Post by forestradio on Mar 15, 2021 21:22:12 GMT -6
New member: I figure it's a matter of time before the FCC starts cracking down on Chinese FM transmitters like they did the UV5R handhelds so might as well get one before they do. A couple things got me thinking about this subject:
Reading someone (forget if it was here or another forum) mention they used a cheap Chinese transmitter as an emergency backup exciter in a licensed FM station when their primary failed and having the station manager comment he'd never heard the station sound better
Which got me wondering just how good (bad) are the FM transmitters in licensed stations, and are we asking too much of a $100 or even $500 Chinese transmitter.
The requirements for a broadcast FM station are specified in Part 73, Section 317:
Any emission appearing on a frequency removed from the carrier by between 120 kHz and 240 kHz inclusive must be attenuated at least 25 dB below the level of the unmodulated carrier. Compliance with this requirement will be deemed to show the occupied bandwidth to be 240 kHz or less.
Any emission appearing on a frequency removed from the carrier by more than 240 kHz and up to and including 600 kHz must be attenuated at least 35 dB below the level of the unmodulated carrier.
Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.
I was measuring the output of my Nio T15B today with an rtl-sdr to see how dirty it is (and to check for spurs in the airband) and decided to plot its spectrum against the FCC limits. I realize this isn't strictly correct because they'd have the transmitter connected directly to a spectrum analyzer, but it at least gives me an idea of what it looks like and is the equipment I have on hand.
I have a spur on both sides about 100 KHz out of band (the one on the right interferes with the HD portion of the adjacent station, which I hadn't realized before because I don't have an HD receiver), but both are well under the Part 73 limit of 25 dB. In fact a Part 73 approved station could completely drown out the two adjacent stations at 35 dB.
Putting aside the audio quality issues for a moment and just focusing on the interference issues, is this a problem better solved by throwing a narrow bandpass filter tuned to the exact transmit frequency on the output to get rid of all the spurs? Would this ultimately be a more cost-effective solution than finding a transmitter without spurs? How does a licensed station deal with this in situations where an approved transmitter is still interfering with adjacent stations? Or is there something I'm missing?
