"Rodney Myrvaagnes" wrote in message
...
On 2 Mar 2004 18:22:07 -0800, (Gw) wrote:
Most VHF antenna's have 0 ohms to ground The resistance you are
reading is the round trip up and down the mast of the coax.
Interesting. What connects the shield to the center at the top? And
how does the shorted cable transfer power to the antenna?
For instance, an impedance matching circuit. RF and DC resistance ar
different beasts. A nice example is a folded dipole as seen with television
reception: the folded dipole is a perfect short for DC, if you measure it
with a multimeter, you'll measure almost 0 Ohms. Still, to RF this folded
dipole has a 'resistance' of 300 Ohms. To prevent confusion, resistance for
RF is called impedance. And with RF, resistance is not always equal to
impedance.
With marine VHF antenna's, the antenna itself is often a 1/2 radiator which
is fed at the end. The impedance at the end is high, a few kOhms. To match
that to the 50 impedance of the cable, a coil of, say, 10 windings is
connected between the antenna end and the ground. The cable is connected
between ground and a turn or two from the ground. This way, you measure 0
Ohms with an ohmmeter, while perfectly matched to 50Ohm at VHF frequencies.
Meindert