toggle quoted messageShow quoted text
On FT8 my RX is set for a bandwidth of 5000 Hz. Reception tapers off near the extremes. Few signals are anywhere near the high end, but some are at or below 200. In those cases, it helps to tune down a few hundred Hz.
73, Steve K2SN
On Feb 24, 2019, at 4:50 PM, Dave AA6YQ <aa6yq@...
# even more AA6YQ comments below
you could improve my simple algorithm a bit. Assume a worst case of a fixed SSB Rx filter at 2400 Hz bandwidth centred on 1500 Hz, one could subtract 600 Hz from the spotted frequency then truncate that to a 2 kilohertz step. Doing that would ensure spots at 7076.6 would be received at an audio offset of 2600 Hz, whereas a spot at 7076.7 would end up at an audio offset of 700 Hz (from 7076) rather than 2700 hz (from 7074) which may be too high for optimum reception.
The only issue with the above is that a spot using the VFO dial frequency, rather than the dial frequency plus the audio offset, would cause issues as the VFO would end up at 7072 for a spot given at 7074. I suspect that would cause more problems that it would be worth and the simplistic truncate to a 2 kHz step is your best bet.
# Bill, you're right that with some rigs, one would be better off working an FT8 signal on 7076.6 or 7076.8 with the rig QSY'd to 7076 instead of 7074.
# One way to handle this would be to enable the user to specify a "max RX bandwidth". If this parameter is set to 3 KHz, then double-clicking an FT8 Spot Database entry on 7076.7 would QSY the rig to 7074. But if the "max RX bandwidth" were set to 2.5 KHz, then double-clicking an FT8 Spot Database entry on 7076.7 would QSY the rig to 7076 (2 KHz above the sub-band base frequency).