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Transmitting FM, AM, SSB, SSTV and FSQ with Just a Raspberry Pi (rtl-sdr.com)
109 points by irfan on Oct 31, 2015 | hide | past | web | favorite | 23 comments

Very cool, but please don't do this without a proper filter, you're spewing garbage all over the spectrum.

Right, but narrow passband filters are not that hard to make.

I see this as a way to get more experimenters to be hams. It also makes it much easier to demo explain / teach / get people excited about SDR and what you can do with them.

Build yourself a low pass filter before transmitting:


Awesome. Now, the critical question is: how to implement a proper filter and a HF afterburner? And can it do live streaming - i.e. use an USB/Pi-Shield soundcard with a Line-In or Mic input?

I could imagine this being a cheap platform for crisis regions where radio transmission channels are run by government (or not existing at all).

I hope the spurious emissions aren't as bad as they look in the waterfall, because they're basically stomping all over the band and all the neighbouring bands and they're closely spaced enough that I'm not sure it'd be realistic to filter them all out. Is this even legal to use?

This is directly addressed in the article:

"Important Disclaimer: While the output power is very small, you should still take great care as the carrier is a square wave, and there is no filtering on the antenna output. So any transmissions will cause harmonics all across the spectrum – possibly interfering with life critical devices. A filter must be used if you actually plan on transmitting with any sort of range further than your room."

Harmonics are trivial to filter - they're at multiples of the fundamental frequency, so 100MHz, 150MHz, and so on. You can just stick a lowpass filter on the output and it'll block all the harmonics because they're so far away from the fundamental. Unfortunately, the waterfall appears to show strong spurs every 30 Hz, and those would be ridiculously difficult to filter out. I think the normal solution is to design your transmitter so that it doesn't do that.

Given how regularly spaced they are they might be other channels (regular TV maybe?)

It looks like they're spaced at 30KHz, which seems to me to be a very tight grouping for any kind of transmission like that (and presumably if that band was that crowded to begin with, the author would have picked a different one).

In working with RF, the test gear you need to see what's going on costs far more than the radio gear itself. The prices go way up as you get into the GHz range.

There's a lot of filter adjustment, antenna tuning, and shielding placement as you look at a spectrum analyzer and see spikes that shouldn't be there. This is why there are little RF modules in cans for many applications. Someone already did that work for you. Some vendors: [1][2]

[1] http://www.rfdigital.com/ [2] https://www.linxtechnologies.com/en/products/modules

For experimental Arduino and Rasberry Pi based RF apps, serviceable RF test equipment isn't as cost prohibitive as it once was. EG, Amazon has a PC-based 24MHz - 1.8 GHz RF spectrum analyzer by Touchstone for USD $80. The functional 100 MHz digital Rigol 1102E scope is useful when you're in EE-mode and still runs for $400. If feeling creative, Hantek arbitrary waveform generators hover around $160, and can use Tektronix software available online gratis, complete with GUI. Basic SainSmart USB PC-scopes with logic analyzer modules hover around $150 (watch circuit power output or you can fry your PC). None of these items are top of the line but they should get the job done at the Arduino or Rasberry Pi level.

24MHz - 1.8 GHz... that's going to be a $10 RTL-SDR dongle with some custom software, which doesn't really have a low enough noise floor or intermodulation to test your equipment for compliance. Though proper RF test equipment does seem to have come down in price a lot faster than ham gear.

True but the projects here are experimental apps limited to 100 mw (theoretically) under FCC Part 15. If you move up to a limited (the old tech minus 5 wpm) Amateur license and start broadcasting at say 50W on 70cm or 23cm then you'll need more sophisticated analytical equipment and software (Clear Waves has a functional kit for $400). As an aside I could never figure out why 13cm became so mainstream in routers, et al, since 2.45GHz is the center frequency of a microwave oven. Looked like a plot to fry the masses /s

Wow, bit banging radio? Neat idea. But there's gotta be USB SDR kits on the cheap, too, right?

HackRF: https://greatscottgadgets.com/hackrf/ ~> $300

HackRF Blue: http://hackrfblue.com/ ~> $200

rad1o (HackRF from spare parts): https://events.ccc.de/2015/07/10/rad1o/ , http://hackaday.com/2015/07/12/cccamp-2015-rad1o-badge/ ~> $100?


That would only give us half duplex. If we want to simultaneously transmit and receive, I guess we could add an RTL-SDR. ~> $10

Plus antenna(s). $ $ :)


The DAB iniatives doesn't seem to work out (eg Canada) though the lobbyists are still strong. Analog radio works even with bad signal quality (https://en.wikipedia.org/wiki/Digital_Audio_Broadcasting )

I am slightly confused by the screen-shot of an ubuntu OS with a windows XP desktop. :)

Its just a vritual machine.

Yeah, how...what is that?

I think it's called 'fusion' or something in VMWare, and in VirtualBox it's called seamless mode

> all you need to do is plug in a wire antenna

Just to make sure I get this right, a "wire antenna" is just a wire, right?

More or less, it really helps if it's the right length (as posted near here). An easy way to make one is to calculate 246/Frequency for a 1/4 wave antenna. So around the two meter / 144 Mhz ham band: 246/144=1.5 feet. The example in the video he's at ~440 Mhz so : 246 / 440 = 0.6 feet ~6 1/2 inches. You can make a "ground plane antenna" with a connector and coat hanger wire that will improve both the reception and transmission. At 50 Mhz where lots of doorbell items are it would be 246/50: ~5 feet of wire. In which case you can go for 1/8 wave length, about 2' 7'.

Somewhere in the posts is a suggestion to build a lowpass filter and that will work. But the Pi puts off some pretty messy spectrum splatter, so before you build a good antenna take a few mins and build a narrow passband filter. It cuts the splatter below (like a lowpass) and above (like a highpass) your chosen frequency.

Better yet, go to QRZ.com and look up a ham in your area to help you out. We have parts in our vast inventory of supplies (aka junk box) to help you out.

If it doesn't have the right length, impedance, and possibly orientation it's going to be significantly less effective. Even at this scale it probably makes sense to distinguish a wire antenna from a wire on that basis.

For more powerful equipment getting the wrong wire geometry could melt your expensive setup and get you in trouble with the FCC, so it makes even more sense to tack on the word "antenna" to mean "I've thought about this chunk of metal outside the DC approximation."

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