AVR-Chat

The performance of an antenna depends on its relative size
compared to the operating frequency. I would not expect a
900MHz Yagi to work very well at 450MHz.

You can make a very simple antenna called a "J-pole" out of
300 ohm twin lead and some coax (to connect to the radio).
You can hang it from a tree branch or almost anything that
gets it up in the air. Here is a web site that tells how to
build one:

http://www.qsl.net/wb3gck/jpole.htm

Simply scale it inversely with frequency. This design is
nominally centered at 146MHz. FRS band center is about
467MHz. So, multiply all of the length dimensions by
146/467). 

Jim
 
On Thu, 3 May 2007 12:21:28 -0700 (PDT)
 kholt@sonic.net wrote:
> 
> 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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