New SDR user - questions on birdies, harmonics and combining

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cypherstream

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Hello,

New user here, new to the forum. Not entirely new to scanning but I am amazed at what I can do with the RTL-SDR and SDR Sharp programs for such a low entry price. I just have a few questions on 3 simple topics, the first two which could likely apply to any radio scanner.

Birdies
Do you encounter a lot of signal birdies? For example off the top of my head at exactly 144.0000 MHz, there appears to be a very strong birdie. Huge signal peak but the audio is nothing. It opens squelch to silent audio. There's a few of these around even in the 1000-1300 MHz range.

Harmonics
Also do you ever pick up signals shifted to alternate frequencies because of harmonics? I have one at 859 MHz and it appears I'm also picking it up at 932 MHz. We also have a local radio station at 102.5 MHz, but it also comes in loud and clear (and in stereo) around 28.5 - 29 Mhz area. Is this a studio to transmitter link or is this just a harmonic image since the tower is on a mountain that's only a 10 minute drive from me?


Combining
A third question... if one of these only has a 2.4 MHz bandwidth, could you combine two to get 4.8 MHz, or combine 3 to get up to 7.2 MHz bandwidth? Just thinking on trying to cover wideband data transmissions to pass through GNU Radio. (Like 3.2 MHz QPSK and QAM64/256).
 

SCPD

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Not entirely new to scanning but I am amazed at what I can do with the RTL-SDR and SDR Sharp programs for such a low entry price.
Welcome and glad you tried out the RTL-SDR! Such an amazing "toy", eh?

I'm no expert on the subject, but I've been using those dongles a lot the past few months. I've uploaded a few samples of my experiments to YOUTUBE if interested: Frank K2NCC - YouTube

Lots, but I also never had 3+ megs of bandwidth to be able to notice them as well either. I also presume that a sub-$20 radio isn't going to give me the best quality. A trade-off worth having for all I do that a $2000 Flexradio couldn't.

Harmonics
Since I live so close to FM broadcast stations, I had to pick up an "FM trap" via Radio Shack to null out those monsters. I set my gain to ZERO and still get them loud and clear. But I can see 5 towers from my front porch. I'm now more concerned about RF damage to my brain than to my radio! ;)

You'll get stations bleeding over into other places across the band. Something I've learned to live with, but again, it's $20, so I don't expect a lot.

Combining
I don't think that's possible. But you can at least use SDRConsole to monitor up to six slices (VFOs) in the same range. Which allows you to monitor, say, six repeaters at a time, or 3 or 4 HFDL transmissions if you're using a Ham-It-Up converter.

You could get more than one dongle, but that still won't give you a multiple in one.

I feel happy to get a 3meg waterfall when most ham rigs can't even see a 3k one! 8)

I tend to stay at 2.8 or below. I've not tried modes wider than 3megs, so someone better adept at that would have to reply. I figure even if you could copy QAM64/256, could you decode it and have the necessary signal quality? I'm not even sure it's within the range of an RTL-SDR anyway is it?

Great questions cypherstream, welcome to the group!

frank in Oregon
 

cypherstream

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Joined
Jun 15, 2014
Messages
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Location
PA
Thanks for the reply! Yeah I can't believe the little investment just to get started with this device. It does provide hours of fun. There's also a program that scans a frequency range and graphs a plot. I did this with cable tv signal, directv signal and the gsm frequencies, just out of curiosity to see the signals signatures. Really neat stuff.

I started with SDR Sharp, and I really like it. I also tried HDSDR but right now I find SDR Sharp easier. There' s a whole world of frequencies below 24 MHz out there too. How low can you go if you get a Ham-It-Up converter? Can you go down to 30 kHz? Can you go down to VLF 3-30 kHz? Just curious what the limitations are.
 
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