Software defined radio 1/4. The hardware

Have you ever considered why it has been predicted that in future radio frequencies will be more valuable than oil?
Even now there are ship loads of information on air, all the time. I wanted to grab some and see the "traffic jam". This guide will offer you a way to follow up my lead.

This is my first post in English. I'll use global language of English to offer guide also for others than Finns. Reason for this guide is that I was able to find many sites about RTL-SDR radios but most of the tutorials seemed lack some aspects for full system. This won't either be complete guide but I'll try to do my best ;)

I will post four chapters:
- 1. Hardware (R820T + RTL2830U)
- 2. Antennas (HF & VHF)
- 3. SDR software (SDR# & HDSDR)
- 4. AIS & ADSB

First of all. What is software defined radio? I'll let Wikipedia answer for that: SDR
Briefly: radio device is only used to receive / transmit RF and to do A/D or D/A converting as computer does all the decoding / encoding of the the signals.

My choose for software defined radio apparatus is the cheapest of them all. It has Rafael Micro R820T tuner chip + Realtek RTL2830U demodulator. Commonly this combination is called RTL-SDR. Good places to by one are Ebay and AliExpress. You can find this kind of receiver under 50€ including shipping but even cheaper option is to buy TV-dongle and make some modifications to it.

Mediatek chips R820T and RTL2830U were originally manufactured for digital television USB-dongles for computers. But some amateur radio operators find out that it is possible to use this hardware also as wideband receiver for other radio signals.

My receiver was advertised as being able to tune from 100KHz  to 1.7GHz. Package included: receiver, 3m (10´) coaxial cable, usb cable and aprox. 30cm (1´) long whip antenna  for "GSM" with magnet stand.

Receiver has SMA female connectors for HF and VHF (UV?) antennas and USB mini-B connector for computer connection. There is also one green led next to USB port to show if receiver is ON.

Receiver is very compact

Datasheets and many sites tells that R820T + RTL2830U combination is not able to tune below 20MHz. After testing my hardware, this seemed to be true. So why in earth is this device being sold as 100kHz to 1.7GHz? I was very keen to be able to receive HF (shortwave) transmission so this bothered me a lot.

I did not receive manual with the receiver and after awhile it became clear that there exist none. No paper manual nor online. So answers did not come that easy.

Neither was I able to find answers by googling. I did find some clues from internet that it is possible to get this hardware tuned below 20MHz by making hardware modification called "direct sampling mode" but it requires some soldering work and extra filters. Another possibility is to buy 75-100MHz upconverter. So I decide to dismantle the receiver to find out it's secrets.

After some hours of internet investigation and exploring the receivers circuit board I find out that Chinese manufacturer had made the direct sampling mode for this receiver all ready at factory. Hooray, no need to warm up the soldering iron or squander some 50€ for upconverter.

Original 1 foot antenna and circuit board 

Circuit board is build so that direct sampling modification is connected to HF connector and VHF connector to tuner chip. After figuring out the use of direct sample mode I found guide how to enable direct sampling in software. I will guide the setup for this in SDR-software post.


*** Forgive me those errors I have made in text and help me to fix them. Please leave comments with link to source if possible.***