FM Deviation?

Discussion in 'Becoming an Amateur Radio Operator/Upgrading Privi' started by K7TUX, May 29, 2009.

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  1. K7TUX

    K7TUX Ham Member QRZ Page

  2. WB1HJS

    WB1HJS Ham Member QRZ Page

    Deviation is how information is conveyed on FM. The signal from your mic is used to cause the carrier frequency of your rig to "jitter" -- deviate -- from the resting frequency. The louder your voice, the farther the deviation from the resting frequency.

    Apparently, there's a problem with that rig having low deviation on 10 meters. That mod would correct that problem, I guess.
  3. K3WRV

    K3WRV Guest

    With AM, we think in terms of percent modulation, 100% being ideal (mostly). FM works differently - as HJS explains. Think of deviation in terms of signal width (Look in the handbook - there are diagrams that will show you how this works that are cleazrer than I can explain.

    The Standard used in the US is something like 5 kHz deviation (we use 15 kHz chanel spacing on 2 meters - hence a 10 kHz signal will fit). In other countries, they use 2.5 kHz deviation to fit in their narrower chanels. On my Yaesu VX-170, I can select between 2.5 and 5 kHz deviation. 2.5 gets reports of very low audio. Whether your rig neeeds a mod or it's just a menu setting, I don't know. Does the manual shed any light? (I know, Yaesu manuals are inscrutible!)

  4. KC5FM

    KC5FM Ham Member QRZ Page


    What this means to you is that you should do the modification before putting the radio in FM mode between 29.00 and 29.7.

    The modification ensures that the radio transmission stays within the 3 kHz allowed on 10 meters. In other words, the citation above explains that FM is allowed on 10 meters but it must occupy a space no GREATER than the A3E or AM full carrier double-sideband mode.

    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.

    The modification results in a 2.8 kHz footprint, according to the documentation. That should be well within the 3.0 kHz allowed. However, it is the control operator's responsibility to insure that the radio is operated within the specifications of design and within the regulations allowed by the Federal Communications Commission.

    Hope to hear you on 29.6 FM soon!

  5. K8MHZ

    K8MHZ Ham Member QRZ Page

    No FM on 10 for Techs is the correct answer.
  6. K7TUX

    K7TUX Ham Member QRZ Page

    Thanks for all the helpful information. I now have a better understanding of my radio's capabilities and what they are for.

    K8MHZ, I am fully aware of the fact that I cannot (at this point) use FM on 10m. As I said before, I appreciate all of the HELPFUL responses. I appreciate the time that everyone else took to post clear, helpful answers for a fairly new Ham.

  7. K7JEM

    K7JEM Ham Member QRZ Page

    Above 29 MHz there really is no limit on the deviation, other than good amateur practice. Normally, deviation on 10M FM is 5KHz peak, which has an occupied bandwidth of about 16KHz. Below 29MHz, FM is allowed wherever phone is allowed, but the modulation index must not exceed 1 at the highest modulating frequency. This effectively limits deviation to 3KHz or less, with an occupied bandwidth of 12 KHz or so. And your rules that you are linking to are 12 years old.
    This doesn't apply to an FM phone emission.
    The mod must be for increasing the deviation, it must come set much lower than that. Even that amount would be legal anywhere that FM is legal.

  8. K0RGR

    K0RGR Premium Subscriber QRZ Page

    Unfortunately, I don't think what you've cited applies to FM phone on the high end of 10. That section says it applies to 'non-phone' emissions. Here's the whole paragraph: (part 97.307 (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. "

    Note that part (2) applies to all the HF bands, not just the high end of 10.

    The part that really applies is the paragraph above that (1), which only applies to the band segment 28.3 to 29.0. It stipulates that the maximum modulation index of an 'angle modulation' transmission must be less than 1. This is true for all the bands below 29.0 Mhz.. It does not apply to the band above 29.0 Mhz.. You can operate FM on any phone band (though Techs are not allowed to on any part of 10), but with this extremely narrow restriction.

    The 'norm' for F3 emissions outside those restricted bands is 5 Khz. deviation. As I recall, the formula for bandwidth is 2 X the deviation plus the maximum modulating frequency. So, we're talking about a 13 Khz. wide signal. The designator used to be 16F3 - 16 Khz width. At 2.8 Khz. deviation, it will still be 2 X 2.8 + 3 Khz or more, so 8.6 Khz. or more.

    The 2.8 Khz. deviation is probably fine for the higher FM band. The 'channels' there are effectively only 10 Khz. wide, anyway.

    I suspect that the factory setting is to accomodate the much lower deviation on the lower part of the band. A deviation of 1.2 Khz. would probably be close there, and that would surely sound very low on the high end.

    It's highly unlikely that you'll ever want to operate FM below 29 Mhz., but if you do, I suspect it will be legal if you change this setting back to the factory setting.
  9. K7JEM

    K7JEM Ham Member QRZ Page

    Should be legal anywhere but 60M with 3KHz of deviation, assuming a max modulating frequency of 3KHz.

  10. K0RGR

    K0RGR Premium Subscriber QRZ Page

    Yes, you're right about that. If the peak deviation is 3Kz. and the maximum modulating frequency is 3 Khz., the modulation index would be 1.
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