What frequency to use? « Jee Labs Talk

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What frequency to use?

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izrun

Hi there. I'm thinking of working on a project and I am trying to decide which HopeRF model to get: the 434MHz or 915MHz (I'm in the US). The usage would be inside, but not through walls but possibly through people, like across an auditorium or large room. I'm looking for moderate range (<50m) and low bandwidth (probably max 2400bps, maybe less). Some packet loss would be acceptable.
I guess the first question is if you all think the transmitter on the JeeNode is strong enough to do this. And, if so, would it be better to go the 434 or 915 route. I know they list 100m but I can't know how true that is. I like that the 434 would work anywhere and people in Europe could use the design without modification so all things being equal I would choose that. However, that's by no means a deal breaker. I've never really seen the pros and cons of using the different frequencies.
Any help would be appreciated. Thanks!

Posted 19/10/2010 19:16:16
jcw

It's hard to come up with range specs, and they seem to be very hard to find on the web as well.
I suspect that this is because it totally depends on the conditions under which these measurements are made. Line-of-sight-out-in-the-field figures are often quoted as over 100m, whereas I'd say it's more like 10m inside through some walls and a re-inforced concrete floor, as I'm using the RFM12B's here at Jee Labs.
If your bit rates are low, you can play games with the frequency shift and bandwidth settings, which I think can have a substantial effect on range. The above figures are with ? 50 kbaud and ? 130 kHz bandwidth settings (the RF12 default).
For 50m in a room with people and no walls, I'd call it a toss-up. Positions will probably make a difference: if you have to go horizontally through a crowd of people, then perhaps the absorption is higher? I'm just guessing here.
What I do know is that lower frequencies propagate further. Then again, all it takes to rule out either frequency band is one device hogging that band and sending out data continuously (our wireless headset does that).

Posted 19/10/2010 21:34:33
jbeale

In my experience, Jean-Claude is exactly right. Lower frequencies have a theoretical benefit for range, but there are so many other factors there's no real way to tell, without testing it in your specific situation. I think the RFM12B has some RSSI signal (Received Signal Strength Indicator). If you worry about interference, you could make a simple spectrum analyzer just by recording the RSSI values across the available frequencies for a few days, and seeing what kind of other signal levels are out there.
Of course, nothing prevents someone from bringing in something really radio-noisy after you've done your test! But I think the better radio microphones may do this kind of thing automatically: listen first, and change channels if it's too noisy.

Posted 25/10/2010 20:26:58