AVR-Chat

Half mile in hilly wooded area is a bit of a challenge with
low power at ANY frequency. Trees tend to absorb pretty
well. 

Problem with low frequencies is that there  is a lot of
interferance. Problem with medium and higher frequencies is
the terrain loss. 

AM won't gain you much. Its simple and there are lot of AM
chips and modules out there, but they are generally good
for a few hundred feet, max. FM and other modulation modes
all have disadvantages along with the advantages. 

You might try building a net of zigbee modules though I've
heard that zigbee networking may not be ready for prime
time.

Get yourself a ham license (no morse code now, exam is
pretty simple). Then you can use a 25W transmitter or more
with ax.25 packet and you will have an off-the-shelf
solution. It would handle your half mile easily. Modems are
readily available and you only need to feed ascii in and
get ascii out. For this purpose, you could even get used
radios (cheaper) because it does not take a lot to make
this work.

Jim


On Wed, 2 May 2007 14:30:59 -0700 (PDT)
 kholt@sonic.net wrote:
> 
> I am also looking for a cheap, simple, low power RF
> system
> that might as well be AM.  I need to send intermittent
> telemetry
> over 1/2 mile of wooded hills, where there is not much RF
> noise
> at the low end.  I am looking for low freq because of the
> hills,
> but would not like to be bothered with very long
> antennas.
> The data at either side of the link is handled by AVR
> micros,
> and is mostly sensor info: water levels, temperature,
> etc.
> I have been experimenting with FRS radios, just because
> they're
> cheap and off the shelf, but they are pretty high freq.
> 
> Ken
> 
> 
> > Well, yes, but....
> >
> > That "transmitter" would have been almost as "dirty" as
> the
> > old spark-gap guys in the early days. Harmonics would
> be
> > horrendous.
> >
> > Now, you CAN do it at low frequencies using function
> > generator techniques to generate low distortion AM sine
> > waves (up to 5MHz, perhaps).
> >
> > Receivers are more difficult because you need to
> > discriminate between the signal you want and the many,
> > many, signals you do not want. Think AM broadcast
> stations
> > as a simple but almost ubiquitous example.
> >
> > However, if the original query is about VHF or UHF
> where
> > there are many fewer interfering signals (except for
> those
> > really strong TV and FM stations and cellphones and
> FMRS
> > radios and mobile 2-way radios and ....), its almost
> > interchanged.
> >
> > You CAN use a very low sensitivity receiver so that you
> > only hear relatively strong signals (such as a low
> power
> > transmitter) that is relatively close. The transmitter
> can
> > rely, to some degree, on transmission-line resonators
> (and
> > thus not use coils in the strictest sense).
> >
> > So, what might be done depends on so many things. The
> > original poster needs to help us with more description
> > about the requirements (range, type of information,
> > information bandwidth, etc).
> >
> > Jim Wagner
> > Another RF Engineer on the list
> >
> > On Tue, 01 May 2007 20:04:30 -0000
> >  "kernels_nz" <kernels@slingshot.co.nz> wrote:
> >> Hi Zack,
> >>
> >> Cant say I fully agree, I cant recall the exact
> detail,
> >> but I have
> >> built a voice-AM transmitter without any coils, I went
> >> something like:
> >>
> >> Buy a 4-pin crystal oscillator outputting square waves
> at
> >> the
> >> frequency of carrier your looking for, I believe mine
> was
> >> 1MHz, then
> >> vary the supply voltage depending on the "voice" input
> >> voltage.
> >>
> >> Cheers
> >> Hein B
> >> Auckland, NZ
> >>
> >> --- In AVR-Chat@yahoogroups.com, Zack Widup <w9sz@...>
> >> wrote:
> >> >
> >> >
> >> > It can't be done. You will need a coil at least for
> >> your
> >> antenna/matching.
> >> >
> >> > How much power are you talking about?  What range do
> >> you want to cover?
> >> > For what purpose?  What frequency?  What are the
> laws
> >> in your country
> >> > regarding license-free transmissions on that
> frequency?
> >> >
> >> > Inquiring minds want to know, especially this RF
> >> engineer.
> >> >
> >> > Zack
> >> >
> >> > On Wed, 25 Apr 2007, azza eldessoky wrote:
> >> >
> >> > > please friends,can any one  send me a circuit
> design
> >> for AM
> >> transmitter or receiver without  any coils .
> >> > >    thanks alot
> >> > >        azza
> >> > >
> >> >
> >>
> >>
> >
> >
>
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> >
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> 

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