Thanks for the steps! I will post results.

Hi John,

Heatmap steps the frequency of the dongle and places spectra with a bandwidth of about 2 MHz side by side. Depending on hardware and how you configure it you might have a center spur that repeats.

You cover 118 to 137 MHz which is nearly a factor of 10 above the bandwidth of the dongle so you loose 90% of the data on each frequency with an associated higer noise floor (5 dB perhaps) that makes weak signals more difficult to find.

It seems you have 10 sub-bands (based on the regions of low loise level.)

I have not followed this thread so I do not know what tuner you are using. The image does not give any hint.

All the dongles have many things that can be controlled by software. As a first step I suggest you use a "normal" SDR software that uses a single center frequency. SDRsharp, Linrad, HDSDR or whatever. Look at the spectrum and play with available settings.

Regards

Leif

Update:

Termination: My dongle has 75 ohm antenna connection so I used 75 ohm terminator. The lines remained. The lines are much diminished or it could be due to fact "background" signal is very prevalent. I have attached this one image for reference. I was expecting to see no signal at all (or no single where the lines are not).

Bandpass filter: ave not done this. I have a broadcast FM notch filter I could try.

RF Chokes: Tried these on 8" usb cables feeding this RTL dongle, another RTL dongle and an 802.11 dongle. No apparent change in lines.

Enclosure: The RTL dongles were already in aluminum enclosures. They are right next to an Odroid single board commuter that has no case. All of this is in a tin box.

Linrad: Have not tried this.

All my debugging was air band. When I switched to 450-470Mhz the problem goes away. Since this is my range of interest I will not be working on debugging this anymore.

I can provide other images if someone is interested themselves.

Thanks again,

John

On 07/15/2015 08:03 AM, Nikolay Dimitrov wrote:

...

Hi John,

Put a 50- or 75-ohm termination on the rtl-sdr antenna connector and redo the plot, to see whether the beat-frequencies are generated inside or outside your dongle.

Next you can put a bandpass filter in front of your rtl-sdr dongle, in order to reduce the out-of-band signals that probably overload your front-end. In practice, we shouldn't be using any RF device without input and output bandpass filters.

Next, you can also try putting an rf choke/ferrite (a common-mode transformer) on the USB cable, in order to reduce the noise coming from the USB-host and through the cable.

Next, putting the dongle inside a metallic enclosure will help screening the RF circuits, and will allow it to receive signals only through the input connector (and preferably through an input bandpass filter). You can create an effective "poor man's enclosure" by cutting and soldering pieces of double-sided PCB.

Finally, you can test your dongle with Linrad with its patched version of librtlsdr. Linrad uses a different gain distribution and there's a big chance that it can satisfy your needs. You can do similar experiments by reducing the RF gain and AGC on rtl_power and see whether it influences positively your measurements. Please try these and share your experience.

Regards, Nikolay

On 07/15/2015 02:52 PM, John wrote:

...

Hello,

When I use heatmap.py with output from rtl_power I get regularly spaced vertical lines that do not appear to be related to any signal. They look they like repeat at the dongle bandwidth (2048000Hz in this case). The crop option for rtl_power reduces the presence but I am not sure f that is intended by that option. Even at -c of 70% they are still there (see attachment).

Is this because of small bin width? If I use a larger bin (32k) they are still there. In this case there is no frequency legend along top so can't compare if they happen more often.

Are these lines expected? Can they be removed?

John