Broadcast Overload on Kiwi SDR

The Kiwi SDR and Wellbrook loop provide a very good single antenna solution for 0 to 30 MHz shortwave reception. The Kiwi is capable of displaying the complete band of 30 MHz, but the disadvantage is the very high power from some of the LW and MW stations can cause the receiver to overload in the evenings.

One approach would be to build a notch filter to attenuate just the one or two strongest signals, in my case this would have been 909 kHz Radio Five Live from Brookmans Park, and 1332 kHz Christian Radio. The problem I had was in the evening this would often change, and at times some of the continental AM stations would become an issue.

The alternative would be a bandstop filter, that would just attenuate the MW band. We also have two amateur allocations at 472 kHz and 1.9 MHz that I would like to receive, with the medium wave band in the middle. The solution would seem to be a bandstop produced by Nooelec. The units retail for around 11 dollars in the USA, you can get them from Amazon or eBay for around 16 GBP with free delivery.

They say on their website “We designed Distill:AM to provide sufficient attenuation for broadcast AM frequencies (>40dB typical) while ensuring adjacent bands, such as 160m, are minimally affected. The -3dB rolloff of the filter is 350kHz and 1900kHz. Minimal out-of-band insertion loss means the filter can stay in place for most any application, though we do recommend removing Distill:AM from your setup when not listening to HF frequencies. As a true bandstop filter, you are able to pass-through DC (bias power) when it is required.”

You can view the data sheet below.

I found 909 kHz reduced from around -20dBm to -50dBm so this helped reduce any signal overload.

You can read more or use the Kiwi receiver here.

The Welbrook look is an active broadband receiving loop antenna.

Nooelec have a range of products online.


The Camb-Hams operated from a new site this year, due to their usual site being unavailable. The location had good unobstructed views, the local area is very flat but it did have such a good elevated VHF take off.

The event falls at the end of the year and providing a chance to catch and discuss the year’s events.

This year the team had 3 van based masts, their own Transit based Flossie with its 22m mast, as well as the recently purchased Mercedes Sprinter ex OFCOM van owned by Colin that comes with a 25m mast.

Geoff G0DDX also had his 12m vehicle mounted SCAM mast.

VHF was operated from Flossie, while HF was from Colin van with the 3e Triband beam at 25m.


The SSB FD rules have recently changed, the maximum power is limited to 100w. This can make it hard going as I would guess some of the EU stations are running a little more power. The contest also runs alongside the worked all Asia contest, adding another dimension to the band.

We also enjoyed a BBQ by torchlight, a field day favourite.

The drone provided some aerial video and pictures from the event.


Thanks for an enjoyable weekend.

LEFARS and SNBCG Big Radio Weekend

Once a year the LEFARS crew and SNBCG come together at the Secret Nuclear Bunker Kelvedon Hatch for a big radio weekend.

It’s the last chance to play some radio over a bank holiday weekend in 2018 before winter arrives!

On Saturday we had a BBQ for 25 people, giving everyone a chance to catch up over a burger.

The radio was split into a number of tents, VHF/UHF and 50 and 70 Mhz for LEFARS, with SNBCG running 2 HF stations. George M1GEO looked after the triband beam 14/21/28 MHz while I concentrated on 7 MHz using my vertical.

Band conditions could best be described as variable, at times allowing some big DX to be worked, and at other times presenting no signals at all due to a solar storm.

7 MHz on FT8 mode on HF helped work in the poor conditions, allowing me to work several stations from Indonesia, Brazil, West Malaysia, Argentina, Cuba, Australia, Oman, China and Arizona USA. Not bad for a 7 MHz vertical antenna with elevated radials.

I took the drone up to capture some nice aerial shot of the setup.

We had attendees from lots of local clubs including Camb-Hams, Essex Ham, LEFARS, SNBCG. A very well attended event, really nice to see everyone having fun and enjoying both the social and radio event.


HF fun with the 18 MHz beam

The addition of a beam on HF never ceases to bring a smile to ones face, the addition of a linear amplifier is often enough to create a pile-up.

