Choice of a frequency and antenna for a wireless sensor

[vic]

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.

 

[Tom Biasi]

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.

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

 

[Baron]

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 ?

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.

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 ?

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 ?

 

[vic]

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.

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.

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.

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

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 ?

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.

 

[vic]

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)

OK. I don't think 144MHz is an open frequency here (Europe), but 433 or
868MHz would do, then ?

As far as your antenna, if you want more gain use a directional antenna. I

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 ?

get the impression you only need to transmit to your own stationary
receiver.

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.

 

[Tom Biasi]

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.

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

 

[Baron]

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.

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.

 

[Baron]

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 ?

vv.

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.

 

[Baron]

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

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.

 

[Rich Grise]

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 ?

Building a Yagi is almost trivially simple - the elements don't even need
to be isolated from the boom!
http://www.google.com/search?q=build+a+yagi

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