The computer device, through it's software allows for a very powerful and flexible tool. Many software packages come ready to run and are open source software, allowing anyone to build on the work of the original, generous authors.
I am using three separate hardware devices, two of which are commonly known as TV Dongles. They are a small usb device that was originally designed to allow computers to receive off-air tv signals. Someone discovered that the chipset being used actually had a massive bandwidth capability, somewhere between 1 MHz and 1900Mhz. An obscenely wide coverage. The third device is a more complex and better made version of this. They cost less than $20 for the two simple ones and less than $50 for the more advanced device.
The upshot of this is a detailed VISUAL representation of what is in the air. The software I am using is called SDR# (SDR Sharp) that displays a 2 MHz spectrum analyzer type plot of the spectrum surrounding a signal (or a place a signal is thought to be) and allows the user to see in real time signals appear and disappear. RF strength against frequency. From this it is possible to determine channel use, approximate strength at a given location and the density of use.
This package also has a display running parallel to the analyzer called a waterfall display. This gives a representation of RF against time. It displays the bandwidth used and, with some practice it is possible to speculate the type of signal being recieved. Analog, digital or an interference or intermodulation. It is with practice possible to speculate is this a Motrbo or a Kenwood signal? Is it analog or digital? TDMA or something else? You can see a screen shot below. The large box has the spectrum analyzer above and the waterfall display below.
The combination of display does require a healthy amount of memory to drive the displays. However they are individually selectable to aid with issues from a slower computer.
I have found this set up most useful in finding new frequencies. The visualization of spectrum and bandwidth has allowed a number of either intermittent use frequencies to be confirmed or to discover whole new users. The data found is then transferred into my trusty Uniden 785 reciever for more intensive reception.
Aside from a number of teething troubles, mostly to do with drivers for the dongles, the system has operated well from day 1. Through the addition of a number of add-on software packages, it has been possible to receive and decode P25, Mototbo, Kenwood NXDN modes as well as a multitude of analog transmissions in the VHF, UHF and low SHF bands. There are web pages dedicated to satellite reception, HF, VLF beacons, you name it. Someone is out there doing it with these little devices.
I have been searching all through the spectrum from 30 MHz on up. AM, NFM, WFM, USB/LSB and some digital modes. I have been able to confirm some new VHF frequencies carrying both MilAir and public safety transmissions. With the add on packages, I have been able to monitor voice transmissions that had previously been impossible to resolve into speech. Many of these are from the down-town systems.
I'll put some frequencies together and let you all have them next time.
