Overdriven audio on FT8, or other data modes

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:

  1. 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.
  2. 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.
  3. 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%.
  4. 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.

Best regards,
David M0DHO

New FT8 data mode versus JT65 and JT9

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.

David M0DHO

WSJT-X and JTDX news from Dave, M0DHO; 50Mhz Meteor Scatter

“I’ve been using WSJT-X and JTDX for about a month. I many cases I could have made the QSOs with CW. However, I’ve worked many more Chinese stations, presumably because they find CW hard for non-English native speakers. There have been a few cases were JT65 or JT9 enabled a QSO that would have been too weak for CW. I’ve worked a number of VKs and one ZL on 80m like that, and a few stations on 6m that would otherwise have been marginal. It’s been very successful for working South American countries on 80m – I’m still trying for Chile, Bolivia, Peru, Suriname, Guyana on 80m, for which getting up at 05:00 seems to be required. It’s the wrong time of year to try it on 160m. There seem to be very few African stations using JT modes apart from the ZSs. Pity because ZS is easily workable using JT on 80m so the others ought to be easy as well.

I’ve tried MSK144 meteor scatter on 6m during the Perseids with good results. But you often have to be patient – it might take 10+ minutes to complete the QSO because the pings might be infrequent. Using the KST chat room helps. At some stage I want to try it on 2m but need to sort out interfacing to my FT847 first.”

Night on the air Part 2; vintage evening; drinks, 24-Aug 7pm

Next RADARC meeting is at Reading Rugby Football Club, Holme Park, Sonning Lane, RG4 6ST, Thursday 24th August, 7pm.

Given folk seemed to enjoy last meeting “Night on the Air”, we’ll do the same again. The reason it’s billed as “Drinks” is so we all get a chance to relax a bit but with a nod to health and safety, I think we can combine the two.

Feel free to bring along any portable gear to play/experiment with.  There was loads of stuff last time. Just be aware there may be 100W of RF kicking about on HF which delicate equipment may not appreciate if connected to any nearby antenna.

For fun, if you have any vintage/veteran gear you might want to
demonstrate to people (not necessarily radio) feel free to bring it.
Simon ZSU and I like Tilley lamps (i.e. paraffin storm lamps) and one
came out to play last time.

In fact – bring what you like. Even if you think it maybe of no interest
to others. You may be surprised.

Start 7pm but feel free to arrive earlier to help with setup or just
enjoy the evening. NOTE I’m going to start an hour earlier than the usual 8pm as it’s getting dark earlier. Don’t feel you have to be there for that time.

If you are new to the hobby this is a great opportunity to see amateur
radio in action and have a go at operating yourself.

We may also have Woodley air cadets joining us.

Should weather preclude outdoor operations we will meet anyway as the bar will be open.

A huge thankyou to people who brought stuff last time.

Looking forward to seeing you there.

73

John
G4RDC

Amateur Radio ISS contact next week at Gilwell Park

A contact between the International Space Station and youngsters at Gilwell Park, north London, is scheduled for Tuesday 8 August (next week), as part of Youngsters on the Air 2017.

The youngsters will take part in a Q&A session with astronaut Paolo Nespoli, IZØJPA, Flight Engineer of Expedition 52/53, lasting around 10 minutes.

The contact will be at 1838 UTC (that’s 1938 local time) on 2m FM, likely on the standard ARISS frequency of 145.800 MHz, and should be readily receivable using handheld Yagis, turnstiles, or even rubber duck antennas over the UK and northern Europe.

A simultaneous HamTV transmission is also planned, with live pictures from ISS coming down via DVB-S, streamed live via the BATC website.

Some more information provided by Southgate ARC here.

Many thanks to the ARISS team for once again coming together to make this event happen – we look forward to seeing the results.

“Night on the Air” 10th August, RRFC, Sonning Lane

Next meeting is at Reading Rugby Football Club, Holme Park, Sonning Lane, RG4 6ST, Thursday 10th August.

It’s “Night on the Air” so we will have a few stations set up – probably a couple of HF stations. Feel free to bring along any portable gear to
play/experiment with. Just be aware there may be 100W of RF kicking
about on HF which delicate equipment may not appreciate if connected to any nearby antenna.

Start 8pm but feel free to arrive earlier to help with setup or just enjoy the evening.

If you are new to the hobby this is a great opportunity to see amateur
radio in action and have a go at operating yourself.

