UNID FT8 6925 USB 0310 UTC 11 MAR 2024

[myteaquinn]

Sounds like FT8 signal being sent. Here are the decodes :

030900 -18  1.3  324 ~  RW9WT KM9E EM79
030930  -9  1.4 1070 ~  Z62NS WA6TMJ EN52
031030 -11  1.3  324 ~  RT6C KM9E RR73

Here is the screen shot decode by WSJT:

[sternradio666]

SDR images.  Stations were probably on 40m.  Your screenshot shows them at 7074, the usual FT8 watering hole? 

Hammy hambones aren't going to use their callsigns out of band!

[myteaquinn]

Been thinking about this post and what I heard. The screenshot is the WJST decoder. The 7074 frequency is an FT8 frequency and I selected it as it was the closest to 6925. It could be an image but I heard it on my NRD-545 first then checked my R75 and AIRSPY Discovery. Hearing it on all three I decided to post. They do share the same antenna so that could be a problem.

I did tune up to 6930 as I thought the signal moved up there. The squiggly line is I think Indy Radio on 6931. Maybe someone played a recording of FT8 transmissions or a problem with my system. I'm not sure what it was.   

[Charlie_Dont_Surf]

If you are using some sort of LNA/pre-amp before all three receivers, then it is possible that you were hearing a mixing product/spur being generated in the LNA. However, if you aren't using an an LNA, then the fact that three receivers (all with three different dynamic ranges and input intercept points and at least two different receiver architectures, etc.) all heard it makes me think something else was going on.

Another thing to note, the thermometer below is red for a reason. It indicates that you had your audio cranked WAY up. Bad things tend to happen when you do that in WSJT-X. I find that I start to lose decodes and weird artifacts appear (which is exactly what we are talking about) if the receive audio is turned up enough that it averages 70 on that thermometer scale. I don't want to think about what 90+ means. Try to keep it between 40-60 on average and in the green.



I did see/hear some CW with an actual DX ham callsign briefly in roughly the 6930-6950 area about a month ago. I checked it on an SDR and it was there too. I assumed something was wrong with the op's TX because I only heard one side of the conversation. I went about my business and it disappeared after checking back an hour later. I forgot to write down the exact details. I think the op was Greek and I vaugely remember this happening during a contest weekend.

[myteaquinn]

Thank you for the feedback Charlie_Dont_Surf.

No LAN/pre-amp in use here., I do have a rather inelegant solution to running three receivers at the same time. I have three antennas feed a MFJ-1704 switch that outputs to an RF Systems SP-1 antenna splitter. One output goes to the NRD-545 and the other output goes to a PL259 "T" adapter to feed the other two receivers. Could this setup be the problem?

I'm running WSJT-X right now and I have the RF Gain control all the way to the 9 o'clock position to get it down to 40-60 range, thank you for that advice.

[Charlie_Dont_Surf]

Thank you for the feedback Charlie_Dont_Surf.

No LAN/pre-amp in use here., I do have a rather inelegant solution to running three receivers at the same time. I have three antennas feed a MFJ-1704 switch that outputs to an RF Systems SP-1 antenna splitter. One output goes to the NRD-545 and the other output goes to a PL259 "T" adapter to feed the other two receivers. Could this setup be the problem?

No. Those are all what are called "passive" circuitry; there is no active amplifier involved. As such, it's extremely unlikely. You would need to put kilowatts of RF power through them to generate the types of birdies we are talking about here and you are obviously not doing that in a receive application.

I'm running WSJT-X right now and I have the RF Gain control all the way to the 9 o'clock position to get it down to 40-60 range, thank you for that advice.

I don't know your receivers specifically but in general I always suggest setting the RF gain control wide open (maximum gain) and then controlling the audio to the computer using the AF (audio) gain, i.e., audio volume. The NRD-545 is by all accounts an excellent receiver that must have AGC that will fold back the RF gain automatically for you when it needs to. I think that you should be able to leave the RF gain setting on the front of the radio all the way up (maximum). This way you should probably get the best signal to noise ratio at all times. You can then use the volume control to provide a sufficient audio level to the computer input.

If for some reason you can't lower the audio input to WSJT-X enough to get it out of the red zone, then, yes, maybe decrease the RF gain slightly, just enough to back off the audio to the point where it is in the green zone.