|
With improved propagation in 2022, we are hearing many European FM signals between 29.0 and 29.5.
You probably thought that was not permitted below 29.5, the "FM and FM repeater" band segment.
This is s voluntary band plan arrangement. Futhermore, there is a popular misconception you hear:
"With the exception of a segment of the 10 meter band, FM phone is prohibited on the HF bands due to its large consumption of bandwidth."
NOT EXACTLY CORRECT.
Here is the right answer: https://forums.qrz.com/index.php?threads/fm-on-hf-bands-other-than-10.28997/
FM is legal on the HF bands. Just be careful to keep your deviation narrow. Section 97.307 (f)(1) of the FCC rules states:
"No angle-modulated (FM or PM) emission may have a modulation index greater than 1 at the highest modulation frequency."
This applies to all amateur phone frequencies below 29.0 MHz. "
ALSO: "When you switch to FM mode, press the "NAR" softkey (you might have to change the softkey list using the "DISPLAY" key).
Your FM deviation will now be ~3kHz which is allowed on HF.
That will set your Modulation Index at 1 (3kHz of freq. response at 3 kHz deviation).
It also narrows the receive filtering so you won't receive so much QRM."
Historically, FM was actually legal and common in amateur use back around 1949.
Check the band assignments in the front of any ARRL handbook from that era.
EXPLANATION OF MODULATION INDEX:
As in other modulation systems, the modulation index indicates by how much the modulated variable varies around its unmodulated level. It relates to variations in the carrier frequency:
where  is the highest frequency component present in the modulating signal  ,
and  is the peak frequency-deviation –
i.e. the maximum deviation of the instantaneous frequency from the carrier frequency. For a sine wave modulation, the modulation index is seen
to be the ratio of the peak frequency deviation of the carrier wave to the frequency of the modulating sine wave.
If  , the modulation is called narrowband FM (NFM), and its bandwidth is approximately 2.
Sometimes modulation index  is considered as NFM, otherwise wideband FM (WFM or FM).
Please see https://en.wikipedia.org/wiki/Frequency_modulation#Modulation_index.
Carson's rule and FM bandwidth
https://www.hamradioschool.com/post/carson-s-rule-g8b06
FM Bandwidth =  ,
where is the peak frequency deviation and is the highest modulating frequency.
|