Noise level - ads-b muta setup


#1

Hi,

Using ads-b mutabilty set up. A question about noise levels. Which is a better noise level figure -4.5 or - -33.5. Obviously when in the positive side the lower number would be better but is the higher negative better???

Jeff


#2

Yes, negative is better. That means e.g. noise level of -30 is better than -20. The lower the noise flour, the better can a signal stick it’s head out of the noise.


#3

The RTL dongle are not calibrate to an absolute signal or noise level. So a value of -30dB is relative to the gain of the RTL dongle. If you change the gain the “signal level” will also change.

The relative values are correct. If you see a plane signal at -20dB and one a -26dB there is a 6dB relative difference in signal strength between the two planes.

Same thing for noise level. If the noise level is at -36dB and the signal level is at -26dB there is a 10dB Signal to Noise ratio.

The absolute value of the noise level is found by generating a known source signal (using a signal generator) to calibrate the scale. Then you can convert from relative to absolute levels.

In practical usage you don’t care too much about the absolute noise level. Relative noise measurements are good enough to know if you are getting interference. Look up on how to do a frequency scan.


#4

Thanks David for your reply. So taking your example of

Using the same signal level figure, that being -26db but a noise level of -20db, would that mean your SNR of only 6db, which would be a better result than your referenced 10db SNR, the lower the SNR the better???

Jeff


#5

If that is the case, the closer the noise level and the signal level are, the better???

Jeff


#6

Sorry to jump in.

The more negative the noise, and the more positive the signal the better, high SNR.

-30 is more negative than -20.

-20 is more positive than -30.

10dB SNR is better than 6dB SNR.

A high SNR does not always mean good reception. If your noise is -120dB and your signal is -100, the SNR is 20dB, but you still need a sensitive receiver to resolve this very weak signal.

The dongle/stick is not the most sensitive receiver, but the signals we are dealing with are strong enough that it resolves them most of the time. In some cases an LNA may be needed.

There is also the case of a receiver that is more sensitive than necessary. In this case attenuation may be needed.


#7

That’s 6th grade arithmetic (in US, in other countries is 4th).
If one doesn’t understand negative numbers, it’s hard to have a meaningful conversation.


#8

Now now… I will make this a bit more simpler, a graphical display. Location rural Australia, reasonably larger town, only RF issue and not sure it is, is a mobile phone tower 4G about 200 meters away. 2 images, some gain settings, 1st image using a Proplus dongle

mean signal level -8.2, average noise level -25.9.

Image 2 using Proplus +

mean signal level -1.7, average noise level -5.8.
Which dongle gives the better detection result. The dongle change occurred during a time when aircraft levels were similar. You will see, there is a major difference in the gap between the signal level and noise level, between the 2 dongles.

Jeff


#9

You should redo the gain settings with the second dongle. It looks way too high.
Look at the posts for gain optimisation.


#10

.
No it is the other way round.
.

Your ProStick (orange) graph is much better than ProStick+ (blue) graph. It seems you used an external Filter with ProStick (orange), and no external filter with ProStick+ (blue).

If my assumtion is correct, then insert the external filter between antenna and ProStick+ (blue), and observe. Most likely the gap between noise level and signal level will increase.

.

Check intensity of Cell Phone signal by RF Scan described in this thread:

Do I Need A Filter.

Here is RF Scan at my location (Urban) using Option-2 (generic dvb-t dongle plugged into Windows computer, No Filter)


#11

No filter used on the Proplus orange.

Jeff


#12

.

  1. If you have a filter available, try it with ProStick+ (blue).

  2. Is there any difference between Message Rate graphs of two dongles?


#13


#14

@jlb56
Your message rate dropped to very low value (right end of the message rate graph). Did it happen when you replaced Orange dongle by Blue dongle?

See below how my message rate behaved when I tried different configuration in 2016

ProStick%20Plus%20-%20With%20and%20Without%20External%20Filter


#15

@jlb56

As the Orange dongle is giving good results WITHOUT FILTER, the Blue dongle should also give good results without filter.

When Orange dongle is replaced by Blue dongle, and all other things remain same, the Blue dongle should give EQUAL OR BETTER results than the Orange one.

