as
well.
In order for an efficient use of FM it appeared necessary to agree upon a few basic parameters which
would determine the optimum filter to be used in the receiver. A 12 kHz receiver bandwidth was finally
chosen. This appeared to be the minimum value giving distinctly better quality than 6 kHz AM without
showing too much of a threshold effect ( at least not much more than conventional AM ). Soon after the
recommendation had been adopted several manufacturers of crystal-filters marketed 12 kHz wide filters.
Almost 30 years later this basic recommendation still is in force, although VFO controlled FM has almost
disappeared ( it still is used with rain-scatter on the microwaves ) and the majority of amateurs use crystalcontrolled
FM transceivers with filters wider than 12 kHz, thus loosing several dB's in communications
efficiency.
It must be noted that the application of the recommendation is not limited to frequencies above 30 MHz. It
equally well applies to the 29 MHz band, although it appears that FM on that band is often received with
filters narrower than 12 kHz.
Three years later, at the 1972 Scheveningen conference, a more detailed standard, largely based upon
the first was adapted, but this time VFO control appeared to be out of fashion and the standard dealt with
"Fixed channel FM stations". The audio response was specified more in detail as 300-3000 Hz with a 12
dB/octave fall off outside this band and 6 dB/octave pre-emphasis.
It appears sensible to combine those two recommendations into a single one at a future conference.
IARU Region 1 page 114/148 Version 5.40
VHF managers handbook