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Old 02-21-2015, 02:17 PM
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dewdude dewdude is offline
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So I've had an SDR for a couple months...

Hey everyone,

I know this is an amateur radio forum; although it does talk about DXing and I've seen a few things related to that....and I can't figure out if this fits in a specific category; plus I wouldn't mind hearing what you guys have got to say.

Going back to the beginning: A few months ago I started fooling around with a couple of AM tuners as I enjoy AM (and SW) DXing; threw up a couple of wires for antennas and played around. I then got more serious by stringing up 212 ft (61m) of wire for the job. I then started playing around with an old Panasonic boombox with cheap SW section on it and really started to miss the RadioShack/Sangen DX398/ATS909 I had years ago but sold when it was apparent BPL was making that useless. That's gone, so I started looking again. So I wrestled with the idea of getting one of the Tecsun units, as a new DSP based 909 was out of my budget. That's when I got to looking at RTL-SDR.

For those who don't know what RTL-SDR; basically some clever hackers/programmers/developers figured out you could make a cheap dongle designed for DVB-T reception act as a general purpose SDR. You're quite limited by the out-of-design use of the Realtek DVB-T chip; but you are able to get up to a usable 2.8mhz of bandwidth; although the ADC only samples the I/Q data from the tuner at 8-bit. It's still better than a lot a of cheap radios. But I got to looking at what you could do with the things and all the various software...and it looked like the most high-tech complicated radio ever! The waterfall display, the multiple decoding modes; there just seemed to be so much more flexibility in how I can tune the signals than I'd get with a hardware radio.

I wound up buying a RTL-SDR bundle since I wanted to go HF; the dongle by itself won't tune below 24mhz (sometimes down to 3 if you're lucky and use new modified drivers for some software), so I added an upconverter to heterodyne 100khz - about 40mhz up to 125mhz; also got the 1:9 balun the comany sold specific for longwire setups. I got all this a week before christmas; so I've actually had a chance to pay with it. My entire out-of-pocket cost was just under $100; a bit cheaper than the better TecSuns and far far more capable (the tuner in the dongle will go as high as 1700mhz).

Guys, this has pretty much really changed how I look at radio. For starters, the ability to tune 100khz - 1.7ghz (even if my sub AM BCB performance is crap) is just...mind blowing. I thought my 909 years ago with it's ability to tune from 150khz to 29.999mhz continuously was cool; this just blows it away. Being able to "see" a nice visual of the spectrum and signal is just outstanding too; I can for example what sideband an AM station has noise on it. That brings me to the IF filter; you can make that puppy almost as wide as you want; plus you have a fine adjustment so you can pick how much of each sideband you want. Of course, my favorite feature has to be "I/Q Baseband recording", where you record the entire I/Q stream that's being piped in to the software so you can work with it later. No more hoping I've got the thing tuned right when I record an audio sample; just record the I/Q stream and spend all the time you want tweaking tuning for a good intelligible sound clip.

Anyway, this thing has just been entirely cool for not costing more than it did. I can scan VHF, UHF, microwave, pipe the audio in to other programs for decoding digital modes, decode digital voice systems..I actually made a small quarter-wave ground-plane antenna for the 1090mhz ADS-B transponders and was pickin up planes 30k, 40k ft up sometimes 60 miles out (my sky view is horrendous and expected nothing out of ADS-B).

Of course, I just love the fact I can open my IF up to 20khz and get some pretty decent fidelity out of the one or two AM stations that broadcast music; yes it'll do that nifty FM Stereo and RDS stuff (I've actually been able to decode SCA with it) and maybe DAB if you're lucky enough to have that (no, no HDRadio).

Anyone else out there played with it any?


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Old 02-21-2015, 03:21 PM
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NoPegs NoPegs is offline
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An RTL-SDR kit has been on my disposable income fodder list for a few months now... As I understand it, practically the only thing it can't do on shortwave is make any more sense of a "numbers station" than the rest of us can.


Your experience has pushed it up a few notches on said list. Thanks for posting over here about it.
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Old 02-21-2015, 04:19 PM
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Electronic M Electronic M is offline
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I heard of this from my antennas elective professor....He actually demonstrated it using Matlab to operate the receiver. I happen to passionately hate Matlab, and unless I clone my hard drive before I (hopefully)graduate next quarter I won't have that software anymore (we get to keep the laptop, but not the software or EVEN the OS).

