After inspiration from this thread I decided to play with rtl_power and inspect the spectrum around here.

I have my RTL-SDR connected to a Radio Shack discone sitting on the roof by means of a LNA. IIRC it's rated for about 30dB but at 15V. I'm running it on a 9V battery so who knows what it's actually putting out. I had to play with the gain settings on rtl_power for a little while until I found a good one.



Using rtl_power, I created a CSV file with signal readings for each frequency. Here's the command I used:

rtl_power -f 144M:148M:1k -g 10 2m_stations.csv

that tells rtl_power to scan 144-148 MHz with resolution of 1 khz and write data to 2m_stations.csv. I started this a little over 3 hours ago and it's still going. As of now the CSV is 28MB. The software does support writing to compressed csv files, which is probably a good idea for larger surveys. If I'm reading the output correctly, it's doing half the band every 5 seconds, so each row of pixels represents 10 seconds to scan 144-148.

Lines in the CSV log look like this:

2015-07-07, 18:38:27, 144000000, 146000000, 976.56, 164, -21.40, -52.10, -52.31, -52.25, -52.91, -52.30, -52.84, -52.14, -52.38, -52.27, -51.65, -51.91, -52.19, -51.98, -52.37, -52.68, ...

That's the timestamp, the frequency min and max (the next line will have 146e6 to 148e6), the step size (just under 1 khz), the number of samples it took, and then the power reading in dbm for that sample.

Using the generically-named heatmap.py you can turn those CSV files into this: (right click -> view image to see the whole size. Despite their resolution they're < 200 kb)



I tweaked the db min and max on the heatmap and came up with this. I'm sure I've lost a few stations in there but it's a lot less noisy.



Check out 144.39 - you can see all the APRS traffic. Outside of a VHF contest, that's probably the weakest signal I can expect to hear on 2m with this antenna. There's a repeater output on 146.67 (clearly the most popular one today).

The command I used to generate the first jpg:

python heatmap.py 2m_stations.csv out.jpg

The second:

python heatmap.py --db -48 -20 2m_stations.csv out.jpg

I have an HF upconverter that mixes 55 MHz to HF signals so it puts them in the range of the RTL dongle. My next goal is to use that to do a 24 hour sweep of 20 meters.

I was looking at RTL today for a simple way to measure FM deviation.

I would have thought someone would have created some software to do that, but it takes a few programs to do it.


When doing a test at a 1KHz tone it is fairly straight forward using spectrum analyzer. It is when you deviate LOL from 1KHZ does a little math become involved.

I haven't seen anyone publish a flowgraph to do that, but it should be pretty simple to setup in grc.

https://www.youtube.com/watch?v=lWcFqsh7ZvQ

Got the upconverter connected but something seems amiss now. It's supposed to add 55 MHz to the received signal but I can't find any of the WWV stations. Oh well, time for bed. Going to go back to receiving 2 meters.

This is great stuff Phurba!

I have a cheap dongle and this would be very interesting to run sampling to see what is active in the area for me.

The obvious repeaters in the area are pretty dead.  So I am wondering if there is simplex traffic around.

Here's my overnight collection.

Still waiting for amazon.....



ETA: What's the echo/mirror around the APRS freq? That's weird. I don't see it near other signals.

When I got up this morning I changed the setup over to 900-928. There's a 900 mhz repeater in the area but I never hear anyone on it. I saw a little ISM traffic but nothing else.

You should come put that up on our 900 MHz site in Lubbock. You'll see nothing but ISM.

It's a shame, we should really use 900 MHz more. I have most of the components for a 900 MHz repeater running at 150 watts but I don't have a good site for it. I've been collecting MTX9000s and EFJ5100s to give away to people to encourage more use of the band.

If I manage to get permission to build on the top of my mountain property...I will have a great site for some repeaters.

Let me know if that works out. I'm working on an open source raspi based repeater controller.

That project is at least a year out. But I will try to remember your interest if I get to fo.

I might finish it by then

It...... has...... arrived..........

In the first couple of screenshots? I think it's just noise from the SDR. I was able to get rid of it by playing with the gain settings.

ROTFLMAO

Well.... that took some fighting to get it to work on the RPi.  Right now I'm trying to sample the 2m band through a G5RV fed with 100+ ft of RG-8. I'm guessing I'm going to be less than successful. Time to set up a temp antenna of some type.

I'm sure you'll get some signals but with an antenna that far out of band and without an LNA it probably won't be awesome.

Already changed. Starting rtl_power over again.


ETA: It's interesting, that's for sure.