SDR - Software Defined Radio
Many radio amateurs interested in situation on the radio bands. To that end, of course, a radio scanner is preffered. However, they are expensive. Sometimes very expensive. Recently, this problem has been circumvented. An unknown radio amateur discovered that very simple and inexpensive device designed to watch TV on a computer, contains hidden options. Inside we have some specialized chips.
It's funny, that this small-sized device, for a price under 20 €, whose primary purpose is to watch TV has a lot of hidden potential inside. Inside of a dongle in probably 99 cases out of 100 we have a standard specialized chips. The direct conversion receiver chip R820 or older E4000 (already discontinued) and specialized demodulator RTL2832U for signal derivation to the USB connector. Using special software this USB device overlaps in very wide frequency range. In practice, from 24MHz to 1766MHz and is capable to accept a wide range of modulation types: CW (telegraph), AM (amplitude modulation) NFM (Narrow band frequency modulation), FM (broadband frequency modulation), LSB (lower single band modulation) USB (upper single band modulation) DSB (digital) and RAW - as in all serious pro devices. It can take a taxi calls, automotive CB radio channels, a simple FM radio stations, aviation dispatchers conversations with airplanes. With special software on a computer we can see the airplane traffic as well as accept weather satellite map transmissions.
Everything is realized with special software on an ordinary computer. The most common is the programs SDR # and HDSDR. They require a newer computer because of high CPU demands. I could not make these programs to operate stably on a Pentium-4 computer.
As already mentioned, the DBV-T dongle covers the range from 24MHz to about 1800MHz. But many people wants to explore the situation on and the low-range radio bands. We would like to see frequencies form the long-wave (LW) to middle wave (MW) and the short wave (SW) range. In the case of the amateur bands, - its really from 0Hz to 30MHz. That's why you need an upconverter.
Upconverter for USB RTL2832U dongle
Soon after I became interested in upconverters, I tried to find anything resonable on a market. There was a wide range of devices, but they were not cheap. By taking a closer look, we can see that almost all of them assembled on ADE-1-24 based chip (Figure 1). Inside this chip you will find four diodes and two small ferrite beads. Such chips has a couple of drawbacks. First of all we lose about 8Db while converting that weak signal. Second - the inverter requires enhanced crystal oscillator amplitude. There also lies another drawback - the converter becomes less resistant to ghost-making channels, the so-called "birdies". I understand the reason producers use namely the passive chip. First, it does not require a power supply, a second - in order to produce higher quality converter one should hand-wrap broadband radio frequency transformers.
For the drawbacks written above, I refused to use passive upconverter. In order not to lose the signal I've chosen SA612AD (older model SA610AD) converter chip. It's a Gilberto cell-based monolithic balance sheet signal converter which is strengthening signal for about 14dB.
For the first test I made a simple non balanced upconverter just to test its performance. The scheme is shown in Figure 2. At the entrance stands a low-pass filter in order to cut the powerful FM station in the city. Its bandwidth is about 10kHz - 40MHz. So that covers a whole range of LW to SW. This device has a 55MHz quartz block which is taken from vintage computer motherboard. All frequencies are shifted up by 55 MHz - for example, 40m amateur band frequency (7MHz) will be received by dongle within its working range, e.g. 55 + 7 = 62MHz.
The first tests showed that such a simple upconverter works quite well. An attempt was to apply Mini Whip antenna (I will talk about it later). The biggest problem while tuning longwave in city is the electrical noise from power lines, electrical devices, motors, etc. which is natural. While in country the interference is reduced dozens of times, - all the bands are full of stations. The main drawback of this upconverter is a significant number of parasitic receiving channels (birdies).
Balanced active upconverter
After a series of experiments I've decided to stay at this circuit (Figure 4). X1 converter input impedance Z is the standard through the entire device, ie, 50Ω. The filter output impedance is at 1,5k it can be connected directly to non balanced chip input (as we see the first version of the converter). However, the balanced SA612AD input impedance is twice as high, ie, 3k. Therefore, in this case it is necessary to do Zin and Zout transformation. This requires the winding balun on binocular ferrite core of about 5-7mm length. Winding direction must be done according to the scheme !
Next the signal is applied to the chip balanced inputs 1 and 2. Chip outputs 4 and 5 connected to another balun, transforming chip output impedance to standard dongles impedance, ie, 75Ω. The second balun is very simple, its primary winding has 13 turns and a secondary 2 turns. Both baluns winded in 0.15 mm copper wire. The chip mixes 125 MHz signal from a tiny SMD quartz module with radio station frequency. It moves frequency we are interested in over the FM band and strong local stations does not preclude the adoption.
The whole scheme, upconverter and active Mini-Wheep antenna supplied from a 12V battery. The antenna is powered from a battery through the signal cables central core. It is not recommended to use any AC or switching 12V power source, namely use the battery.
Mini Whip antenna
It's hard to believe that such small antenna can operate efficiently over a wide frequency range, but it's a fact. Antenna popularized by PA0RDT. More about this antenna theory we can read http://wwwhome.cs.utwente.nl/~ptdeboer/ham/tn/tn07.html We can see how Dutch university SDR receiver, which is using this kind of antenna works: http://websdr.ewi.utwente.nl:8901/
If you are not able to build a Long Wire antenna, the mini antenna is a great option. You just need to keep proximity from large trees, power supply cables and try to have a lot of free space around antenna.
The antenna diagram (Figure 7) close PA0RDT proposed. Difference is that in this case modern SMD transistors are used and output parameters tuned according to 50Ω circuit connecting cable impedance. In this case, instead of an usual J-FET transistor, two gate mosfet BF998 transistors, who have protective diodes inside are used. The output transistor - UHF medium power transistor 2SK5551. Antenna PCB (Eagle)
A bit about mini whip antenna installation: the main antenna plate hermetically sealed in a plastic container. The PCB BNC connector is connected to the required length 50Ω characteristic impedance cable type RG58U. The cable is put inside the plastic mast and then is led into the room. Do not use a metal pole! It is recommended to put on some ferrite chokes on the cable inside of the room. It is also highly recommended to connect cable metallic shield to a ground - it provides significantly reduced interference, which in our environment is really good enough. We may connect metallic converter housing to grounding directly.
In the urban environment, using Mini Wheep antenna for lack of space is problematic too. In the city is a lot of interference, and concrete-steel buildings that absorb the signal. But I tried to use micro version of Mini Whip. I raised micro whip antenna on the house wall of a concrete building. It's strange, antenna works quite well, I thought that would be much worse. Electronics encased in a plastic rod inside (pic. 10). The device is 1.80 in length, at the very end inserted printed circuit and copper strip 200mm length and 6mm in width. Electronics board 7mm wide, SMD assembly, so it easily fits into rod tip. It can be used not only with USB dongles but also with other receivers. Here we see a small video. Micro whip antenna works in LW band. Reception conditions in the city is difficult, anyway antenna signal is quite strong.
Using Long Wire antenna
Instead Mini Whip, a fully fledged Long wire antenna can be used. Tests with this antenna has shown that simply connecting to 50Ω converter input will do no good, because the impedance mismatch, which will make input inpedance bridge the antenna. Long wire impedance is (depending on the wire length and height) approximately 400-600Ω, it is therefore necessary for alignment of specific resistances 9:1 with a help of wideband transformer, which operates in a long line way. The transformer is wounded on a 2-3 cm diameter ferrite core 0.4-0.5 mm thick wire, three strands at once, 11 turns. Ferrite mark is not critical. Here we can see the photo transformer using ferrite 3cm diameter ring. In this way adjusted Long Wire antenna is slightly more resistant to interference than the Mini Whip.