Is there other windows compatible software out there to work these units?
If there is what do you use?
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Old 02-21-2015, 08:21 PM
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dtvmcdonald dtvmcdonald is offline
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I use SDR#, which is pronounced SDR sharp.

You can also use Gnuradio. There is a downloadable
.iso file that you put onto a small USB key and
it boots into Linux. Actually there are several of these.
I use the Ubuntu one. You can browse the web while the radio runs.

Google SDR radio plus the other terms.
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Old 02-23-2015, 12:33 PM
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dewdude dewdude is offline
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Quote:
Originally Posted by NoPegs View Post
As I understand it, practically the only thing it can't do on shortwave is make any more sense of a "numbers station" than the rest of us can.
That's about the truth. I mean, the unit is rather limited in performance due to the lack of serious filtering for the MW/HF bands. It's FM performance is outstanding; I picked up a 100kw FM station out of Raleigh, NC when I was in Lynchburg VA over christmas. 122 miles seems like a pretty good copy for an FM station using just twin-lead folded dipole on a 2nd floor deck. I still question that reception; I couldn't find any repeaters but that station was coming in abnormally clear for quite a few hours. I'm wondering if the elevation I was at in comparsion to the transmitter had something to do with it; a quick check on google earth didn't reveal any thing between me and Raleigh; and the antenna down there had a 1900' HAAT. Still, radio-locator map didn't show the signal anywhere near me. I've done 60 mile FM reception from stations with a 1m whip with great success.

The 8-bit ADC of the RTL chip also limits your noise floor; there by limiting your reception. There are probably some weak stations down in the "noise" a more proper radio with better dynamic range will pick up.

But, despite the 8-bit limitation; having a device that'll go from 100khz to 1.7ghz and "decode" anything you've got software for is amazing. Wanna pick up ADS-B transponders? no problem. Need to decode some weefax? Go for it!

Quote:
Originally Posted by Electronic M View Post
I happen to passionately hate Matlab, and unless I clone my hard drive before I (hopefully)graduate next quarter I won't have that software anymore (we get to keep the laptop, but not the software or EVEN the OS).

Is there other windows compatible software out there to work these units?
If there is what do you use?
There's loads of windows software for these things. dtvmcdonald mentioned SDR#, my current program of choice. No idea why the screenshots didn't show up; they did when I posted. They all have a learning curve; but no where near as steep as MatLab or GNURadio...you don't need to know any programming...just pick your mode and frequency. But MatLab has been popular with this hardware because a $10 piece of hardware now allows you to feed real-world RF in to MatLab for designing receivers.

But there's also software like HDSDR, SDR-Radio, and a few other special-purpose programs that will interface with this. I can't really program to save my life; but I do just fine with SDR#...which is adding features that are rapidly making it one of the better choices for SDR use. The only time I boot something besides SDR# is when I want to listen to an SCA on FM.

I pulled a couple of pirates out of the air on HF the other night; and during the snow on Saturday pulled a 500watt am station from over 100 miles away.

But, like NoPegs said, about the only thing they can't do is help you understand numbers stations or encrypted comms...but you can at least look at them.
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File Type: jpg sdr2.jpg (142.3 KB, 14 views)
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Old 02-23-2015, 06:10 PM
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10$ a piece! That is less than half of the cheapest price I could find them for last year....I think it may be time to bump that up the buy it list a few notches.
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Old 02-23-2015, 06:33 PM
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NoPegs NoPegs is offline
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Quote:
Originally Posted by dewdude View Post
That's about the truth. I mean, the unit is rather limited in performance due to the lack of serious filtering for the MW/HF bands. It's FM performance is outstanding; I picked up a 100kw FM station out of Raleigh, NC when I was in Lynchburg VA over christmas. 122 miles seems like a pretty good copy for an FM station using just twin-lead folded dipole on a 2nd floor deck. I still question that reception; I couldn't find any repeaters but that station was coming in abnormally clear for quite a few hours. I'm wondering if the elevation I was at in comparsion to the transmitter had something to do with it; a quick check on google earth didn't reveal any thing between me and Raleigh; and the antenna down there had a 1900' HAAT. Still, radio-locator map didn't show the signal anywhere near me. I've done 60 mile FM reception from stations with a 1m whip with great success.

