Back in May I posted about my first tentative experiments in the world of radio. Since that first post I’ve made slow but steady progress spending time learning a lot about different frequency allocations, modulation types, antenna designs and so on but being careful not to get too carried away as I’m prone to do!
Switching Back to Linux
Initially I was using an application called SDR# or “SDR Sharp” to tune my RTL-SDR receiver. SDR# is a .net framework application for Windows but not being a Windows user I followed some instructions to get it working on Linux using the Mono framework. This worked but I found the performance nowhere near as good as on Windows so I began looking for alternatives.
I found another application for Linux called Gqrx which performs the same function as SDR# but is pure Linux. It is essentially a front-end for the GNU Radio suite which is a huge collection of tools for building software radios using modular components. Gqrx itself is written in QT but has no KDE dependencies so it is simple to set-up on my Mint 15 Cinnamon installation, to do so I followed these instructions to compile the latest development version of GNU Radio and Gqrx from source that have support for the r820t dongle I have, the current Ubuntu/Mint repositories contain an older version of GNURadio which does not have support for the r820t but should work with older e4000 TV tuners.
While Gqrx is a simpler application than SDR# it works well and it’s fast. It can tune quickly and easily like SDR# and can demodulate all the usual AM, FM, upper/lower sideband (SSB) and CW signals.
I haven’t worked out if it can zoom into the Gqrx can zoom into the frequency band like SDR#, but it isn’t as intuitive or flexible, this feature is useful for identifying narrow bandwidth signals and distinguishing modulation types.
Happy in Linux I spent my early days listening to the local 70cm band repeaters and occasionally I’d discover nearby transmissions on the 2 metre band by hams or walkie talkie users on PMR446 at 446MHz. I could never hear anything on the 10 metre band at ~28Mhz but got a lot of noise from commercial transmissions, I found out I was setting my antenna gain too high and lowering it reduced a lot of the interference but I still get “ghost” FM signals around 26MHz which I believe might be resonance from transmissions in the main FM radio bands because I have direct line-of-sight from the local transmitter only a few miles away across the valley so they are probably a little too strong and bleed across the radio spectrum.
I was initially using my home’s spare TV antenna which is a common “Yagi” type which are designed to be directional, they focus their reception power wherever they are pointed but are not very good hearing anything behind them, so I made my first investment upgrade and bought a scanning antenna that is omni-directional and easy to move about to get a good signal.
While this has all been very fun there was never a lot of activity going on and in my research I kept hearing about all the fun hams were having on the HF bands which I didn’t have access to. The RTL-SDR dongle can received frequencies from 25MHz up to 1.8GHz but there’s a lot of activity below 25MHz where signals travel much further distances so people communicate across the world, I felt the need to investigate this!
To get my RTL-SDR “shack” tuning into these lower frequencies I needed an upconverter. This is a device that mixes a signal to shift it up the frequency spectrum so it can be handle more easily by the receiver. I made my 2nd investment upgrade and bought a “Ham it up” v1.2 upconverter from NooElec in the United States and it’s a thing of beauty!
The upconverter is open source hardware, the designs are available online to fabricate yourself which is interesting but way above my head. There’s an RF input on one side for the antenna, IF output on the other side to go to the receiver, power is supplied via a USB A-to-B cable from your computer and a switch is there to activate the upconversion or simply pass-though the signal untouched without needing to disconnect it when not in use.
This model uses a 125MHz clocking chip which is replaceable, with this any signal that comes in is shifted up 125MHz on the receiver.
This isn’t 100% accurate and some fine tuning is required using a known frequency, I use Absolute Radio’s 1215KHz signal to adjust the offset, on my hardware is this 124.99000MHz which doesn’t seem like much but 10KHz s the difference between finding a signal or not when the signal bandwidth is very narrow. Gqrx and SDR# both have the ability to offset the frequency reading so they show only the RF frequency on the air not the IF frequency that the dongle is receiving which is 124.99MHz higher.
Update – 25th January 2015
My earlier problems with the offset were due to inaccurate frequency reporting by the RTL dongle and not the crystal which was very accurate.
First, you also need to set a frequency correction value, this is unique to each RTL dongle so to find your value see this later post on the topic. This value is entered in the ‘Freq. Correction’ box in the ‘Input Controls’ side panel.
Next, set the offset, still in ‘Input Controls’ edit the value “LNB LO”. For a 125MHz shift set the value to “-125.000000”, note this is a negative value because your tuning frequency is shifted down. If your upconverter is using a different crystal set the value as appropriate. Once done re-tune to your desired frequency and you’re all set.
Yet more antennas!
But let’s not forget the antenna! My house Yagi and scanning antenna both work only on VHF/UHF bands so are not sensitive to the relatively low frequencies of HF, for this I needed a new antenna, so I built one! I chose a simple design, a random longwire antenna, for which I borrowed 20 metres of steel garden wire, removed the insulation from one end and jammed it in the RF input on the upconverter, and it works!
True to my expectations I’ve discovered a lot of activity on HF, I can hear the normal AM commercial broadcasts on Longwave and Mediumwave, I can also hear a lot of world radio stations on various Shortwave bands. Shortwave radio is unfortunately in decline but there are still a lot of stations to be heard, I have not had this set up long but already I have heard broadcasts as far as Beijing and Botswana over 5,500 miles away!
What is this strange place?
Being nearly 30 I am still too young to have experienced Shortwave’s peak, I didn’t know much about it before this experience but I’ve found it rather interesting in my unashamedly nerdish way. Compared with normal commercial AM, FM and DAB stations, Shortwave is bizarre. Because signals travel internationally transmissions are only part-time and sometimes overlap on certain frequencies. Tuning into shortwave is also a mixed bag, every time you turn on the radio you get something different, not just because of the intermittent broadcasting schedules but also because of variable propagation conditions. Signals travel different distances depending on the time of the day or the season of the year and the influence of these factors varies depending on which shortwave band you’re listening to, so your listening experience can be very random!
Most Shortwave listeners are either international travellers or people who follow a hobby called ‘Shortwave listening’. They aim to optimise conditions for listening to as many stations as possible as far away as possible. If you can hear them some stations ask you to write in and report where in the world you heard them from known as a QSL report, I heard one station already reading out the QSL reports they had received from listeners and they were certainly a far-flung group!
Many broadcasters are national institutions such as the BBC putting across their Government’s view of world events to foreigners. This is clearly used as a propaganda tool and so it’s sometimes jammed by oppressive states to block out the signals, North Korea are apparently famed for this unsurprisingly. Religious organisations also use it to preach their message, pirate radio stations are set up by revolutionaries and militaries broadcast warnings to whole continents or just to their agents in code.
The BBC world service have been reining in their Shortwave broadcasts as they are expensive to operate and there’s a lot of ways to receive the BBC in most places in the world today however they have begun to transmit on Shortwave in digital format using Digital Radio Mondiale (DRM). This is similar to DAB used on VHF radio but designed for shortwave, this seeks to address low quality audio you frequently receive on shortwave bands so it can become a viable alternative to FM/AM while retaining its very wide coverage benefits.
However DRM seems be in the catch-22 situation that DAB had for a long time, there’s few broadcasters because there’s few listeners, there’s few listeners because there’s few radios, there’s few radios because there’s few listeners and broadcasters. Mind you the DAB radio in my kitchen tunes into DAB stations daily so it’s not impossible to break the cycle, it’ll just take time and effort to get there. Why do I mention this? Because there is a software DRM decoder called Dream that I can add to my RTL-SDR shack to listen into the growing number of DRM stations but that’s an experiment for another day!