Discussion in 'Amplitude Modulation' started by K5UJ, Jul 29, 2016.
New radios won't be in a cabinet, they'll be in a solid block of epoxy.
They can pry my boat anchors from my cold, silent key...
Seriously, if you don't like the ARRL's strident, overplayed demand for unprecedented bandwidth specifications, please consider cooking up a Reply Comment to refute their points.
Another glitch in the new ECFS: when I first open the website and view the page, some of the comments are missing from the list, including mine and Rob's. Then I click on page 2, let that page load, then go back to page 1. When it re-loads, the rest of the comments are there. The first time it happened, I was wondering why they posted the comments of others who had submitted later than I, but not mine; maybe I had clicked on something wrong when I submitted it.
I don't think that new ECFS "upgrade" is quite ready for prime time.
I'll give them credit for recognizing that an absolute lack of bandwidth controls would be a potentially dangerous thing in the future. And, reading through the comments, I can see their reasoning of "we want a limit, but we don't want hyper-regulation, so put in a broad limit and let hams self-regulate".
However, that only works if we have an agreed-upon bandplan, and if the regulatory environment makes it clear that, while bandplans are voluntary, they do factor in heavily in adjudicating complaints about harmful interference.
That doesn't seem to be the case. Which is why I originally put in a comment suggesting a 500Hz limit below the ACDS frequencies, and why I may make a reply comment reiterating that notion.
The way to maintain something like the existing bandwidth controls without the baud limit, would be to define the bandwidths of RTTY/digital signals in terms of NECESSARY bandwidth, rather than imposing strict limits to OCCUPIED bandwidth.
The occupied versus necessary bandwidth thing is a complicated issue. Here are some spectrum analyzer measurements that will hopefully clear things up.
First trace is occupied bandwidth of a cable TV 64QAM signal. I chose this signal because we know the necessary bandwidth is 6 MHz. Also, the Rigol DSA815 works well for wide band signals. The 99% occupied bandwidth is 5.35 MHz.
Second trace is -26 dB emission bandwidth of the 64QAM signal at 5.866 MHz. The -40 dB bandwidth is 6 MHz.
Third trace is occupied bandwidth of a DVB-T2 signal for a 6 MHz channel. The 99% occupied bandwidth is 5.766 MHz.
Fourth trace is -26 dB emission bandwidth of the DVB-T2 signal at 5.833 MHz. The -40 dB bandwidth is 6 MHz.
1) The occupied bandwidth depends on the shape of the signal. Clearly the 64QAM signal with it's root-raised-cosine roll-off measures differently than the DVB-T2 OFDM signal. This is what they're talking about in 47 CFR 2.202 when they say "In some cases, for example multichannel frequency-division systems, the percentage of 0.5 percent may lead to certain difficulties in the practical application of the definitions of occupied and necessary bandwidth; in such cases a different percentage may prove useful."
2) 99% occupied bandwidth has nothing to do with how many dB the signal is down at some frequency offset. The claim that the FCC has made a 6 dB error is a canard.
3) For narrow band signals on HF, the occupied bandwidth will be close to the necessary bandwidth. Unfortunately, the Rigol DSA815 doesn't have enough resolution bandwidth to accurately measure 3 kHz or less signals.
4) The term occupied bandwidth is often misused. Instead of the strict 99% definition, folks are really talking about the necessary bandwidth.
It's not a canard, it's elementary arithmetic. Whether intentional or by miscalculation, there clearly is a 6 dB discrepancy between the Part 2 (general rules and regulations) definition and the Part 97 (amateur radio) definition.
§ 2.202 (a) Occupied bandwidth. The frequency bandwidth such that, below its lower and above its upper frequency limits, the mean powers radiated are each equal to 0.5 percent of the total mean power radiated by a given emission.
§ 97.3 (a) The definitions of terms used in part 97 are:
(8) Bandwidth. The width of a frequency band outside of which the mean power of the transmitted signal is attenuated at least 26 dB below the mean power of the transmitted signal within the band.
First, let's consider a few power ratios expressed in fractions, percentages and dB:
1/100 = 1% = 20 dB
1/200 = 0.5% = 23 dB
1/400 = 0.25% = 26 dB
Let's use a 6 MHz bandwidth signal as our example. Let's assume the 6 MHz to be centred about a 100 MHz reference frequency. Let's also assume the radiated power to be 100 watts and that the shape of the signal to be symmetrical, although the above definitions could apply to any signal regardless of shape.
Using the § 2.202 (a) definition, the lower frequency limit is 97.0 MHz. The upper frequency limit is 103.0 MHz. If the total mean radiated power is 100 watts, we have a mean power of 0.5 watts radiated below 97.0 MHz and another mean power of 0.5 watts radiated above 103.0 MHz. That leaves us 99 watts of radiated power within the 6 MHz band defined by those frequency limits. The mean power radiated below 97.0 MHz plus the mean power radiated above 103 MHz equals a total of 1 watt, or 1% of the total mean power radiated. Expressed in decibels, the mean power of the transmitted signal below its lower, and the mean power radiated above its upper frequency limits, are each attenuated 23 dB below the total mean radiated power. Adding these together, the mean power radiated outside the frequency limits of the 6 MHz bandwidth is attenuated 20 dB below the total mean radiated power.
Using the § 97.3 (a)(8) definition with the same upper and lower frequency limits, let's assume the same 99 watts of radiated power within the 6 MHz band limits. If the mean power radiated outside the bands limits is attenuated 26 dB below 99 watts, the total power radiated outside the limits is 1/400th of 99 watts, or 0.2475 watts, which is 6.064 dB below 1 watt, the mean power radiated outside the limits as defined under § 2.202 (a).
I suspect this discrepancy is due to an error in calculating the addition of decibels of attenuation, since there is no reason to define bandwidth in the amateur rules differently from the definition in the General Rules & Regulations. Written in terms of dB, § 2.202 (a) would read: The frequency bandwidth such that, below its lower and above its upper frequency limits, the mean powers radiated are each attenuated 23 dB below the total mean power radiated by a given emission. The error was in adding the mean powers above and below the bandwidth limits. Since we are talking about power levels below a reference level, -23dB of radiated power, plus another -23 dB of radiated power, add up to equal -20 dB, not -26 dB.
You're not getting it. Occupied bandwidth is measuring dBm/Hz, not dB.
So, why do the definitions of bandwidth in 97 make references to dB? I don't see any mention of dBm/HZ in either the Part 2 or Part 97 definitions of bandwidth.
You are confusing bandwidth with power density.
The definitions in Parts 2 and 97 state nothing of the kind. They define occupied bandwidth in terms of the sum of total radiated power from the lower frequency limit all the way down to DC, plus from the upper frequency limit all the way to infinity. This total radiated power outside the limits is compared to the total radiated power within the limits, expressed as a ratio either in terms of percentage or decibels.