AVR-Chat

Thanks, all, for the advice.  I probably will try the Maxstream
devices, although the freq. might be a bit high for my steep terrain.
The FRS devices are around 450Mz, and just barely work, at about 100mw.
I haven't tried a directional antenna with the FRS, but I have a yagi
that works well for our old analog cell phones (900mhz).
Ken

> 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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>> >
>> >
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