I'm new to the forums so apologies if this has been answered before but I've done a search and can't find anything on this topic...
I'm considering buying a kit to build a Solar PV monitor and I wondered if there is any advantage or disadvantage to choosing 433 or 868Mhz? I'm aware that 433 band may be more congested than 868 but that's less of an issue as I'm in a fairly isolated location so probably only my devices in use anyway - and I don't currently have that many (that I know of).
I currently have two existing wireless energy monitors and the signal from one seems to go through the walls far better than the other. Don't know what frequencies they use but if 433 or 868 has a better signal strength I'd prefer to know which.
Also, I have some "professional" wireless alarm gear that uses 868 (digitally encrypted signals apparently). It can detect "jamming" but I presume as long as the OpenEnergyMonitor wasn't transmitting constantly I presume there wouldn't be any conflicts? I did some Googling on that and discovered that there's some sort of regulation that devices using 868 should never transmit for more than a few milliseconds per second anyway?
Anyway, any advice on pros/cons of choosing 433 over 868 would be appreciated.
We have never noticed a difference in range. I think due to physics the greater the frequency the better the penetration. All devices on the ISM bands should only transmit for a few mS. We have not had much experience using the OEM system in areas with busy RF's. Most of our deployments so far have been quite rural.
I've been searching here and I couldn't find any order of magnitude for the range of the RF link between emonTX and emonBase, be it here, or there, for instance.
What kind of range can you expect ? Could this be a concern in big buildings, or large houses with thick concrete walls ?
Thanks for any feedback.
Take a look here: http://jeelabs.net/projects/cafe/wiki/FAQ
Note also that you should be able to improve the range by reducing the bit rate. There's been a recent discussion mention about this, sorry but I can't find it just now. here: http://openenergymonitor.org/emon/node/1535#comment-8237
For the specific example we are talking about, lower frequency equals better penetration. See page 43 here:
Not sure you would notice the difference in your average house though. Might depend more on what the neighbours are using. The variables are many from what I can tell.
CAUTION: That data sheet is NOT for the RFM12B module!
Though the table of losses will apply, it is still necessary to take antenna performance into account to get any sort of estimate of the reliable range - and I cannot recollect seeing that for the RFM12B.
Thank you guys for these answers. I was concerned about the transmission in a house through several walls and ceilings. I guess it's generally fine.
Besides, lowering the bitrate should not be an issue when monitoring a house : most measured values are quite low-freq and you don't need thousands of samples by second (unless you are really interested in electricity peak consumption, maybe).
I discovered the range improvement with lower bit rate after replacing a Maplin monitor with emon. Both systems use a version of the RFM12 module but the Maplin system had far greater range so I disassembled the PIC code in the Maplin transmitter and found that the only difference was the bit rate. That's how I ended up with the odd value of 3.918kb/s, it was the value used in the Maplin unit. I didn't take this further to see if lowering the bit rate even more would increase the range since this was enough for my house.
Does slowing down transmission also improve the reliability (so ACK's won't be necessary) ?
Maybe interesting to do some more range/reliability tests with this. OEM packets are so small transmission speed isn't an issue here (according to me).
The maximum packet size is 75 bytes (66 bytes of payload plus header and crc), and I think I've read somewhere that any one transmitter must not hog the channel for more than 1% of the time (on the 868 MHz band). So at 4 kb/s, one max-sized packet occupies the channel for 150 ms, so one full-sized packet every 15 s - sounds good enough to me.
Is it 4 kilobytes/s or 4 kilobits/s?
Just checked the regs for the 433MHz band and they allow a 10% transmit duty cycle so no legal issue there.
The 868MHz rules are much more complicated but seem to be either 0.1% or 1%
document is here if anyone wants to check for themselves....
B/s = bytes per second, b/s = bits/second. (B is bigger than b !). No, I don't like it either, but that's the way it seems to be done.
Lowering the bitrate will in essence reduce the required RF-signal bandwidth .
The range improvement is due to the fact that available RF-power is divided on the bandwidth. (A narrow signal will travel further than a broader one, if the power is constant.)
If you need (or want) both power and bandwidth to be untouched, you'll have to change antenna.
Open-source tools for energy monitoring and analysis. This project uses the GNU General Public Licence