What's SDR then? Well, low cost, flexible and open to expansion for a start.....

Software Defined Radio or SDR for short is a pretty much the cutting edge of radio receivers. It consists of a relatively small device to convert RF to a data signal and a bunch of software to filter, resolve, display and in some cases decode the recieved signals. It also consists of a pc/Mac/tablet to run the software. The 'defined' part refers to the infinite and adjustable stages of filtering the other part of the system allows to filter and to resolve the signal into understandable information.

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. 

Low Band ducting!

The Uniden 785 was sitting on top of the fridge as usual. The RG-8 feed from the roof mounted antenna was capturing various activity around Denver. There were helicopters and at least two F-16s out of Buckley in the air.

The scanner was running through my programmed frequencies. It stopped on a very distinctive and new signal. An analog voice channel in English discussing (firing) locations on a range. Reference was made to various activities. It was just about 8.15 mountain time. I stopped the scan on 32.300 in FM. The voices were clear and consisted of a control station and a number of out stations. These seemed to be somewhat mobile. The signals were very strong and clear. One peculiarity was a short data tone every 5 seconds or so. A Havequick sync tone? Then something surprising. The voices changed to French and an exercise continued with various coordinates being passed in French. An F2 ducting event? The English voices had a distinct Canadian accent at times. 

The conversation continued. The Uniden was put into the car to allow various chores to be performed. The dedicated 40 MHz NMO mount antenna was put into service and the signals were almost as strong as the multi-band on the roof. Between transmission blocks I began searching. On 32.550 a signal that sounded like a PD dispatch. A NY or NJ accent was noted. The range activities continued until after 1830 local.

Later in the afternoon, a helicopter was found approaching Buckley from the South on 41.750 FM. This is a new frequency for me as I mostly hear their traffic on 32.300 FM, 32.500 for Buckley 'Outlaw', 32.600 and 32.750, all FM. That day the Command Post was active on 122.850 AM and the F-16's were in the Sandbox on 138.950 in FM.  Tower was monitored on 121.000 AM. I tend to listen to VHF as it makes the AirLife and Flight For Life helicopters movements monitorable as they pass in and out of Buckley's airspace on their way in and out of the group of hospitals on the East side of the City.

Well, hopefully this has been of use. More to follow as it is found. Keep searching!

What's This All About

MONITORING 

I started being aware of radio at a local fair in a park where a Jamboree On The Air station had been set up. It was 1969 or 1970. I found the old boy with his valve "twin set" and a G5RV type antenna strung up in the trees fascinating. From that humble station he was working other stations within a couple of hundred miles with ease.

Our local police had just begun trialing small hand-held radios as well as the large, trunk mounted valve radios in their vehicles. With a little care  I learned it was possible to hear the base station dispatching the cars and officers. 

I was hooked to the world of radio, ham radio and utility monitoring.

My present interests lie with military land, sea and air monitoring. This blog will attempt to bring together many sources of information that exist both on paper (!) and on line. The object is to assist the newly interested listener in finding interesting things to listen to in the greater Denver area.

The frequencies and talk groups listed will be either ones that I have been able to verify or ones that have been passed to me from other sources. The idea is not just to copy and paste the pages of Radioreference.