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  • From: Clansgian AT wmconnect.com
  • To: homestead AT lists.ibiblio.org
  • Subject: Re: [Homestead] Internet connection,
  • Date: Sat, 26 Apr 2008 21:31:31 EDT


> Ron says it is just a single wire antenna, with insulators in the
> attic-grounded only through the stereo itself.
>

Grounded through the stereo itself is what you want. The antenna connection
on the back of the stereo is for the FM only. It needs a 72 ohm load to
operate at the max (or to operate well at all). What you probably have in
the
attic is a dipole antenna, a piece of wire in two pieces with the lead to the
stereo between the two pieces. The dipole well resonate at one frequency
(band of
frequencies). Roughly divide the frequency you are listening to (in MHz)
into 468 and this gives you the lenght of the wire. Divide that in two and
you
have the point where the impedence of the antenna at that frequency is about
50
ohms. So for the FM band centering around 100 MHz the antenna is only 4.6
feet long, two arms of 2.3 feet each. But to get even more signal gain and
usually match the impedence better, you can make it any length that is an ODD
multiple of the fundamental frequency. So making the 'arms' of the antenna
6.9
feet each works even better and 11.5 feet even better.

The wire from the atenna to the stereo needs to be either a coaxial cable or
else that flat ribbon wire that is two wires molded into plastic some 1/2"
from each other.

--------------------------------

The antenna for the AM part of your stereo is not connected to the antenna
posts in the back. It is a ferrite rod with a coil of wire around it. You
have
to make an internal connection to the lead wire of this ferrite rod antenna
in order to enhance the AM reception. It's true that just connecting an
external antenna to the FM imporves the AM but that's only because you are
bringing
a large piece of metal within the electrical field of the ferrite rod
antenna.
It isn't very selective or efficient.

Using our above formula, if you are tuning in an AM station just above
500kHz, the dipole antenna would be nearly 1000 feet long. Not practical.
So one
runs a 'long wire', just a piece of wire as long as is possible and
practical,
I've even seen insulated fences used for this. The impedence into the AM
receiver will be mismatched so we put a coil and variable capacitor (based on
another formula) in series with the antenna between the antenna and receiver.
By
adjusting the capacitor, the load created by the long wire, coil, and
capacitor can be made to exactly match the impedence of the reciever. Thus
the
desired AM signal is enhnaced and fed into the reciever while all others are
attenuated.

------------------------------------------

For the FM bands and also short wave bands, you can make an antenna array (an
array type antenna) with the object of giving the antenna a directional
'gain'.

The most potent type is the quadrilateral array or quad-antenna. It looks
like a box kite. Formulas forthcoming if anyone is that interested. Another
type is the Yagi-antenna (named after its designer). Most of those TV
antennas
for sale that look like a boned fish are Yagi antennas.

With an array antenna, when the antenna is pointed toward the desired radio
station (even hundreds of miles away), the radio signal is multiplied in the
antenna by simulaneously receiving the singal on the main element, and then
the
other elements (called parasitic elements) reflect the signal back onto the
main element in phase. It is quite very effecitive.

--------------------------

The time may come when we need to know things like this. </HTML>




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