The 8-bit ADC of the RTL chip also limits your noise floor; there by limiting your reception. There are probably some weak stations down in the "noise" a more proper radio with better dynamic range will pick up.

But, despite the 8-bit limitation; having a device that'll go from 100khz to 1.7ghz and "decode" anything you've got software for is amazing. Wanna pick up ADS-B transponders? no problem. Need to decode some weefax? Go for it!
So what if I whack a rough-cut bandpass/high-pass/low-pass filter in the antenna chain? That is to say if I'm cruising the large bands, and I put a brick wall that starts at 12MHz will that assist? (Considering the antenna best matched for those bands is very large, it will pick up every damned signal it can.)

I didn't know there was a Live-CD distro for these. That sounds like it could be very handy. I have several machines awaiting a reason to be used, I can just build off of the Live-CD distro on a laughably small hard drive.

FWIW I believe your FM DX experience is completely within the bounds of possibility. The thing to look for is were there any other stations on that same frequency that logically should have been competing for your RF front-end?
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Old 02-24-2015, 12:57 PM
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dewdude dewdude is offline
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Quote:
Originally Posted by Electronic M View Post
10$ a piece! That is less than half of the cheapest price I could find them for last year....I think it may be time to bump that up the buy it list a few notches.
Depends where you buy them. I bought a bundle from NooElec; and I was right at $100 for the upgraded "R820T2" dongle and the Ham-It-Up board with the cable to connect the two, the enclosure for the Ham-It-Up, and a SMA-to-SMA cable. I sent them some great feedback about a customer service rep that sent me links to pages of reviews of these things; and it wasn't a sales pitch; it was genuine "just to help you make the best decision you're comfortable with, even if it's not us"; so they wound up giving me their 1:9 balun for free. This was a premium price; but they do sell good stuff.

there are currently two tuners in these units. the R820T and R820T2; they're both completely compatible with each other, but the T2 is noted for having a bit more sensitivity. The $10 chinese dongles are ok; but it's questionable iff they have any antenna protection.

But, yeah; for what it can do the price is outstanding. I remember just about 5 or 6 years ago looking at some of the SDR solutions and they were all thousands of bucks. When this came out it literally changed the game; and despite the limitations, it's become very popular for projects simply due to the price.

Quote:
Originally Posted by NoPegs View Post
So what if I whack a rough-cut bandpass/high-pass/low-pass filter in the antenna chain? That is to say if I'm cruising the large bands, and I put a brick wall that starts at 12MHz will that assist? (Considering the antenna best matched for those bands is very large, it will pick up every damned signal it can.)

I didn't know there was a Live-CD distro for these. That sounds like it could be very handy. I have several machines awaiting a reason to be used, I can just build off of the Live-CD distro on a laughably small hard drive.

FWIW I believe your FM DX experience is completely within the bounds of possibility. The thing to look for is were there any other stations on that same frequency that logically should have been competing for your RF front-end?
It's possible if you did that you'd get better performance. I know I hooked in to the IF stage of my Sansui 200 and did in fact notice some performance increase on weaker stations. It's often been said these things would do a lot better if you weren't piping such a wide signal in to it. There's a dude on IRC who's talked about this several times; but he's not around to get more dtails about.

There's a Live-CD for them? Doesn't surprise me; but people have said gqrx isn't that great of a program and a lot of people wish SDR# was built for Linux.

Here's what's interesting about the WRDU DXing. I looked up on Radio-Locator as to what was close-by on 100.7. There were no stations within 60 miles. There was a station down south of Roanoke, about 66 miles away putting out 820 watts. That's down in the mountains though; so I suspect that while they had a height advantage; mountains were blocking it. The only other closest station was 73 miles north in the mountains.

