I also share the concerns about hotspots, new users, and those who want to build a repeater when all they have for test equipment is free DMM from Harbor Freight.
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However, a few may discover a passion for RF and want to 'learn the ropes' - hopefully, with real study, not by watching a 2-minute youtube video.??
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For those folks and a lot of other amateurs and professionals, bandwidth is a topic they haven't had much experience with.? Mostly we use commercial gear and trust that the manufacturer took care of it.
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The repeater builder site and this discussion list are the pre-eminent resource to shine a light into these dark corners.
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In that interest, let's talk about bandwidth in the context of channel spacing, bandplans, and frequency coordination.
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First - occupied bandwidth [OBW]
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Occupied bandwidth is a regulatory requirement that is specified for equipment in some regions, such as the USA. [47CFR2.202(a) & 47CFR2.1049]
It is defined by the ITU-R as a maximum bandwidth, outside of which emissions do not exceed a certain percentage of the total emissions such as 1% - in other words, the OBW contains 99% of the total power.?
Thus, the 99% occupied bandwidth is 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.
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There is also the term Authorized BW which is the maximum bandwidth that the modulation is allowed to occupy. This is essentially a reduction in the Channel Spacing
for example, for 15khz channel spacing the authorized bandwidth is 11.25 kHz
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Then, there is the bandwidth indicated by the emission designator.? For analog FM it is determined by Carson's rule based on a single 3 kHz frequency causing the indicated peak deviation.
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Thus, for Analog FM (5.0 kHz peak deviation)?
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Emissions designator: 16K0F3E?
Occupied bandwidth:? 12.59 kHz
Authorized bandwidth: 20 kHz (25 or 30 kHz channel spacing)
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Accumulating a peak hold trace on a spectrum analyzer will certainly tell a lot about a transmitter's bandwidth utilization, but, in fact - OBW is a defined concept with a specified measurement procedure.
The actual measurement procedure uses a transmitter test pattern comprised of two tones, one at 650 Hz and another at 2,200 Hz The goal is to simulate normal modulation for generating adjacent channel power data.
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So, if we know the OBW of a transmitter do we know how well it will play with neighbors (i.e. adjacent channels)?
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Not really!? 99% of the power seems like it wouldn't leave enough to be a problem.? How much trouble can be caused by a half percent on either side?? Maybe a lot.
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1% is actually just -20 dBc so 1/2% is -23 dBc. If we have 100 watts, -23dBc is a half watt.? If that half watt is in the neighbor's receiver passbband - how much trouble could we cause with a half watt into a DB224?
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Hmmm.? There must be a better way.? There is.? More on that next time.
