David Forsyth wrote:
Bob, is this something I could possibly be able to use in this application?
Where does one find such high-impedance coax? As an alternative, would it
be possible to put an active device up near the antenna that would act as a
buffer to feed a low-impedance line?

Dave

The trick in this recommendation is that the car radio feedline is a
specific length (a few feet)and its capacitance is taken into account.

-Bill




"Bob Lewis (AA4PB)" wrote in message
...

If the antenna is the typical end-fed wire then the impedance will be
very high. Most AM radios have a high impedance input designed to
match a random wire like that. If you use coax, the capacitance of the
coax (center conductor to shield) will essentially short most of the
signal to ground and little will get to the receiver. If you've ever
opened up the coax used on car radios you'll find that it is a special
high impedance coax. A small wire fits loosly thru an insulated tube
with the shield around the outside. This keeps a relatively large
spacing between the center conductor and the shield in order to
minimize capacitance.