With luck, we will also have a demo of an “Antenna test range”.

On the military kit side, with luck, we will also have a “Clansman”
dipole and 30-70MHz vertical antenna/dummy load atop. I suspect there will be some other “green” radios on parade.

Should weather preclude outdoor operations we will meet anyway as
the bar will be open.

Looking forward to seeing you there.

73

John
G4RDC

Thursday 27th July – ‘Fix it’ / Alignment evening

Next meeting is Thursday, 27th July, 8pm Woodford Park.

It’s a ‘Fix it’ evening (Alignment evening in old money) so bring along those radio/electronics items needing attention. Even if we can’t fix items on the evening we may be able to work out a plan for them.

I’ll try and make sure we have some test gear available (scope, maybe spectrum analyser, meter etc.)

Computers also welcome – I’ll bring a VGA/HDMI monitor.

As per last meeting, would be great if you can let me know what you’re bringing first but no problem if not.

Even if I have to use many kettles, we’ll do coffee, tea and biscuits. The urn was indisposed last time.

ISS SSTV 2m FM transmissions this weekend

This weekend, ARISS is celebrating its 20th anniversary with a series of SSTV transmissions from the ISS – International Space Station.

Transmissions will be on 145.800 FM in PD 120 or PD180.

These should be easily receivable on a handheld tuned to 145.800 with anything better than the stock whip. Last time around they were loud! A 3 element tape measure yagi will work very well for this, and it’s not much effort to knock one together. Full quieting signal should be achievable.
Images can be decoded live during the passes on a smartphone using, for example, “SSTV Slow Scan TV” by Black Cat Systems for iPhone, or “Robot36 – SSTV Image Decoder” on Google Play for Android. A cable will help, but holding the mic up to the HT speaker should work okay. Alternatively recording the audio through whatever means will allow you to play around decoding later.
Passes for the weekend can be found on http://www.heavens-above.com as well as other sources. Be sure to set your location. Some key passes:

Thu 23:23:21
Fri 00:58:35
Fri 22:32:15
Sat 00:06:25
Sat 23:14:23
Sun 00:50:21
Sun 22:22:34
Sun 23:58:03
Mon 01:34:30

Event is scheduled to end at 18:00 UTC on Monday 24 July.

Most of the passes this weekend are visible, so assuming no cloud, antenna pointing shouldn’t be a problem 🙂
 
Most favourable pass taking into account the time looks like Sat 23:14:23 BST.
 
For those of you who have not spotted ISS in the night sky as yet, at its peak it is second only in brightness to the moon in the night sky, appearing as a solid bright white point typically low in the south-western sky from the UK, appearing to move across the sky about the same speed as an airplane at altitude. At its highest point, on a high pass, it really is hard to miss – easily brighter than Venus or Jupiter. Some passes it will fade into shadow, some it will pass across the whole sky brightly reflecting full sunlight, well into the late night down here.
 
If the same format is followed this time around as last, there will be 2.5 ish images per pass with a few tens of seconds gap in between, with no carrier.
 
Happy sat spotting…
Update
In the end I slept through most of the passes! However here’s the one image I did capture:
I put together a simple cable to connect my Wouxun handy to my iPhone based on this circuit,  used an inexpensive Nagoya NA-771 whip, and for aiming, I listened for ISS on a second handy on its stock antenna, held closely and parallel to the Wouxun, broadside to ISS. Almost perfect copy, up to at least S6 or so. Great fun!
Did you capture anything? Tell us @MX0AAA!

Clansman SURF unit – another interesting object

For anyone into military radio, Patrick kindly pointed out that these Clansman SURF units are available quite cheaply on well know auction sites currently. It is essentially a variable bandpass filter I believe, designed for using VHF manpacks together in close proximity. It is a beautiful piece of engineering if slightly radioactive!

I love the worm drive – a variable capacitor and variable inductor is driven from the tuning knob via this drive.

Looks like new inside.

Some Clansman fun at the RADARC barbecue

Sorry about the bright pink basket!!

We had some Clansman fun at the Barbecue – thanks John, Patrick and Alison for helping out – learnt a lot about the Clansman kit. The mast is a Clansman 5.4 metre with Clansman dipole (eventually set correctly for 60m!) and an elevated groud spike antenna on top for 4 metres.

Website for the Reading And District Amateur Radio Club