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]

"vic" <news@bidouille.org> wrote in message
news:49579caa$0$18986$426a34cc@news.free.fr...

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]

vic wrote:

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 ?

Thanks.

vv.

--
Best Regards:
Baron.

 

[vic]

Baron wrote:

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]

Tom Biasi wrote:

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]

"vic" <news@bidouille.org> wrote in message
news:4957c6a3$0$1636$426a34cc@news.free.fr...

Tom Biasi wrote:
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.

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

 

[Peter Bennett]

On Sun, 28 Dec 2008 19:28:00 +0100, vic <news@bidouille.org> wrote:

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.

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
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Baron]

vic wrote:

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.

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.

 

[Baron]

vic wrote:

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 ?

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.

--
Best Regards:
Baron.

 

[Baron]

Tom Biasi wrote:

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.

--
Best Regards:
Baron.

 

[Rich Grise]

On Sun, 28 Dec 2008 17:34:03 +0000, Baron wrote:

vic wrote:

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 ?

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