Dr. Alison Johnston G8ROG will be with us on Thursday 28th September to tell us all about the “high quality” voice communications now possible on HF.
Venue: Woodford Park Sports Centre, Woodley, 8pm, as usual.
Modern digital modulation techniques allow more information to be
exchanged in the few kilohertz of bandwidth we typically use on SSB, for example. This enables near-FM quality.
We will cover a bit of theory, the practicalities of interfacing the
necessary computing equipment (maybe even mobile phones???) with (HF) radios and there will be a demonstration using a couple of HF sets.
For me, HF DV is in the true spirit of amateur radio and is an exciting
development as the technology is open to all. If you wish to experiment with the software, you can. As far as I am aware it is not covered by any patents.
Refreshments to be served as usual.
A huge thankyou to Alison for supporting us. Her knowledge of radio is, shall we say, expansive to say the least, as you may be aware. So if you have related queries I’m sure she would be interested to hear them over a cup of tea.
Looking forward to seeing you there.
Next meeting is on Thursday 14th September entitled “The events leading up to and including the Battle of Britain by Chris Wren MBE”.
Venue: Woodford Park, Woodley, 8pm.
Refreshments to be served as usual.
Chris is the curator of the WW2 underground bunker and museum at RAF Uxbridge built in 1939 some 60 feet below ground. He will introduce personalities of the time including Douglas Bader who had a flying accident not far from the our very own Woodford Park on the Woodley airfield and the most successful fighter squadron at the time.
Churchill visited the bunker during one confrontation and enquired what reserves we had – the answer was “none”.
I look forward to seeing you there to look back on those perilous times and be inspired by those incredibly brave men and women to who we owe so much.
I was on 20m working EU and DX stations using FT8 this afternoon. Just after 16:00, I noticed all signals faded quite markedly and within a minute or two. It was sufficiciently ominous for me to check on http://dx.qsl.net/propagation/index.html and http://www.swpc.noaa.gov/products/goes-x-ray-flux. A couple of minutes later, I saw an X class flare reported. That explained the HF fadeout. About 5 minutes later I saw it was rated as X8.2, only a little weaker than the X9 flare we had on the 6th September. It will be interesting to see if the CME was Earth directed, and if so, how much aurora we’ll get for VHF propogation fun.
This evening I came across a terrible signal on 15m FT8. The audio from a Cuban station sounded distorted. I won’t name and shame the station. The audio fundamental was about 600Hz, and the waterfall showed strong signals at 1200Hz and 1800Hz matching the 2nd and 3rd harmonic of the audio signal. This caused significant QRM across a significant part of the FT8 segment. Almost certainly the station was overdriving the audio, either causing distortion from their soundcard, and/or distortion in the rig’s modulation chain.
There are several things you can and should do to avoid this, especially when using significant power and/or gain antennas:
- Don’t use full output from your soundcard. Set at no more than 50% volume level. Play a pure tone at that level and listen to it on the soundcard output as you adjust the level.
- Some rigs, such as the Elecraft K3, have a dedicated data audio input and data mode that removes audio processing like compression or other ALC artifacts. Use this if possible.
- For rigs without dedicated data modes, disable all audio compression and set the RF power to a desired amount, say 25W, and start with a low audio signal so the RF power is much lower than that. Then increase the audio level using the soundcard volume and rig audio level until just below the desired RF output while keeping the sound card level in the range 10% to 50%.
- Use an audio fundamental of at least 1400Hz. If necessary, tune the rig to adjust the dial frequency. For example, say you are using FT8 on 20m where the band segment starts at 14.074Mhz. You see a free frequency on the waterfall of 400Hz, and you want to start calling there. If you transmitted a 400Hz tone, then the 2nd, 3rd and 4th harmomics at 800Hz, 1200Hz, and 1600Hz will all be in the audio passband of the rig. Hence the rig will transmit the desired signal at 14.074400, as well as undesired signals at 14.074800, 14.075200, and 14.075600. To avoid this, retune the rig 1kHz lower, to 14.073Mhz and use an audio frequency of 1400Hz. This will generate the same RF for the audio fundamental, but the 2nd and higher audio harmonics will be mostly outside of the rigs audio passband and will be suppressed from the RF output.
I’ve been trying JT65 andf JT9 for about 5 weeks now. I thought I’d give FT8 a try. This is a new data mode designed by Joe Taylor, K1JT, and Steven Franke, K9AN. You can find more information at http://www.arrl.org/news/ft8-mode-is-latest-bright-shiny-object-in-amateur-radio-digital-world.
One of the strengths of JT65 and JT9 is that they are very sensitive, JT9 being slightly more so than JT65. However, each transmit or receive period is a minute. So it takes at least 4 minutes to complete a QSO. In many cases, the signal to noise ratio of stations is well above the minimum for a successful decode. So FT8 was designed for that, using 15 second periods rather than 1 minute. So one big advantage is that in many cases you can complete a QSO 4x faster with FT8 than JT65 or JT9.
Unfortunately, there’s a downside. Some JT65 and JT9 decoders, such as those in JTDX, are very good at decoding multiple replies to your CQ, even those on the same frequency (to the nearest Hz). It does this through multiple decoding passes where decoded stong signals are then deducted from the recorded data to expose weaker signals beneath. However, the same does not currently appear to be the case with the FT8 decoder in WSJT-X 1.8.0-rc1. So while you can see a reply on the waterfall, you often get no decode. This is particularly the case if your signal is strong at long distance and attracts multiple replies. Even with single replies, often decode fails despite a visible trace on the waterfall.
For working DX on 80m, I’ve found JT65 and JT9 is much better than FT8, being able to decode signals only just faintly visible on the waterfall. So which data mode you should use depends on what you are trying to achieve. For faster QSOs with decent strength signals where multiple repies are not the norm, then FT8 is a better choice. But for working DX with marginal signals, where your patience might be better rewarded, JT65 and JT9 might be better.