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[Clansgian AT wmconnect.com]

> 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.

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