Picking Up TV Signals On A Ham Band?!

[Gary P]

Was removing some unused channels from my master CHIRP file yesterday. Thought I'd share something interesting I learned about 445.5 Mhz last year...

PICKING UP TV SIGNALS ON A HAM BAND?!

By Gary W9ST

As you have probably learned at foxhunt trainings, we can use the third harmonic of the VHF frequency to hunt an overly strong signal and pick it up in UHF band at a reduced strength. (A fox at 145.0 Mhz X 3 = 435.0 Mhz). This is typically at least -40 dB down from primary signal for an FCC-compliant transmitter.

Well, I once programmed a few UHF simplex freqs into my mobile rig. (I picked 446.0, 446.1, 445.5, 446.5, etc). Oddly, squelch would break every so often while driving and I'd hear a quiet patterned noise on 445.5. Sounded like a scratchy open mic, with periodic whistles, but definitely a carrier, not background static. Turns out, I was picking up HDMI signals from buildings I was driving by!

THE THIRD HARMONIC OF THE HDMI PIXEL CLOCK

What?! Well, the third harmonic of a standard 148.5 MHz HDMI pixel clock (commonly used for 1080p @ 60Hz video displays) is... 445.5 Mhz. Right in the middle of the simplex 70 cm ham band. Poorly or improperly shielded HDMI cables can leak both frequencies (and other harmonics). Tune your HT to these and walk around your house with squelch off... there's probably something you can pick up!

While 445.5 is an easy to remember freq in the simplex range of the UHF band, it's best to avoid using it, unless you're out in a natural area away from electronics.

SOLUTIONS TO INTERFERENCE ON 445.5

If you want to get rid of the interference without buying a new better-shielded cable, simply increase screen refresh rate (75 Hz, 120 hz, etc.) or change the screen resolution to something other than 1080p. That moves the pixel clock harmonic away from the ham bands. If you must stay on 1080p at 60 hz, looping the HDMI cable 4 times though a 60 mm ferrite ring close to the HDMI output port can eliminate the interference. Snap-on ferrite chokes can also fix it, but you may need 4 or more of them.

HACKING HDMI DISPLAYS WITH RF YAGIS?!

Insanely, it's actually possible to decode these RF signals into ghostly B&W representations of the original video signal. Most demos take place within inches but ressearchers used Yagis to decode signals from at least 20 meters away! So, not only are shoddy video cables mucking up the RF environment, they can even be a security risk.

LINKS:

Oscilloscope before/after torroid install

https://qrm.guru/hdmi-interference/
(Note: because this Aussie ham uses 50 hz not 60 hz, their frequency is slightly different than USA.)

Waterfall screenshot
https://www.sigidwiki.com/wiki/HDMI_RFI_Clock

How to set it up using DragonOS (an RF/SDR-focused Linux distro):
https://radiohackers.com/eavesdropping-on-hdmi-using-rtl-de-voussdr-and-tempestsdr-b173f8cbf0f5

Reconstruction of images captured by TempestSDR and Yagi up to 20 meter away
https://ceur-ws.org/Vol-4092/paper30.pdf

Video of TempestSDR capture with antenna laying on HDMI cable:
https://www.reddit.com/r/RTLSDR/comments/qa4mye/yet_another_tempestsdr_demo_with_a_hackrf_much/