Turning off PI HDMI for mast mounted installations



Although i have not done much noise/emissions testing on this topic, have we considered recommending turning off HDMI to reduce emissions from the Pi, when people are considering mounting the Pi, LNA and SDR on the mast?
i.e. 891,000 MHz and others

Although by disabling HDMI it may make it harder to diagnose if issues occur, it may also reduce power consumption for stable remote nodes. I keep a spare SD Card available with HDMI activated for any occasions where SSH doesn’t work.

Issue this command from a SSH session.

     /opt/vc/bin/tvservice -o

Results in.
Powering off HDMI

This may need to be made permanent, as I expect HDMI reactivates after reboot.

To power back on.

     /opt/vc/bin/tvservice --preferred

Results on.
Powering on HDMI with preferred settings

In some circumstances you may need to also issue the following for the screen to display properly.

   fbset -depth 8; fbset -depth 16




Interesting concept.
Would this make any difference when a HDMI cable is not plugged in?
Is there any noise just from the Pi’s HDMI circuitry?



I haven’t had much or a chance to play. If Ethernet connected, turning off Bluetooth and WiFi would also reduce emissions.

I’ve turned HDMI off on both of my nodes (299 and 334), which are both mast mounted and use 2.4G to connect to my home WiFi.
I’m just scheduling some observations that I have already seen, to see if the noise floor drops a little.

I was also going to put it into test mode and see what affect it has on and off against a known beacon and also tune it to the known HDMI frequencies to see if ti makes much difference.



The difference in construction and shielding of some cables seems to vary greatly and therefore leak EMI more in some.
If the video chip doesn’t have sync with a cable or monitor, as you have inferred, the chip may not generate a signal and therefore maybe doesn’t generate the EMI.

As it seems there are differences in EMI from the cable and station, we could come up with a series of tests, based on particular hardware configurations and deployment methods for people to assess the quality of the station - obviously providing tweaks that could help.

Once active tune to known PI, Pager, FM/TV broadcast and other known terrestrial signals.

  • ramifications include, not being able to detect some satellites if the noise floor is too high or the SDR front end is being saturated.
  • remedy, consider a notch filter, different antenna design, etc.

We could provide what is expected, based on the existing community stations, what will be seen, what cant be seen (eg. good ISS or SSTV from a turnstile), the mandatory need for a LNA if you are interested in observing a wide range of sat’s, etc.

Some of this practical information might help set some expectations and amount of effort/cost required.
Not everyone has the facility or budget to get a rotator. What are the other options what could be considered to increase the attractiveness of the community.

Just some thoughts.



It is necessary to generate a heat map before and after. And compare the result