Questions about AM mode

Discussion in 'Amplitude Modulation' started by KI5OMM, Jul 2, 2021.

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  1. W7IMM

    W7IMM Ham Member QRZ Page

    You're absolutely right!

    And in actuality, with a modern SSB rig, that is operating correctly, you could in fact operate directly on a suppressed carrier frequency of 7300 kHz using LSB (or 7125 USB) because the oppsite sideband (and other spurious products) would be adequately "suppressed" (more than 43db) below the total output making it technically "legal" (even though those spurious signals above 7300 might be detectable with a sensitive receiver during optimum band conditions)

    I am NOT of course, suggesting anyone do this since very few people have a calibrated spectrum analyzer connected to their transmitter. (sorry people, an ICOM IC-7851, KW TS-990, FTDX-9999 , Flex 6xxx, K4, or any other ham-toy, etc doesn't qualify, nor do ANY of the remote SDR receivers)
     
  2. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    That was what the FCC used back in the late 50s to cite W3PHL, who ran double-sideband reduced carrier at 600 watts DC input with several kilowatts of modulation, using a high-level balanced modulator. Under the regulations back then, his "legal" power was 600 watts input (about 400 watts carrier output), thus circumventing the legal limit, so the FCC cited Fred, who largely operated on 7290 or 7295 kc/s, for having detectable sideband products above the band edge of 7300. At that time there were no specified limits to spurious sideband products in the rules, so the FCC was able to claim that any detectable sideband products above 7300 constituted illegal out-of-band operation.

    With the currently enumerated standards for spurious sideband products written in the rules, the FCC might no longer be able to use that tactic as long as a station is in compliance with the technical standards, but Fred's case was cited by the FCC when they were first proposing that pee-ee-pee nonsense back in 1983.
     
    N3RYB likes this.
  3. W7IMM

    W7IMM Ham Member QRZ Page

    I think you may be right.

    If the FCC wanted to cite someone for this, I think they'd have to prove the offending transmitter was transmitting spurious signals above the limit (43db down?)

    1500w is about 62 dBm so if opposite sideband and/or other spurious signals must be at least 43dB "down" from that, it would follow that spurious signals could be somewhere around 19 dBm (62-43) or a little less than 80 mW. (do I have this right?)

    So on a 'good' propagation day, 80 mW would absolutely be "detectable" on a remote receiver or even across town etc.
     
  4. K5UJ

    K5UJ Ham Member QRZ Page

    I had a problem last fall with my 75 m. 3-400 rig, but I didn't realize it was the rig at first. It showed itself as reflected power so I simply figured I had some kind of antenna problem. Something changed outside; maybe one of my antennas had started behaving parasitically and was putting power back into my transmit antenna, or something else outside had changed. I couldn't find anything wrong and I was about to embark on a test of grounding, when about a week later I just happened to run a different rig on 75 m., and at some point noticed that there was no reflected power. I immediately knew I had a rig problem and the rig problem was likely a harmonic. So, the tube gear can be remarkably clean until they aren't.
     
  5. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    If the spurious signal is 43dB down, and you are coming in on a receiver with a reasonably linear s-meter at 40 dB over s-9, you will be clearly audible at slightly below s-9, since by definition one s-unit is alleged to represent 6dB. The signal would be clearly readable far beyond "across town".
     
  6. W7IMM

    W7IMM Ham Member QRZ Page

    Right. But I don't think the FCC would cite anyone for this if the actual "detected" signal was in fact 43dB (or more) below the main signal.

    It would have to be actually interfering with another service for them to take the time to even investigate. I'm going to guess that even it that was happening, and after they determined the "offending" transmitter was meeting technical standards, they'd probably tell the "victim" to "pound sand" (or say sorry, that ham is within tech standards", etc)

    They have really much bigger "fish to fry" anyway.
     
  7. KA4KOE

    KA4KOE Ham Member QRZ Page

    If you have a PEP meter or an oscilloscope, monitor the output of the 25W carrier on a 100W rig. Some solid state rigs have ALC issues with AM where it is impossible to fully modulate 100%. On an oscilloscope, the waveform will double in amplitude for a 100% modulation with a test tone. If your rig does this, then reference this article by W8JI:

    AM.jpg
     
  8. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    A good idea, the external ALC carrier level control; I hadn't thought of that. I would trust the oscilloscope a lot more than I would a hammy hambone meter. A professional lab-grade meter might be OK.

    Maybe run the carrier a little below 25%, to achieve some extra positive peak capability if your natural voice is highly asymmetrical.

    I usually advise others to turn the ALC off when running a plastic radio on AM, but evidently some rigs don't have that option.
     
    AG5CK likes this.
  9. N2DTS

    N2DTS Ham Member QRZ Page

    I have not seen a rig you could turn off the TX alc.
    The RX agc yes, not the TX alc.
    That would be very nice.
    In the old days, you could just adjust the output power setting to 150 watts and run 25 watts carrier without getting into the
    alc on radios like the ic 735 and ts440.
    Now its mostly software I think.
    Some kenwoods make nice AM, but that is about it for ssb radios.
     
  10. WA5VGO

    WA5VGO Ham Member QRZ Page

    To disable the ALC on vacuum tube rigs all you need to do is short the ALC input jack to ground. I’m not sure if this works with modern radios.
     

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