If youโre intending to transmit on the VHF band, youโre probably going to reach for a handheld or some kind of rackmount rig in your ham shack. But you neednโt bother with all that complexity, when you can use the computer on your desk to spit out such signals using a simple browser tool from [Efe].
The concept is straightforwardโ[Efe]โs tool manipulates pixel clocks in order to create spurious transmissions from your computerโs graphics hardware. The math pencils out pretty easilyโmultiply the horizontal resolution by the vertical resolution by the refresh rate, while paying attention to the precise timing of the video standard your monitor is using, and youโve got your transmission frequency. For example, for a screen displaying 1080p at 60 Hz, with the CEA-861 timing standard, your horizontal and vertical resolutions are 2200 and 1125 respectively when paying attention to the requisite blanking intervals. Multiply those by 60 hz, and youโll find youโre creating a signal at 148.500 MHz. Leverage this by displaying the right pattern of black and white pixels to maximise changes in voltage state on the HDMI or DisplayPort lines, and you might create a strong enough signal that you can actually pick something up. [Efe] created a tool to display these patterns to send simple Morse code messages over VHF just by flickering your screen just right.
You can test the transmitter tool for yourself here, right in your browser. Youโll want to hold your radioโs antenna nice and close to the monitor to see if you can pick up much of a signal. After all, the monitor, connectors, and cable are all built to optimize for clear signal transmission to the display, while preventing signal from leaking out to interfere with surrounding equipment.
Of course, a fair warningโyouโre not supposed to intentionally transmit on bands youโre not licensed for, even if itโs incredibly weak and unlikely for anyone else to notice in a scenario like this. Still, itโs an interesting project that shows you just how electromagnetic interference can leak out of just about anything under the right conditions.