AVR-Chat

Probably not legal per US FCC Part 19 for Ham Radio (I am a licensee).

You should be able to get that half-mile easily with a pair of 1W
902-928MHz serial port extender radios. I've used these. And a small
yagi for one or both isn't expensive if you need its gain. Maxstream
sells these - as 100mW and 1W. I don't recall what the FCC Part 15
limit is in that band; it's usually much higher with highly
directional antennas. And higher yet for freq. hopping radios as are
the Maxstream. These kind of radios are popular in in the SCADA
business - telemetry of traffic signals, water pumps, electrical
transformers and the like. 

I got 6 miles line of sight with a pair of 100mW radios and 4 ft. long
yagis, and with 2MHz bandwidth with 1+ Mbps data. The ones I mentioned
above for serial port extension (like 19.2Kbps or so) would do much
better due to the lower modulation rate and 1W. You can save some $ by
purchasing a PC board without enclosure and power supply.

steve

> 
> On Wed, 2 May 2007 14:30:59 -0700 (PDT)
>  kholt@... 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@...> 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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