I did some further tests here just now and was getting FT8 decodes reliably down to -20dB. This seems to be about the threshold for successful decoding here.
This is about 10dB below what I can see on the BATC WebSDR waterfall. SIgnals decoding at -10dB are just visible.
73
Charlie
Hi Robert
Yes - I have one PC connected to the TX and another connected to the RX and can decode myself with no problems. In the first round of tests (with the locked LO but poor signal quality) I got some FT8 decodes down to -20dB. I'll try some more tests tomorrow morning (I may have to find some more attenuators to put in the TX path :-). DT generally between 0.4 and 0.5s.
I usually test with dial frequency mid band in the NB digital modes segment, QRM permitting (have not had any QRM yet).
73
Charlie
The drifty xtal oscillator chain has now been replaced by the original 10MHz locked synthesiser, with a redesigned loop filter. It now has a much cleaner spectrum.
After switching back to the locked LO, I found that FT8 was then very reliable.
73
Charlie
DD4YR I was waiting for a confirmation email from that site but never got it (also checked spam folder). I chatted to Jean-Pierre on batc wb chat and he said I had been registered OK for two weeks. I then tried logging in and it worked.
73
Charlie
PS I am making progress learning MMSSTV....
Heard someone say they thought the recommended max level was 10dB above the beacons yesterday, rather than same level as the beacons.
Yes, agreed you need a very stable link to be able to work with FT8. My unlocked LO is marginally good enough for FT8 (and JT65A). I decoded you today a number of times with JT65A. My RX is GPS locked here.
I found today that with my slightly drifty LO, I can work with JT9F fast with 5 and 10s periods. I received myself via the BATC Web SDR for these tests. It seems very stable. Also tests showed JT65C was better than JT65A.
Also tested JT9E (fast), MSK144 (wideband), and ISCAT B. All seem fine with 5 sec periods.
Given that in most cases S/N margins are good, we may not need to use modes with 60s periods, and use of these will be compromised if frequency stability is going to be problematic for many stations.
I managed an easy QSO today with OZ1CTZ using JT9F. I also managed a QSO on FT8 but this was more difficult.
Of course as technology improves, FT8 will still be interesting as you can pack so many users into a very small bandwidth.
73
Charlie
Hi Digi folks
Following change of LO here to a clean one that drifts more, I am trying JT9F 10s periods and it is working very well. Drift is probably too much for FT8 as it is also problematic with JT65A. 10s periods are good too as it progresses QSOs faster!
73
Charlie
Thanks, Robert
Slight problem here - my 10MHz locked LO for the upconverter is dirty and I changed this afternoon to an old free running xtal chain. SSB signal now much better but I think the stability is now not good enough for FT8. Tomorrow I fit a crystal heater and see if that makes it good enough.
I can receive OK, so happy to try to decode you. With the present LO, JT65 should be OK.
73
Charlie
Achim
I have measured TX power here now. To get a comfortable but not excessive signal back, I am running about 200mW into 1dB loss to the feed with 3m 0.3 f/D dish dish and 2.25t helix feed.
Transponder noise is now about 15dB - seems less than yesterday, but I may not have the 10GHz feed exactly at the focus.
Testing now with FT8 running 200mW is showing decodes at +7dB S/N
73
Charlie
Hi Achim
I will try to make some measurements today. Power here yesterday was in the region of a few hundred mW to 3m dish. I'm planning to add a coupler/power meter at the dish to measure it.
It was a very interesting day yesterday! 12 QSOs on SSB and also decoded myself with power backed off at least 16dB with FT8, QRA64D and JT65A.
73
Charlie
Hi Viljo
Yes, digi modes seem to work well. Decoded myself yesterday with FT8, QRA64D and JT65A - started a thread on this yesterday. I'm available to try two-way tests now.
73
Charlie
For WSJT-X modes, it depends on the mode in use. Signals levels are normally referred to as signal to noise ratio in 2.5kHz bandwidth. JT65 (1m periods) works down to about -25dB, while QRA64 (1m periods) can go 2-3dB weaker. FT8 (15s periods) can go down to about -20dB. SSB becomes difficult to read at about 0dB, so with digital it would be possible to use very low TX ERP and still make QSOs.
There are other digital modes that may also work well for satellite use. Some of the JT9 fast modes could be interesting, with periods of only a few seconds possible.
Tests done so far here have been back to myself - one PC connected to uplink radio and another to the downlink radio. I set TX power to give me about -12dB for initial tests.
73, Charlie
Also JT65A is working well:
Some early results with digital modes. So far I have tested QRA64D and FT8. Both are working fine.
1741 and 1743 are QRA64D, the rest are FT8.
73
Charlie
Some screencaptures of today's video at different MER values obtained by offpointing the dish:
https://drive.google.com/drive…haePGJUwGNi96?usp=sharing
Picture started to break up at 0.5dB, solid at >=1dB
Good signal here today also. MER of 10dB with (severely underilluminated) 3m dish. Attempts to use my narrowband downconverter were partially successful, with 7-8dB MER but intermittent picture/sound. After switching to Octagon LNB the signal was solid. Poor performance of the narrowband downconverter probably caused my the image filter (pipe cap type) being too narrow and creating distortion.
It is and I share your concerns. Any ideas to mitigate.. paint?
Paint might work, or perhaps better make the helix out of copper or brass and solder it to the connector. I have made helices out of either thick copper wire or even 141 semirigid. You can bend copper wire around the N type connector centre pin and make a solid solder joint.
73 Charlie
No need to worry about circularity (axial ratio) unless it is VERY bad. The losses are generally small / negligible , as shown here:
Hi Iain
Not sure about the gain of the Kuhne preamp either. I have two other commercial ones, DU3BC and F1OPA. Both have inbuilt bandpass filters, but cover up to 10.5 no problem. The DU3BC has slightly less rolloff at 10706, so was useful in finding the EB initially.
Not sure either of the design of the DB6NT transverter. If it uses pip cap image filters, it would be quite easy to retune to 10488.
Charlie