It was clearly WRDU 100.7; I was getting RDS data that confirmed this, there's also this: http://dewdude.ath.cx/WRDU.mp3 - and that was literally just a 300-ohm folded-dipole made for twin-lead (which was half-wavelength for 88mhz, so it's a japanese FM band antenna) that was simply attached to the railing (push pins through the holes in the insulators) and running back to my SDR through extra twin-lead and running through the 9:1 balun...I didn't have an F-to-SMA to use a 300-75 balun...not to mention the SDR stuff is 50ohm input. I had zero complaints about that performance.

The question that's burning me is whether there was something going on letting that station come in; or if it was just a LOS thing. The tower down in NC for that station is in a pretty flat area and has a HAAT of like, close to 2000'. From what I could tell on Google Earth; I was about 800ft elevation with a clear LOS down to Raleigh. So...maybe that station in that particular area isn't really a record but just a fluke of line-of-sight.

I was down at my grandparents last month with the setup; they live just outside of Fredericksburg VA. Their kitchen basically sits at the second floor on the back of the house; so I was sitting there with my 1m magnetic whip on the top of the chest freezer. When I was doing a reception log; I actually had to tell radio-locator to start showing me stations past "fringe" because that's what was coming in...I'd stop on a signal...figure out what content it is...look at the list..and nothing. I eventually got the point where I just have it show me FM stations within 200 miles.

I'm not questioning whether I picked up WRDU or something else; I was able to identify it both from a sound clip and RDS data; but it's just got me wondering if it was a crisp winter night that did it; or just the fluke of being on the side of the mountain facing the transmitter with little in between us but air.

As far as stations competing; I don't believe this thing responds to FM-capture like normal radios. I would get ghosts of strong FM stations; sometimes 4 or 5 on the same frequency. There was a time or two I heard two FM's competing...and it wasn't capturing either one. SDR seems to break some rules like that.
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Last edited by dewdude; 02-24-2015 at 01:00 PM.
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Old 02-24-2015, 05:36 PM
JBL GUY JBL GUY is offline
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I built my own HF converter and incorporated an active, tunable RF pre-selector. I included switched gain, attenuation and bypass. The bypass mode was for use as an HF spectrum analyzer.

I made sure the LO in the converter was very clean before driving the mixer stage.

The noise figure of the pre-selector was less than 1 dB.

I ended up selling the whole setup to a friend and have been contemplating ordering another dongle and building another pre-selector.
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Old 02-25-2015, 12:49 AM
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dewdude dewdude is offline
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I'd be interested in the design/schematic of your pre-selector.
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Old 02-25-2015, 07:18 PM
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dtvmcdonald dtvmcdonald is offline
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Quote:
Originally Posted by dewdude View Post

As far as stations competing; I don't believe this thing responds to FM-capture like normal radios. I would get ghosts of strong FM stations; sometimes 4 or 5 on the same frequency. There was a time or two I heard two FM's competing...and it wasn't capturing either one. SDR seems to break some rules like that.
It does do capture properly IF ... everything is linear before
something sufficiently narrowband.

The IF bandwidth on the chips is set insufficiently narrow in
much software! HackRF
on SDR# is particularly egregiously hideously stupidly bad.
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Old 03-03-2015, 10:34 AM
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I'm not sure about the HackRF; I do know on the RTL-SDR dongles there's been talk about there's an additional IF in the R820 tuner they haven't "hacked" yet. I want to say the HackRF is based on the same things.

There still doing development on the stuff though; even after two years they keep coming out with improvements in drivers.
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Old 04-30-2017, 11:43 PM
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I've been using RTL2832U based dongles for SDR work since late 2013 and have now graduated to an SDRplay RSP1 which uses a 12 bit ADC giving a better noise floor then the RTL based dongles and I use SDRuno, HDSDR and SDR console V3 as my receiver GUI and I have something that approaches the quality of professional receivers costing kilobucks for a fraction of the price.
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Old 05-01-2017, 11:57 AM
madlabs madlabs is offline
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I got a RSP2 about a month ago. Love it. I also have a Nooelec dongle with a ham it up upconverter That one I have connected to a 'scope probe and use it as a poor man's spectrum analyzer.
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Old 05-09-2017, 09:50 AM
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I enjoy checking the low bands and AM broadcast band from around the world on websdr.
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