How far off frequency? Like 28097 kHz or 28099.1 kHz? I know that 28085 / 28.085 MHz is Channel 19 "up two bands" on export radios - 27.185 MHz - 27.635 MHz - 28.085 MHz + 10 kHz switch gives you 28.095 MHz.
Export radios that cover 25-30 MHz or so (usually 25.615 MHz - 30.105 MHz, 26.065 MHz - 28.755 MHz or 25.615 MHz - 28.305 MHz) are very popular with fisheries.
Of course, they could also be running one of the Chinese fishery 27.500 MHz - 39.500 MHz FM fishery radio system, 480 channels 27.5 MHz - 39.475 MHz. 25 watt radios, FM voice 16K0F3E/16K0G3E emissions with 25 kHz steps (27.500 MHz = channel 1, 27.525 MHz = channel 2, etc.) So 28.100 MHz FM fits within that channel plan perfectly as far as frequency and mode. I could see one of those radios being 500 Hz off-frequency or so without any issue since they're running regular "20 kHz FM" or "25 kHz FM" - what's used for amateur radio and VHF low band land mobile communications.
https://www.hfunderground.com/wiki/index.php/CB#China.2FHong_Kong_Chinese_27.5-39.5_MHz_HF_and_VHF-low_Fishery_Marine_Band_FM_Fishing_RadioThe FT-801 radio and similar radios are not doubt sold by fisheries equipment suppliers in Portugal, Brazil, etc.
Apparently QRM on the 10 meter band from AM/FM export radios covering 25.615-30.105 MHz and FM fishery radios covering 27.5-39.5 MHz is extremely problematic for ham operators in Asia, since when you look at those channel plans (10 kHz steps 28.005 MHz, 28.015 MHz, 28.025 MHz, 28.035 MHz, etc. AM/FM/SSB and 25 kHz steps 28.000 MHz, 28.025 MHz, 28.050 MHz, 28.075 MHz, 28.100 MHz, 28.125 MHz, 28.150 MHz, 28.175 MHz, up through 29.575 MHz, 29.600 MHz, 29.625 MHz, 29.650 MHz, 29.675 MHz, 29.700 MHz, 29.725 MHz, 29.750 MHz to 39.400 MHz, 39.425 MHz, 39.450 MHz, 39.475 MHz, etc.)
see:
https://simonthewizard.files.wordpress.com/2014/12/ft-801-fishing-boat-rescue-etrust.pdfFT-801 Fisheries Dedicated Radio Dual Signaling fishing boat mobile transceiver
*With distress call, select call, group call, all call, sea and other communications functions call etc.
*National unity networking: able to access the nation's coastal fisheries establishment of several shore station for receiving emergency aid management and undo edits sea conditions forecast.
*Ship's position information be sent automatically and display functions.
*Full band multi-channel 27.5 ~ 39.475 MHz, total 480 channels to work.
*Fishery dedicated communication channel.
Output Power: 25 W
Modulation Limit: 5 kHz
Frequency Range: 27.5 - 39.5 MHz
Channel Spacing: 25 kHz
Number of Channels: 480
Modulation Method: FM (16K0G3E)
Power Supply: DC12V or DC24V
Signal Format: 300 baud rate ASK / 1200 baud rate MSK
Appears to have 10 kHz bandwidth receiver (for FM communications 27.5 - 39.5 MHz) as well as a medium wave and shortwave broadcast receiver covering 500 kHz - 9999 kHz (receive only). Marine antennas for the 27.5 MHz - 39.5 MHz band are available on the Chinese market.
Fishing boat special radio FT-801 uses high-quality parts to improve transmitting and receiving performance, intuitive LCD Chinese display screen, and easy operation. According to the needs of fishermen users, this unit adds an AM radio function to facilitate listening to weather broadcasts; Setting 4 Communication channels - 4 channels on duty (channels 221, 231, 236, 238), greatly improving the data signaling connection rate; built-in GPS module to complete positioning and navigation functions.
Fishing boat special radio FT-801 is a new type of wireless telephone with high reliability, multi-function, intuitive display, simple operation and good communication effect. Frequency range: 27.5-39.5MHz Channel grid: 25KHz Number of channels: 480
It would appear that the Chinese have a pretty extensive network set up for this band, much like the Japanese and South Koreans with their 26 MHz / 27 MHz AM and SSB fishery radio systems. Multi-channel dual watch function for channel 221, channel 231, channel 236 and channel 238, which, if straight channel numbering is used, equal 33.075 MHz, 33.275 MHz, 33.400 MHz and 33.450 MHz.
Therefore, if these guys were using FT-801 radios or similar Chinese spec gear on 28.100 MHz FM, that would equate to channel 25 on these radios.
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