FM747 Deviation mod?

[KC2UGV]

So, since an FM deviation above 1 can't be used under 29.00 MHz, is it possible to turn the deviation down on the FM747 board (Installed in a FT840)?

What equipment should I have on hand (Other than a dummy load) to test my deviation before going "live"?

And, if I have do this, does anyone use FM on the lower HF bands? I've not found any, even in the "outside the ham bands" area, to my knowledge.

[PA5COR]

Just some repeaters high in the 29 MHz band here but very sparse, all direct qso's is USB.
Under 30 MHZ the deviation here should not exceed 2.5 KHz and above 5 KHz for 2 meter and above ( 12.5 KHz channel spacing).

Modulation index β is defined as:
In other words, if we modulate an FM transmitter with a 1 KHz audio tone, and slowly increase the deviation, we see the carrier drop in level, and disappear when the deviation is 2.40 KHz and then increase, to disappear again when the deviation is 5.52 KHz, etc.
If we instead modulate the transmitter with a 3 KHz tone, the first null is seen with 3.0 KHz * 2.4 = 7.2 KHz deviation, etc.
To clarify one point, the total power in an FM system remains constant, regardless of deviation and modulating frequency. However, the total power is spread out amongst the carrier and the modulating sidebands in a complex fashion, described by the Bessel functions. At a Bessel null, 100% of the power is in the sidebands, with the carrier reduced to zero.
http://www.repeater-builder.com/proj.../devmeter.html

The poor ham's deviation meter ;)

http://www.fmsystems-inc.com/manuals/BESSELart.pdf

There are ways to make a deviation monitor from an FM receiver that has a Foster-Seeley type of FM discriminator.
Basically you hook an oscilloscope to the output of the discriminator and then set the trace on the oscilloscope so that the on-frequency signal is centered.
Then you input signals 5 KHz to either side of the desired frequency and set the gain so that the line produced is a set number of divisions away from the center (i.e. 5 divisions on either side).
When the transmitter is modulated, the resulting trace on the oscilloscope will be on either side of the center. If 5 divisions are used (or 10, or whatever), then 1 division on either side represents +/- 1 KHz deviation (or 1/5 of the divisions originally set).
The IDC (instantaneous deviation control) in the transmitter is the adjusted for whatever deviation you wish. Since +/- 5 KHz is pretty much standard world-wide, you would adjust the maximum peak of the signal to come just to the 5 division mark (or whatever was set). This makes sure that the deviation of the set doesn't exceed +/- 5 KHz, or 2.5 KHz under 30 MHz..

If you wish to reduce the deviation to less than +/- 5 KHz, then you adjust the peak to whatever level corresponds to that deviation on the oscilloscope.

Unfortunately, most amateur transceivers do not use the true FM discriminator these days and thus you really need to use a deviation monitor. Some people try to adjust the deviation by ear but usually this does not produce that good results.
Fooling around with the deviation adjustment of an FM transmitter is not something that should be done unless you really know what you are doing and have the proper test equipment to measure the results.

[WØTKX]

http://www.repeater-builder.com/proj.../devmeter.html

I've not heard any FM activity other than 10 meters but some folks play around with Korean era military AN/PRC-8 radios on 15 meters. Watch yer bandwidth.