SparkFun Forums

[reklipz]

Hey all, I'm looking for some advice about wireless solutions.

I'm looking to create a handful of networked devices that would be used in a wooded area. The size of the wooded area is about 30 acres, but designing for something larger, say 50 acres, would be preferable.

The devices would all need the ability to communicate with a central host, but not necessarily directly (some protocols might support some sort of "daisy chaining" or packet forwarding features, which is fine). The devices also don't need to be able to push large amounts of data, just simple status notifications and the like.

I was looking at using Zigbee, and the XBee Pro sports 100mW power, claiming 15mile range (best case, LOS, I would assume). I've never used the Zigbee protocol, but the protocol doesn't matter so much to me, cost (assuming the range I need) is the main driving factor.

Perhaps using the Nordic chips with an amplifier would also work? Again, this just drives up the cost, but the Nordic chips are fairly cheap to begin with.

Thanks for any suggestions!

-Nate

====

Looks like the Nordic chips are a dead end, not very easy to add an external PA.

Perhaps some sort of ARM core with integrated 802.15.4 would be a better solution...

[stevech]

in wooded areas, likely 2.4GHz anything will have short (500 ft?) range, depending on foliage density/season.

for 30 acres wooded, you'll have to go to MURS (160MHz) band $$$ radios for unlicensed. Or get a Ham License (ARRL.org) and take that learning path and use 150MHz - but for non-profit only.

Re 1W at 900MHz (say, Digi's high power 900MHz products), you may have success. But I did a 900MHz project where one stand of dense trees zapped the coverage.

re 30, 50, 100mW. The difference among these is just a few dB. The path loss can be 120dB or more so what counts is lots and lots of antenna gain. Hard to get at 900MHz, big antennas.

Antennas are much better than Tx power amps- as these distort waveforms and don't get as much link budget benefit as antenna gain. Low bandwidth helps a lot too.

So VHF is the way to go- but at low data rates.

[riden]

...Or get a Ham License (ARRL.org) and take that learning path and use 150MHz - but for non-profit only. coverage... So VHF is the way to go- but at low data rates.

I did want to point out that the purpose of the amateur radio (ham) service is to facilitate communications with other amateur radio operators and not for things like home automation, etc. Technically speaking, I believe that using those bands for this particular purpose wouldn't be legal, ham license or not.

You might be able to cover the area with a network of transceivers relaying packets, but it is an expensive and potentially unreliable solution, due to the environmental issues and the dependency of one node on others. As usual, Steve has provided excellent advice, and I agree with his conclusion about having to go with VHF or below. It is amazing how much foliage can zap a signal, especially once the frequency gets about 450 MHz. A few years back before cable/DSL was available in my area, my broadband was supplied by a 2.4GHz access node using a high gain antenna on my end. I was talking to the node less than 2 miles away (which I could see from the antenna location). I had to reorient the antenna each spring and fall because the leaves on the (few) trees between my house and the remote site all but obliterated the signal. In the spring, I pointed the antenna to a nearby water tower to use as a passive repeater, and in the fall, I pointed it directly at the node. It was a crazy situation, and I'm glad I don't have to worry about that anymore.

I haven't checked the frequency allocations recently, but perhaps with the switch to digital TV in the U.S., more frequencies in the upper HF and lower VHF will be opened up for unlicensed use. At least, that was what I remembered was going to happen with part of the spectrum.

[reklipz]

So VHF is the way to go- but at low data rates.

Thanks stevech, lots of good info in there.

I'm fairly new to the wireless side of things, I've only played with the "RF Link" modules SparkFun provides, nothing real technical. It looks like I've got a lot of learning to do, hopefully I can do so and have this project up and running by summer 2010. At least, that's the plan.

Before I go and spend some $$ on a few books and equipment, any recommendations on good resources?

Also, I'm hoping to keep cost down to around $100 a unit (cost of parts. components(ICs + antenna) + PCB + battery...). If prior experience tells you this is not at all feasible, please let me know, I'll delay learning wireless tech for some other time.
(50mW XBee + $20 antenna ~= $70, + PCB + uC goods...).

Thanks much everyone!

[waltr]

I agree with the other posters that foliage greatly attenuates UHF and uWave transmissions. Otherwise ZigBee would do what you want.

There may be some 27 and 49MHz low cost, license free transmitter/receivers that would work.

[mdipperstein]

There are people in the wireless sensor community that do things like this all the time. I think they usually use bluetooth networks and pass the data along some hierarchical path.

-Michael

[MichaelN]

I'm planning on using Atmel's AT86RF212 for a project, which has the same pinout as their AT86RF230, but operates in the 900MHz band instead of 2.4GHz.

900MHz would be a lot better than 2.4GHz, but nowhere near as good as VHF as far as trees blocking the signal. You would need to do actual testing onsite to determine if any particular frequency or module was appropriate. There are formulas for calculating propagation loss, but things become very difficult to predict when you put trees in the way.

Essentially, you need to find out if:

Transmitter Power - Receiver Sensitivity + Transmitter antenna gain + Receiver antenna gain - Propagation Loss > 0

to determine if a link is feasible.

I like the AT86RF212, since it has a "link budget" of up to 120dB (transmitter power 10dBm, receiver sensitivity -110dBm at low bit rates). Using (say) 6dBi whip antennas in theory allows for more than 130dB of propagation loss, although in practice a margin would need to be included.

