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Starting in simple software defined radio

Software defined radios (SDRs) have taken amateur radio transmitting and receiving by storm. They have greatly reduced the cost of setting up a wide-coverage HF, VHF and UHF station and made it easier for people to operate their gear remotely via the internet. Low cost spectrum displays have led to use to see as well as hear band activity. Plus there's great scope for personal involvement, education and experimentation with basic SDR construction being well within the reach of the average ham or electronics maker.

Keep reading for a few ideas with what you can do with SDR.

1. Listening to online receivers

No need to build or buy anything to do this. Just visit websites like SDR . HU to tune other people's SDRs they've generously put online for public use. This lets you explore radio conditions and activity from near and far. If you've got an amateur radio or CB transmitter you could even transmit a test signal to see if you can hear yourself at a distant location.

The Stoke on Trent SDR is an example of a radio club project in the UK. SDR . HU is a central site for over 100 SDRs around the world. Note that their antennas and locations vary - some are in noisy spots while others are in quieter places. If you have one nearby try tuning in the same signals on them as on your own equipment and compare the results.


2. Buy a cheap SDR dongle for VHF/UHF and add an upconverter for HF

Listening to online remote receivers is great but if you want greater personal involvment there's no substitute for setting up your own SDR-based receiver. The availability of cheap RTL dongles that allow VHF/UHF radio reception on a standard home computer has made this an easy, cheap and fulfilling project. Apart from the computer the items you need include an RTL2832 USB stick, software such as SDR# (SDR Sharp) and a vertical antenna suitable for VHF/UHF reception.

While some RTL2832 come with supplied software and a small antenna, don't use these. The small antennas are insufficient for good reception, especially for the low power CB, amateur, emergency and aircraft signals that we're interested in. Instead put up an outdoor antenna such as a discone or a 1/4 wavelength ground plane for 144 MHz (which will also pick up UHF signals). This should be fed with coaxial cable to the USB stick's antenna socket. Or at least go outside and use a better portable antenna like below.


When you're starting out with SDR dongles first try your local FM stations to get a feel for using them and the software. These are high power and are on all the time. Experimenting with various settings can be highly educational. Below are the results I got when tuning across the 88 - 108 MHz band. Adjusting gain is particularly important as you want sufficient gain but also not so much that it overloads and prevents some signals being heard.


Later you can try other activity like amateur 144 & 432 MHz FM and SSB. And other communications services that may still be transmitting analogue FM. Search frequency lists for your area for clues as to what's around. The video below is where I receive a 1296 MHz amateur signal by travelling to near where a known beacon is transmitting from. Again a dedicated antenna would have given even better range. The demonstration also illustrates the limitations of my particular dongle (there's better ones around) as regards to frequency accuracy and stability.


Possibly the most fiddly aspect is loading and setting up the SDR Sharp software. Some pointers are at the RTL SDR quick start guide. First try receiving known strong local stations such as 88-108 MHz FM broadcast stations. Then experiment with reception of other signals such as amateur beacons and repeaters.

Later on you'll want to broaden your horizons try HF reception. As the cheaper SDR dongles only do VHF/UHF you'll need an upconverter to convert HF signals up to VHF. Upconverters are available in built form or as a kit. They can also be made from scratch as an intermediate-level project.

Consider resources and products like those below if you wish to get into SDR.


3. Build your own

If you don't have an RTL SDR another way to start receiving signals is to make your own HF to sound card adapter / converter. These are super simple to build but only cover narrow sections of one or two popular HF amateur bands. The other big performance compromise is because they only have a single output there is no image suppression. Consequently the same signal appears at two spots on the display and you might get 'image' interference from other signals. Dodge this by changing your converter's frequency slightly and retune to compensate through the computer.

These videos show some experiments. I've had great results with SDRadio software by I2PHD. VK5TM presents a circuit and description.

While not for serious reception results are still surprisingly good for the few parts required. Later on you will want to build a better version that suppresses image responses. Instead of a single output this type of converter has two outputs. The signals from each output are slightly different from one another (ie in quadrature or a 90 degrees phase difference between them) to allow the software to differentiate the image from the desired signal. This phasing method can be highly effective but requires a good sound card with stereo inputs to work.



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4. A little trick that makes tuning an SDR easier


5. A diversion from radio

Tuning into radio stations isn't the only thing you can do with SDR. In conjunction with a small transmitter you can use it to produce cool voice and sound effects. Hours of fun for science fiction fans and movie makers!


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