Ham radio operators have a long history of using pan adapters to visualize an entire range of the radio spectrum. Traditionally, an adapter was essentially a spectrum analyzer that shows a trace where the X-axis is the frequency, and the Y-axis shows the signal strength at any particular frequency. You can quickly find either busy frequencies or empty frequencies at a glance.
Although the pan adapter has been around since the 1930’s, they aren’t as common as you’d think with regular analog radios. However, if you’ve used an SDR (Software Defined Radio), a spectrum display is par for the course. [Mehdi Asgari] did what a lot of hams have been doing lately: he married an SDR and his traditional receiver to provide a great pan adapter with very little effort.
To understand this hack, you have to remember how a superheterodyne receiver works. A mixer adds (or subtracts) a variable frequency with the frequency of interest. This shifts the received frequency to a fixed frequency where the radio can easily amplify and filter the signal. [Mehdi’s] radio, an Icom R72, uses a fixed frequency of 70.45 MHz. This is known as the intermediate frequency or IF.
[Mehdi] used a PlaySDR, although even the inexpensive RTL-SDR dongle will work at that frequency. The trick is finding a place to tap the IF from the receiver without hampering the radio’s operation. He found a likely place to tap the IF and used a small resistor to ensure the SDR input didn’t load the radio’s IF stages. Then it is a simple matter of setting some SDR software to the IF frequency, and you have a pan adapter. You could even use GNU Radio to do something custom if you wanted.
This isn’t an original hack. However, every radio needs a little different method for tapping the IF. The video below, for example, shows a similar hack with an RTL-SDR dongle and a Kenwood TS-570.