Multiple false signals

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snifftheglove
Posts: 11
Joined: Mon Jan 01, 2018 10:32 pm

Multiple false signals

Post by snifftheglove » Thu Jan 25, 2018 4:24 pm

Hi,
I do not know the correct termanology but I shall give an explation of the problems.

I have just brought a SkyScanner Mobile Antenna to use with by RTL-SDR.

Now when pluggin it in to the V3 I get lots of peaks, I thought great lots of stations but it seems that alot of them are just the same station.

For example I have the local FM station tuned (104mhz) but there are several peaks each side of this frequency and it's all the same stations. I have turned down the gain quite low and there are now only 4 peaks showing in total but clicking on the peak it is clearly the same station.. The range of the frequency screen is from 102.50mhz to 105.50mhz
Is there a prnonoma that reflects or mirrors a signal into another signal some khz away

snifftheglove
Posts: 11
Joined: Mon Jan 01, 2018 10:32 pm

Re: Multiple false signals

Post by snifftheglove » Thu Jan 25, 2018 11:20 pm

Here is a screen shot of SDR#

The actual station (BBC Hereford and Worcester) which is being broadcast from the Malvern transmitter on 104mHz which is approx 1 mile from me.

So, with 104mHz tuned on the SDR# and is a nice strong signal of -10, now if I manually scan to the left I then pick up a further 4 signals (100.2mHz, 100.9mHz, 101.5mHz and 102.1mHz)slightly weaker than the 104mHz. Why am I seeing these on the V3/SDR# when they are not actually being broadcast on these frequencies and I can not pick these signals up on my pocket radio (that's why I now they are not being broadcast on these frequencies (100.2mHz, 100.9mHz, 101.5mHz and 102.1mHz)
See the screenshot.
Thanks
BBC.png
BBC.png (266.47 KiB) Viewed 3797 times

rtlsdrblog
Site Admin
Posts: 2699
Joined: Mon Nov 19, 2012 11:54 pm

Re: Multiple false signals

Post by rtlsdrblog » Sat Jan 27, 2018 11:36 pm

What you're seeing is overload of the dongle. Basically if signals are too strong the dongle can overload and cause images of the strong signals to appear all over the spectrum. This is simply the nature of how radios and SDRs work.

If you want to avoid it, you can turn down the gain, use filtering or attenuation, or spend money on buying a higher end SDR with a higher ADC resolution. ADCs with higher resolution are more resistant to overloading, but accordingly cost more.

snifftheglove
Posts: 11
Joined: Mon Jan 01, 2018 10:32 pm

Re: Multiple false signals

Post by snifftheglove » Fri Feb 02, 2018 6:04 pm

Thanks for the reply and explanation.

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