Choice of a frequency and antenna for a wireless sensor

Hi,

I'm currently designing a wireless weather station/generic sensor system (I know it can be bought for a fraction of what the components and time will cost me, but I'm doing it because it is fun.). I'm wondering what is the best frequency to choose for this application. What I need is

20-30m of range, and ability to pass 1 or 2 concrete walls. I'll be using ready-made modules (not enough knowledge in RF to build my own from scratch), which I found for 433MHz, 868MHz and 2.4GHz. Which one should I choose considering ability to cross walls and possible interference from other systems ?

Issue number two is the antenna, the only design I know is the 1/4 wave antenna, is there anything better I could build easily to improve the gain ?

Thanks.

vv.

Reply to
vic
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When I used to mess with this stuff I have found the best for going through buildings was 144 MHz. Long waves don't fit through openings and short wave bounce off. (Broadly stated, I admit) As far as your antenna, if you want more gain use a directional antenna. I get the impression you only need to transmit to your own stationary receiver.

Tom

Reply to
Tom Biasi

I suspect that the frequency you use is going to be the least of your problems ! The unlicensed 432/3 Mhz band should give you more than adequate range within the power constraints for that band, as would the

2.4Ghz band.

The modulation scheme you choose is more likely to be a greater issue. The receiver is going to have to pass that on to be decoded into reproducing the data the you want transfer.

There are many better antenna than a simple 1/4 wave ! But, for example, do you really need a four or eight element yagi for a 1 milliwatt signal a hundred feet away ?

--
Best Regards:
                     Baron.
Reply to
Baron

What if I am limited to, say, 1mW ? That's the kind of power small integrated modules are capable of. I forgot to say that the transmitters are solar powered so power consumption in an issue.

Sure thing. Actually the modules I plan to use all include a modulation/demodulation circuitry, and some even directly encapsulate the data.

As I said, I don't know much about antennas. So your choice of answering my question by another question is perplexing. I can only say "I don't know". Let me rephrase my question : is there a better unidirectionnal antenna than the 1/4 wave ?

vv.

Reply to
vic

OK. I don't think 144MHz is an open frequency here (Europe), but 433 or

868MHz would do, then ?

I'm a bit reluctant to use a directionnal antenna since the sensors will be subjected to wind, etc, and may move over time. Is there such a thing as an unidirectionnal antenna with better performance than the 1/4 wave, or is the 1/4 wave the "perfect" antenna ?

That's true. Actually transceivers are quite common, and I figured some kind of acknowledge would be a nice feature, but it's not a must.

vv.

Reply to
vic

Your directional won't be that narrow that small movement will be a concern. At the frequencies that you are considering the antennas would be quite small. Maybe you would consider a dish to dish data link setup.

Tom

Reply to
Tom Biasi

A 1/4 wave antenna is generally considered to be an omnidirectional antenna - it radiates (or receives) equally well in all directions at right angles to its element. You can make antennas more "efficient" by making them more directional. (A unidirectional antenna would radiate in only one direction - Yagi or beam antennas are considered unidirectional.)

The radiation pattern of a 1/4 wave antenna is shaped something like a donut, with the antenna passing through the hole. Stacking two 1/4 wave antennas end-to-end, and feeding them with correctly phased signals, will flatten the donut pattern, and extend the donut outwards

- more energy will be directed at right angles to the antenna, at the expense of the vertical spread.

Look for amateur radio publications on VHF antennas to get a good explanation of antenna performance.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
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Reply to
Peter Bennett

Sorry if if that came across badly ! The intimation was that a quarter wave and a milliwatt should be fine at that close range ! Poor receiver sensitivity could be the cause of a problem if you needed a bigger antenna.

--
Best Regards:
                     Baron.
Reply to
Baron

A "Colinear" antenna can be omnidirectional and can be set up to provide

6 - 8 Db of gain or more ! But you sacrifice simplicity for complexity in construction. Also you may be in danger of exceeding the allowed ERP limits if you use one for transmitting.
--
Best Regards:
                     Baron.
Reply to
Baron

Hi Tom, In general I agree with you ! But if Vic is to consider dishes then he needs to go up a lot more in frequency ! Say to around 5 or 6 GHz or even 10 GHz. Once he does that, he is then out of the unlicensed bands.

--
Best Regards:
                     Baron.
Reply to
Baron

Building a Yagi is almost trivially simple - the elements don't even need to be isolated from the boom!

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If you're worried about the wind jostling it, just put it under a plastic cover of some sort, to act as a sort of "radome".

If you're worried about the wind jostling the whole assembly, you need to redesign your support structure. ;-)

Have Fun! Rich

Reply to
Rich Grise

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