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.



HF Digital Voice (HF-DV) communications by Dr. Alison Johnston, G8ROG

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.



The events leading up to and including the Battle of Britain by Chris Wren MBE

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.

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 and 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

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.



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.

Website for the Reading And District Amateur Radio Club