I’ve been tuning the gain on my system for a while, initially using gut feeling and seeing the impact on daily positions and aircraft reported.
More recently I’ve been using the excellent graphs1090 and during a spell of relatively flat propagation conditions I’ve more or less settled on my optimum gain setting.
In my case, at my particular location, --gain 20 gives the best results. I’m sure every other site will get a different optimum value.
However what I didn’t expect, is that a 1 db change on my gain manifests in such a noticeable CPU utilization pattern difference.
Before ~ 11.21 I was running with --gain 21 (notice the finegrain cpu utilisation up/down)
After ~ 11.21 my “optimal” --gain 20 was set (notice the much smoother CPU utilization!)…
Now I have no idea if this is just a graphing artifact or if the slightly higher messages > 3dBFS causes more processing for the CPU, but I do find this interesting nonetheless!
The graph for the message rate also appears smoother after the gain reduction and I certainly don’t suffer lower number of positions reported…
Yes. I have had the same effects with other metrics too. I suspect it’s an ‘interference pattern’ caused by sampling vs rendering, and gain adjustment is just one way to push the alignment of those into different ‘phases’.
I did qualify that with “Higher and lower gain results in more and less CPU utilisation in the graphs.”
Try as I might, I was unable to reproduce the effect today
It happens now and again when I change the gain around my prefered optimal setting.
While the CPU utilization does go up and down with the gain changes, I cannot reproduce the sawtooth pattern seen previously (much to my frustration) - so I’ll put this down to nothing to worry about.
Thanks guys for your input. At least I’m not the only person to have witnessed it.
For the graphs he gets with a gain of 20, he has an additional external LNA.
Examples:
rtl-sdr LNA + rtl-sdr v3 dongle: optimal gain usually between 20 and 35
uputronics LNA + Fa prostick+: between 15 and 30 (i think)
Have you looked at his >-3dB percentage? It’s clear he’s at least in the correct ballpark.
That’s why i’m curious why you would suggest a gain that’s somewhere else.
I get the impression you assume everyone is using the Fa Prostick+.
@wiedehopf you are correct to think I have uputronics LNA (actually its the SAW version)!
The other items for the record are:
ADS-B Antenna 1.2m, 8.5dBi (Pimoroni)
~5.5 m Aircom Premium (The DX Shop)
FA Pro Stick Plus (Moonraker [uk] Ltd)
RPi 4B with passive cooling in loft https://smile.amazon.co.uk/dp/B07XL17RQ4
Antenna is mounted on chimney stack on a 3m pole as per my user picture at https://uk.flightaware.com/adsb/stats/user/hphillip
@hdtvspace The choice of gain 20 suits me and it’s right that any more gain will cause an excessive amount of >3dBFS messages. I seem to do well compared with nearby sites
Although I’m listed as nearest EGDT (Wroughton) I see plenty of light aircraft activity from Cotswold Airport EGBP especially on fine days, which is only 10 NM away. Gain above 20 means I loose considerable local traffic
My average max range is about 210 NM with a peak of 248 NM in the last 24 hours.
The predicted 1090Mhz horizon for my location and elevation using HeyWhatsThat is remarkably consistent with the results I see in practice. To the South and South-East towards France I am limited by geography to a range under 200 NM.
I get better results to the North and through the M4 corridor East towards The Netherlands.
The site seems to be working well, I get plenty of ADS-B positions and my rank is creeping up - I’m hoping to get in Top 50 UK before the end of the month, weather permitting
