New SDR user - questions on birdies, harmonics and combining

[cypherstream]

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]

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

Birdies

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]

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.