SparkFun Forums

[BudgetEngineer]

You could easily run a standard serial stream over it, just hookup the serial connection to the mic/ heaphone jack (make sure to use TTL level voltages to avoid damaging the radios). You may also want to rig up some directional antennas, since it should be line of sight the whole way up. Directional antennas will also help finding it once it hits the ground.

Nope

What do you suggest? (not trying to be sarcastic or mean)

[wiggles]

You could easily run a standard serial stream over it, just hookup the serial connection to the mic/ heaphone jack (make sure to use TTL level voltages to avoid damaging the radios). You may also want to rig up some directional antennas, since it should be line of sight the whole way up. Directional antennas will also help finding it once it hits the ground.

Nope

why not? you wouldn't be able to do high speed serial, but low speed should be doable, and it would also be half duplex. I see no reason that wouldnt work, I've seen many projects with serial being stored as audio streams.

With a two meter radio (146 mhz) at 5 watts and a homemade yagi I can hit the ISS which is a much greater distance than this balloon can manage.

[wiggles]

How about this? list price of $70 (available at digikey) and a range of 40+ miles.
http://www.rfm.com/products/spec_sheet. ... rd=DNT900C

[wiggles]

heres an old project that requires a couple more more IC's but uses standard walkie talkies:
http://books.google.com/books?id=zAsN0X ... ie&f=false

[stevech]

The Link Budget Math

f = 900MHz (actually, 902-928MHz in the US)
Pout = 1 Watt (30dBm) assume product with this power output.
Transmitting antenna gain: -3dB loss, because antenna on balloon is not optimal in size, etc.
Path length = 30 miles (158,400 ft)
Path loss at 900MHz = 125dB (assume no path obstructions - no trees, buildings, etc. No Fresnel zone loss (skyward path)
Receiving antenna gain = 6dB - at 900MHz, this is a 2 ft. long yagi
Receiver sensitivity for adequate bit error rate = -102dBm - or whatever product spec says

We have
30dBm - 3 - 125 + 6 = -92dBm received signal strength
102 - 92 = 10dB of fade margin in received signal strength. OK, but we want at least 10dB for antenna misalignment, etc.

Not practical to get much more antenna gain on ground side without a too-directional/large antenna.
Increasing from 1W to 2W on transmitter improves by just 3dB.
Best to hope for a receiver that actually gets to -110 or better with a decent error rate.

Now for the ground to balloon RF link- uplink command or simply protocol ACKs...
Assume same 1W transmitter. The math then is the same for up and down link - reciprocal.
The lower the data rate, the better, for receiver sensitivity vs. error rate.

MURS radios are 150MHz, not 900MHz. The 30 mile path loss changes from 125dB to 109dB. But antenna gain is harder to get at this lower frequency due to antenna size.

We're talking about data radio/modems here. Feeding serial data into analog radio's mic jack won't work. No more so that connecting your PC's serial port to your telephone line without using a modem.

Laws of physics at work.

[wiggles]

Alright, my mistake, you need a few passives or an IC or two between the serial and audio jack to act as a modem.

[UhClem]

[quote="stevech"
We're talking about data radio/modems here. Feeding serial data into analog radio's mic jack won't work. .[/quote]

Actually, it can and does.

There is little difference between using a 1200 bps modem that selects one of two audio frequencies and sending the raw NRZ data to a FM transmitter. Both change the frequency of a carrier. FSK modulation of FM transmitters is used frequently for high data rates although the receiving hardware is pretty specialized and expensive to support those rates. The only real trick is to control the DC level. Although you could use a bi-phase modulation scheme at half the data rate, simply randomizing the NRZ data is sufficient. RNRZ is simple to produce and decode with shift registers and XOR gates. (Details in IRIG Telemetry Standard 106, Appendix D)



Long ranges are easy to achieve with APRS. I had APRS data received by a ground station over 100 miles away (Great Bend, KS) without even trying and all I had was an Alinco DJ-C5 300mW transmitter.

[itikhonov]

According to the FCC: 95.194 (FRS Rule 4) FRS units. section (C)"You may not attach any antenna, power amplifier, or other apparatus to an FRS unit that has not been FCC certified as part of that FRS unit. There are no exceptions to this rule and attaching any such apparatus to a FRS unit cancels the FCC certification and voids
everyone's authority to operate the unit in the FRS."

Yep because FRS units use single antenna for reception and transmission. And also radio with 36 miles distance is not FRS probably.

[BudgetEngineer]

Yep because FRS units use single antenna for reception and transmission. And also radio with 36 miles distance is not FRS probably.

The radio I intend to use is the Midland GXT1000VP4 36-Mile. It is FRS/GMRS.

Correct me if I'm wrong, but the vibe I'm getting is that I can send serial data over a conventional, off the shelf, two way radio. Is a modem necessary? I seek the simplest possible setup, but will forgo simplicity for reliability.

Thanks for all your help!

[itikhonov]

Quick googling reveals that you need a license for operating GMRS radio. May be simply get HAM license and use HAM radios instead?

[wiggles]

http://sree.cc/electronics/arduino-as-an-fsk-modem

put on arduino on both sides of some cheap handi talkies, dont forget some code/hardware for PTT.

Overall the simplest is the 900mhz transceivers I linked to earlier. They would also likely be the most reliable as they allow an external antenna and require no extra modems or hardware (except possibly a rs-232 to TTL voltage converter) between serial lines and transceiver.

[Roko]

This guy has a video on youtube of APRS being pushed through an FRS radio...
http://www.youtube.com/watch?v=sqv3RZ0kSZ8

Implementation Details here:
http://rev0proto.com/wiki/index.php/Rev0Trac