
Transmitting FM, AM, SSB, SSTV and FSQ with Just a Raspberry Pi - irfan
http://www.rtl-sdr.com/transmitting-fm-am-ssb-sstv-and-fsq-with-just-a-raspberry-pi/
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pi-rat
Very cool, but please don't do this without a proper filter, you're spewing
garbage all over the spectrum.

~~~
AstroJetson
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.

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radiof
Build yourself a low pass filter before transmitting:

[http://www.ebay.com/itm/LFCN-HFCN-FV1206-Filter-Design-
Kit-M...](http://www.ebay.com/itm/LFCN-HFCN-FV1206-Filter-Design-Kit-Mini-
Circuits-LTCC-Filters-2-Pre-
soldered-/281754737238?hash=item4199e41a56:g:x-gAAOSwjVVVrdYO)

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mschuster91
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).

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makomk
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?

~~~
morganvachon
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._ "

~~~
makomk
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.

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

~~~
GFK_of_xmaspast
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).

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Animats
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/](http://www.rfdigital.com/) [2]
[https://www.linxtechnologies.com/en/products/modules](https://www.linxtechnologies.com/en/products/modules)

~~~
escherplex
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.

~~~
makomk
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.

~~~
escherplex
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

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

~~~
mitchtbaum
HackRF:
[https://greatscottgadgets.com/hackrf/](https://greatscottgadgets.com/hackrf/)
~> $300

HackRF Blue: [http://hackrfblue.com/](http://hackrfblue.com/) ~> $200

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

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frik
Interesting.

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](https://en.wikipedia.org/wiki/Digital_Audio_Broadcasting)
)

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

~~~
themodelplumber
Yeah, how...what is that?

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

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bajsejohannes
> 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?

~~~
AstroJetson
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.

