FlightAware Discussions

1090 MHz interference from HDMI adapter?


I have been trying to diagnose weekday depressed message rates from the beginning, and only noticed a quick jump every time I shut down my work laptop in recent days. (My antenna is right by my own work area. But I had suspected everything from my kid’s school iMac in another room to my neighbor’s WiFi or computer before suspecting my own :innocent:, partly because I do not often shut it down, thanks to COVID work-from-home.)

After additional exclusions, my focused turned to external video display. Initially I thought it could be RF leakage from USB A slots on this no-name (“Model: CB-C34”) adapter, or the much bigger monitor circuits with plastic casing. But as I tried to “shield” the open USB A slots with aluminum foil, I found the depression deepened. When the foil merely touches the aluminium case, it has the same effect as when it wraps the case. This suggests that the adapter, with all the right certifications including one from FCC, has a totally float metal case! Either float or worse, connected to the wrong wire. If I connect the exposed metal from the HDMI A plug with the adapter’s case using a metal rod, an appreciable increase in message rate occurs. (So, float.)

I found a link to this Spektrum from abcd567 in Diagnosing interference discussion. Before I can figure out how to use it on macOS - or how to use it at all, any information on RF characteristics of HDMI (or USB C for that matter)? Why does it affect 1090 this much? (In addition to suspecting my neighbors, I had also suspected my own 2.4 GHz WiFi. Switching to 5 GHz made no difference.) Anyone else experiencing 1090 interference from video equipment? (My HDMI TV, for example, doesn’t seem to have any effect even when I put antenna right beside it.)

HDMI can be a source of EMI for sure. Here is a decent article I came across sometime back that may be of interest here:

USB3.0 is also ‘loud’ - to the point many wifi router manufacturers refrained from adding USB 3.0 all together on their routers for a time. Most have made a mends in the meantime with better shielding and redesigned PCBs, but it can all be an issue.

Monitors, TV’s, laptops, microwave ovens and pretty much anything else you plug in will usually have some sort of emissions. It’s probably why we’re all keeling over from cancer and dementia… What was I talking about again?

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I get interference from my bedroom TV. I have a piaware that sits in out bedroom just behind the TV. Even with a cavity filter, I can see a large dip in traffic whenever the TV is turned on.

Good article. It also implies that the leakage is broadband. If this is the case, a narrowband filter should help. Whereas it’s curious a standard body in hi def digital video doesn’t define how to ground cable shield, I was rather abhorred when contending the possibility that the outer metal case may not be grounded at all.

I wonder if there is significant difference in TV technology. Mine is an older, 720p plasma. I recently installed a 15 MHz bandpass filter on the antenna closer to TV. But before that, I don’t remember it being affected by TV.

Now that I start to understand viedehopf’s graphs1090, these effects can be quantified. In this plot, noise rose 10 dB after plug on, depressing usable dynamic range from 30 dB to 15 dB.


The “shield up” point was when I used paper clips to connect the metal case and exposed metal on the HDMI plug. The effect wasn’t as pronounced as I hand-press a metal rod.

In comparison, the second receiver 15m, two or three dry walls away shows no obvious change. (The two plots are from the same time period, using different local time.)

This is the one close to TV AND with the narrowband filter. So I can do a similar test by swapping the filter with the other antenna and switching TV on and off.

That’s a completely flawed assumption.

In fact a filter can only help against noise out of the band that you’re trying to receive.
Thus if a noise source is broad band and that includes what you want to receive the only fix is to suppress the source or move further away from it.

I put my dongles in metal cases to reduce the likelihood of noise getting into them directly. A filter will not help with this.

Luckily, your wonderful graphs1090 makes testing easy in this 4-step sequence:

  1. Turn on the leaky converter to reproduce effect of the interference.
  2. Insert 15MHz bandpass filter between the antenna and the FA blue stick to test noise level change.
  3. Remove the filter to reconfirm high noise level.
  4. Turn off converter to reconfirm normal noise level.

That’s exactly why a narrowband filter helps suppressing broadband noise. In that big rod vs short wire antennae thread, LawrenceHill points out that the blue stick’s built-in filter is behind the preamp. My high-gain rod is picking up a lot of broadband noise to overload the preamp. After applying the narrow-band filter, the station currently with the rod began to outperform the wire antenna station even though the rod is in a really bad position.

Broadband means that energy is spread out, therefore energy in 1090 MHz ± 7.5 MHz is small. If digital video leakages has a strong peak near 1090 MHz as I first feared, the filter will not be useful. Of course, this source is too close, so the filter’s protection is limited. I’ll ask for another converter and see if it has better shield.

But you mostly care about the energy in the band you’re receiving.
The other bands are only an issue if the amplitude is massive and it overloads components.

For broadband noise covering the band you want to receive, only that part is relevant.
Or rather if the broadband noise gets so strong it overloads the receiver … it will have destroyed your signal in band long before that.