The drastic drop in message rate in your case shows that reverse has happened. The possible reasons I can think of are:

  1. Blue dongle is not properly plugged into Pi. Unplug and replug firmly.
  2. Antenna cable is not properly connected to dongle. Tighten the nut.
  3. The Blue dongle is taking more current than Orange one, and you need a better power adaptor. If you are using a long dc cable between adaptor and Pi, try to use a shorter length.
  4. The Blue dongle is defective.

#16

Two things you should also consider.

  1. A signal to noise ratio (SNR) of about 1-2dB can be decoded. The problem with a signal so close to the noise is that a microsecond noise spike can mess up a bit in the message and the message will be thrown away as un-decodeable.
    https://en.wikipedia.org/wiki/Signal-to-noise_ratio

  2. Adding parts to a RF chain will change the signal and noise levels. I have seen a few cases where the blue prostick preforms a bit worse than the orange. This is because the prostick plus filter adds about 1dB of noise but rejects 30dB of out of band noise. The problem is that if the noise is already very low you are adding 1dB of in band noise and not decreasing the over all noise as much. The SNR can be worse with a filter than without.

You can sometimes see the same thing with adding an amplifier. It will amplify the signal and also amplify the noise. You want the signal to be increased more than the noise to get a better SNR.

The SNR is the important measure of a radio system!!!


#17

Now same vane but different approach. I do have maybe 20 - 25 metre coax run but a very low loss, expensive cable. It is written in various sites, to try and get the dongle as close to the antennae as possible. What is the best way to overcome such a coax length and getting the dongle close to the antennae. I am running a satellite mast head amp amplifier https://www.matchmaster.com.au/amplifiers-and-modulators/10mm-sa18/
which frequency range covers ADS-B. Now I could easily mount the dongles close to the antennae, but powering them the issue. I suppose I could mount the PI3 up there to in a weatherproof box and run network cable down to the modem, that cable length wont be an issue, not sure about powering it though as apparently the 5v is critical and not sure about voltage loss to the nearest power point. I have a similar length network cable running from a blitzortung blue lightning detector on the roof, to the modem.

Jeff


#18

The longer your coax cable the much more complex the other parts need to be. That is why people say get the receiver as close to the antenna as possible.

I don’t really want to go into “link budget” but you can get a rough idea of how a system will perform by adding up Noise Figure values, gain values, and filtering.

Here are the two system you are trying to compare.
Antenna —> coax cable --> amplifier --> receiver
vs
Antenna —> amplifier --> cox cable --> receiver

Your matchmaster amplifier has a 5dB noise figure (it adds this much noise) but it also amplify signals 20dB.
Your coax cable has an attenuation of 3dB.

So your are adding 3dB losses (coax losses) + 5dB (amp noise figure) and then the amplified signal
vs
5dB losses (amp noise) before you amplify the signal. Coax losses don’t matter much after amplification.

The prostick with built in amplifier has a Noise figure of 0.8dB and a gain of 18dB.
So in case 1 you will see 3dB (coax loss) + 0.8dB (amp noise) with the amplification of the signal
vs
0.8dB (amp noise) before you amplify the signal if you place it near the antenna

Two alternate solutions:

  1. Run power over ethernet (search on amazon). Place the weatherized box near the mast and run a ethernet cable to the box. No need run a dedicated power line since you are sending power over the ethernet.
    POE runs 24V on the ethernet cord and then down converts the power to 5V 8V or 10V. Make sure you get a POE system that will output 5V.

2)RTL-SDR blog just released a new mast head ampliifer (not built into the receiver) that will amplify the signal before the coax losses. It is also very low noise with extra filtering and powered through the coax cable.


RTL-SDR developed a dongle that will inject 4.8V on the coax line to power mast head amplifiers with a “bias-tee”.


#19

Thanks David, food for thought. ABCD I have run that RF scan, simple dongle into an indoor antenna.

As you will see, some 900mhz but large 950mhz. Subsequently thinking about the linked 1090 pre amp, with ceramic filter from Nervis link on the FA build an ADS-B receiver. Now I am thinking the current masthead amp would not only be amplifying the 1090 signal, also be amplifying the950 mhz mobile phone tower signal as well, so a specific 1090 mhz amp and associated filter may be the go??

Jeff


#20

My personal experience: When replacing the dongles in the same location, the gain for the Blue one needs to be lowered by some 20dB.
Also, if gain of Blue is left on “AGC” it would not work at all in my location.