[stevech]

There are people in the wireless sensor community that do things like this all the time. I think they usually use bluetooth networks and pass the data along some hierarchical path.

-Michael

Bluetooth is 2.4GHz as is 802.15.4. The ZigBee option of 802.15.4 uses mesh networking - where a daisy-chain of mesh nodes (repeaters) retransmit. This is rarely practical.

The path loss in heavy foliage is much, much greater than a line of sight link budget (20log(f) + 20log(d) + n)

For a one-hop link, you'll need to look at VHF as mentioned above. The 43MHz band is unlicensed in the US but there are few products available. The regulations for "CB" 27MHz may forbid data.

[riden]

For a one-hop link, you'll need to look at VHF as mentioned above. The 43MHz band is unlicensed in the US but there are few products available. The regulations for "CB" 27MHz may forbid data.

I wonder what the U.S. regs are for the frequencies used for RC (27 MHz, 49 MHz, and 72 MHz). Clearly some type of data is involved when controlling RC devices, but I don't know what is and is not permitted, besides non-interference with other devices.

[reklipz]

Been reading a bit more:
http://en.wikipedia.org/wiki/TV_Band_Devices

Looking at 902MHz devices:
http://www.atmel.com/dyn/products/produ ... rt_id=4349
http://focus.ti.com/docs/prod/folders/print/cc1101.html

I'm not quite sure where I would start with developing/implementing the protocol layers upon 802.15.4, perhaps there are already solutions for popular micros, or I'm missing something and things will turn out really simple. I have no idea how I would utilize the chipcon device.

Thoughts?

[stevech]

network routing above the complete MAC layer in 802.15.4 - needed only if you are doing complex topology. Alone, '15.4 does all framing, error correction, clear channel assessment, source/destination addressing.

If you save $5 and buy less than '15.4, you get the fun of implementing all of this yourself. Hence the popularity of Digi XBee, S1 Jennic, et al.

[wiml]

As MichaelN says, 802.15.4+ZigBee can be used in the 900MHz band as well (eg, these Meshnetics modules which I haven't used but look nice). Attenuation at the lower frequency should be less than for 2.4GHz, though still not as good as at VHF, but it may be good enough for your purpose.

If you're building a sensor network / mesh routed thing, the total area covered is less important than the density of nodes, since each node only has to be able to reach the next one. Of course this also means that when a node fails it makes some number of other nodes unreachable as well.

There's a moderate amount of academic research going on in the field of low-power sensor networks, which has led to some commercial spinoffs (eg Crossbow's MICAz products) which could be useful. If you decide to roll your own you might also want to dig through some academic papers on the subject to see what hardware they've used.

[reklipz]

I don't know how I missed MichalN's post about the Atmel AT86RF212, but that was the one I was looking at as well. Two people concluding the same thing is a good sign, I think.

Also, I noticed that ZigBit module as well. It looks nice, would save on a lot of implementation as well. I may try to save a few bucks and try to port one of the stacks to whatever processor I end up using in the application. I've yet to decide, but a small ARM would be nice, I've been wanting to learn that(those) platform(s).

Looking at linux-zigbee

[mitchind]

in wooded areas, likely 2.4GHz anything will have short (500 ft?) range, depending on foliage density/season.

for 30 acres wooded, you'll have to go to MURS (160MHz) band $$$ radios for unlicensed. Or get a Ham License (ARRL.org) and take that learning path and use 150MHz - but for non-profit only.

Re 1W at 900MHz (say, Digi's high power 900MHz products), you may have success. But I did a 900MHz project where one stand of dense trees zapped the coverage.

re 30, 50, 100mW. The difference among these is just a few dB. The path loss can be 120dB or more so what counts is lots and lots of antenna gain. Hard to get at 900MHz, big antennas.

Antennas are much better than Tx power amps- as these distort waveforms and don't get as much link budget benefit as antenna gain. Low bandwidth helps a lot too.

So VHF is the way to go- but at low data rates.

Wow - great info Steve! Really appreciate you sharing your experience with these radios and scenarios.

I'm just going thru the range-testing myself using XBee XSC 900 Development Kit radios. Want to use them in a heavily treed area needing about 800m range - to send basic yes/no data. Also finding that even the 100mW radios take the life out of a 9V battery in a mere 24 hours - even in cyclic sleep mode.

Beside a solar powered charger for the remote radio, it sounds like I'll need to look at lower freq radios or larger antennas too.

What would be the largest antenna for maximum power at 900 Mhz - around 1 foot high?

Also - any suggestions for proven long-range VHF radios at Sparkfun?

[stevech]

900MHz - I don't recall the FCC radiated power limit regulation. Probably something like 4W if the antenna beamwidth is narrow. Look in FCC part 15 or ask the vendor.

I've used Yagi's.
You should do some measurements with omni antennas before deciding on what antennas to purchase.
I've used Yagis like this to get 6 miles over water - where the far end was an omni (on a boat)

http://www.moonblinkwifi.com/pd_16.cfm
http://www.l-com.com/category.aspx?id=2071

I don't think that SFE sells any MURS band radios
I've worked with this vendor
http://www.raveontech.com/rv_m3_m.html