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Author Topic: Is it legal to transmit FM on 20 meters?  (Read 1141 times)
N0NS
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« Reply #15 on: February 25, 2008, 02:55:18 PM »

All, many thanks for the detailed information!  

It is greatly appreciated!  73  Joe
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W8JI
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« Reply #16 on: February 25, 2008, 04:55:15 PM »

The rule is pretty plain. You can run FM if the modulation index is no greater than 1 at the HIGHEST modulating frequency and the BW isn't wider than an A3e signal.

The problem isn't the index of 1, which is easy to obtain. The problem is the bessel function of FM which with a modulation index of 1 would result in the second harmonic of the modulation frequency being about -19dBc and the third harmonic being -34dBc.

This means with a modulation index of 1 and 3kHz maximum audio frequency the sidebands 6kHz up and 6kHz down from the carrier would be -19dB, and the sidebands 9kHz up and 9kHz down would be only -34dBc.

This fails to comply with the rule about not exceeding A3e bandwidth and also would not be very good operating practice on a crowded band.

There is also such a thing as common sense in radio. Common sense would dictate not running ESSB or FM on crowded bands. Keeping FM bandwidth legal would be pretty tough.

73 Tom


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WI7B
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« Reply #17 on: April 02, 2008, 02:55:01 PM »


This is what Part.303(f) says (beow).  Note, there is NO requirement for a phone emission ALONE to meet the A3E requirement in (2).  The only reference in (2) to phone emission is a MULTIPLEXED image and phone emission.

Depsite the prejudice against, FM-narrowband transmission is permissible on the HF bands.

73,

---* Ken

"(f) The following standards and limitations apply to transmissions on the frequencies specified in Sec. 97.305(c) of this part.
    (1) No angle-modulated emission may have a modulation index greater than 1 at the highest modulation frequency.
    (2) No non-phone emission shall exceed the bandwidth of a communications quality phone emission of the same modulation type. The total bandwidth of an independent sideband emission (having B as the
first symbol), or a multiplexed image and phone emission, shall not exceed that of a communications quality A3E emission."
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