If you look around at some of the amateur radio publications you will often see people complaining about the conditions or the lack of propagation. Undoubtedly we are at the bottom of the sunspot cycle, and HF propagation is much more of a challenge than sunspot maximum, but you would be forgiven for thinking  HF was dead.

Home for the weekend

The truth is even in this solar minimum it’s still possible to work DX, all be it you will need to try a little harder. If possible I try to use a beam, and the addition of a linear but you still need to be able to hear the lower powered stations.

The transceiver was a newly purchased Icom 7300, second hand but in excellent condition. The radio performed flawlessly and often had to cope with multiple stations calling simultaneously.

At times it was hard to work out what JA I was working at any given time!


Last weekend team SNBCG decided to have a weekend of radio fun on 144 MHz MS and 18 MHz HF. I operated the HF station, while George M1GEO operated MS.

The beam is a DK7ZB design, you can read about the construction project in this article.

FT8 was the favoured mode, giving me the best chance to work some DX.

Over the course of the next few hrs I decoded a large volume of exotic calls, not all working me but it did demonstrate what is possible with a reasonable setup.


25 USA, 42 JA (Japan), not forgetting to mention XE (Mexico), BD (China), FR (Reunion Island) and ZS (South Africa)

As traditional with field days, they often finish in VU.

Mysterious HF Number Stations

Much has been written about number stations over the years, although little is known for fact.

We do know they often broadcast on a schedule, so with a little planning, it is quite easy to receive them for yourself. Online SDR receivers take this to a new level, and now with TDoA, we are able to learn something about the geolocation of the transmitters.

It’s probably no surprise to learn many are now being geolocated to the area of Russia and Poland.

I remember the first time I ever stumbled upon a mysterious signal reading out numbers, this was BG (Before Google) so it was hard to identify or read about the history of these stations.

Needless to say, I started to log the station’s frequency and content, and would often find the same stations time and time again.

BBC Radio 4 program first aired in 2005. provide excellent service, collating lots of information in one place. They also link to various SDR around the world, providing you with a unique insight into this mysterious world.

The strange thing is they are still around today, despite us having the internet and 101 ways to encrypt and send messages these days. The attraction of HF is it leaves no digital footprint, and messages can be broadcast to 1 or many recipients over 1,000 of miles.

The Conet Project spent many years recording these strange stations and later selling these as audio recordings on CD’s

You can listen to many of these stations on my SDR, I have been added them to the frequency dial to help identification. Over the coming months, I want to collect some recordings and add them to this page.

The BBC has published a few articles on the subject one is “The Ghostly Radio Station that no one claims to run” and you can listen to the UVB Buzzer below.

Why not check out the geolocated maps, and view the site in Google street view.

UVB-76 Buzzer (Geolocated) or maybe (Geolocated)

6802 kHz (CW)

7600 kHz (Voice)

9147 kHz (AM)

10343 kHz (CW)

11581 kHz (USB)

Using the KIWI for TDoA (Time Difference of Arrival)

Pirate radio stations around 6MHz have been using that part of the spectrum for many years. They often populate that part of the band, and in the main don’t seem to cause the primary users any issues. The primary users sometimes put a STANAG signal on top of the pirate music station, I doubt they even notice the radio station.

The KIWI now has a Time Difference of Arrival function, you can use this to help identify the likely location of an HF transmission.

This evening 6316.5 KHz was active with a music station, and I guessed this may be located in the UK. I selected 3 KIWI receivers located around the SE of England and achieved this fix.

The accuracy can be very good, but much depends on the location of the receivers relative to the transmitter. I repeated this one a few times, and the location did change but always remained in and around Coventry, Solihull area of the West Midlands.

6235 kHz Energy FM (pirate radio relay)

The signal was very strong in the evening, indicating the station was likely to be 100 to 500 KM. The TDoA located it as seen below to Cirencester near Swindon UK.

A further example can be seen in this map, resulted from TDoA Radio Caroline.

Want to try it yourself?

A good place to start is to read this article and my KIWI SDR

(apologies for the SMPS QRM on my SDR its outside of my control and under investigation)

© 2015 Dave, M0TAZ