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Leif Asbrink leif at sm5bsz.comHi 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 >