Transmitting FM, AM, SSB, SSTV and FSQ with Just a Raspberry Pi • Hacker News Board

Re: Transmitting FM, AM, SSB, SSTV and FSQ with Just a Raspberry Pi

#21

Post by AstroJetson » Sat, Oct 31, 2015, 8:48 PM UTC

bajsejohannes wrote: post #17

> 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.

Re: Transmitting FM, AM, SSB, SSTV and FSQ with Just a Raspberry Pi

#22

Post by escherplex » Sun, Nov 01, 2015, 2:00 AM UTC

Animats wrote: post #19

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…

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.

Re: Transmitting FM, AM, SSB, SSTV and FSQ with Just a Raspberry Pi

#23

Post by makomk » Sun, Nov 01, 2015, 10:38 AM UTC

escherplex wrote: post #22

Animats wrote: post #19

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…

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 Tektro…

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.

Re: Transmitting FM, AM, SSB, SSTV and FSQ with Just a Raspberry Pi

#24

Post by escherplex » Sun, Nov 01, 2015, 12:23 PM UTC

makomk wrote: post #23

escherplex wrote: post #22

Earlier quoted context omitted.

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 Tektro…

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