Category Archives: Member stories

Sharp Resonance: A Low cost Audio filter from Screwfix; mag loop update

Tony Canning G2NF has an intriguing evening lined up for us next meeting:

Thursday 12th October, Woodford Park Leisure Centre, Woodley, 8pm

Some of you will remember his superb talk on ‘Tricks with Coax’.

He states:

“Given my interest in coax related stuff and various tricks, I have been
playing with an audio filter that can give the user with no IF 200Hz
filter a similar tool in a pile up, and like wise for the SSB operator
an audio notch filter. There is no eltrickery, smoke mirrors or power
supply involved. I think its a great resonance demo, as the relationship to coax filters and tuned circuits is uncanny.

Can be made from house hold items or from Wickes / Screwfix or BnQ etc.

This is not about Audio DSP but my simple efforts do appear to offer
comparable performance to some at a fraction of the price but much more fun building and learning.”

Continuing the theme of sharp resonance, I’m trying to twist G2DD’s arm (Loz) into bring along the coax based magnetic loop antenna he’s been working on as I know a few people would be interested in having a go at making something. Or indeed – if you have been working on one feel free to bring it along and tell us what you’ve been up to!

Refreshments to be served as usual with an increasingly wide selection of biscuits of variable age.

73

John
G4RDC

X8.2 Solar flare

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.

David M0DHO

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.”

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!

My venture into 4 meters

On the 11th of march I bought a Simoco SRM9000 from Mike G4CDF. (Thanks Mike!) I finally had a 4 meter rig and he also let me borrow a 4m loop to get me started until I could make my own. I was pretty much working to get it all setup as soon as I arrived back home.

First question when putting any amateur station together is of course how on earth will this all go together and where can I put it?!

The antenna itself is not small and would not be the kind of thing I can just hang in my window. It also has a pole fixed to it for mounting vertically with a horizontal spacer between it and a mast. This made things more difficult as it could therefore not just be laid down on a first floor roof ridge which is close enough to my shack. Instead I settled for persuading my parents to let me stick my military fibreglass mast up on the corner of the house. Amazingly, it blends in quite well and just looks like a piece of guttering down-pipe that is rather small…. Oh and it also has a large chuck of metal hanging off to one side!!

So I have an antenna sorted and a mast for the antenna to go on. I setup the radio connected to the power supply that I have running my other radios on the desk and it immediately sprang to life. Great… I have a radio, an antenna, a microphone and speaker and power. I knew I was missing something.

Aaaaah…. Coax!!!

Next problem – how to get coax into the shack without drilling holes in the wall or doing anything else that seems to annoy my parents! So here is my solution:

It got a little cold last night because the window is now open just a crack for the coax to come through so i enlisted the ever useful duct tape method!!

Finally I was ready to get a QSO on 4m so I then experimented with all the buttons and I think I hit scan and heard Alison G8ROG doing a test. I quickly replied and sort of knew that I would manage to contact her given we are just across the valley from each other. Perfect 5/9 signal!!!

I was given various pieces of advice including about the local nets and events throughout the week and frequencies (Thanks Alison!!)

The important ones to remember are:

  • 70.425 – Local net frequency
  • Nets are on Tuesday evening’s at 20:30 and most members are from RADARC but it is not exclusive.
  • GB2RS news is also read out on a Sunday morning with a net on the same frequency afterwards.
  • Vertical Polarisation is the norm but some use horizontal

So my next steps now are to re-program the radio and have a browse through the settings and of course join in the nets regularly.

If you are interested in getting into 4 meters then there is a wealth of knowledge within the club. Just come along to one of our meetings and ask! We will be happy to help!

There are also some great resources for the Simoco SRM9000 series of radios on the Simoco post linked here

Listen out for me on the weekly nets and I look forward to speaking to you!

73 de 2E0JPM