"john doe" wrote in message
...
Thanks Roy!!!

Is it possible to cancel the capacitive reactance of the antenna by
placing an inductor in series with the radiator??

i.e center of coax connected to one end of a coil with the other end of
the coil connected to the radiator. Coax shield connected to ground plane
radials.

de ka2pbt

John DK2 PBT

I'm no Roy. But, I have thought about Smith Charts alot. I wonder if
you'd consider trying to identify where the antenna's input impedance must
be located so that it might be matched with a series inductor. I'd be way
out of line to assume that you want to look at a Smith Chart for knowledge
about the effectiveness of that series inductance. But, the Smith Chart
can be a real big aid in estimating 'what it takes' to match any antenna's
input impedance.

Jerry




"Roy Lewallen" wrote in message
...
Let's suppose you have a vertical antenna with a base feedpoint impedance
of, say, 75 - j300 ohms (75 ohms resistance in series with 300 ohms of
capacitive reactance), typical of a thin 5/8 wave vertical. A network
consisting of 1275 ohms in *parallel* with 319 ohms of capacitive
reactance has the same impedance. (Note how the Xc isn't much different
from the Xc of the series circuit in this case.) If we put an inductor
with 319 ohms of inductive reactance in parallel with the antenna (that
is, from the base feedpoint to ground), the reactance of the inductor
cancels out the capacitive reactance of the antenna, and we're left with
1275 ohms of pure resistance from the antenna base to ground (that is,
across the inductor).

We can use the inductor as an autotransformer. If we tap up on the
inductor some fraction k of the whole way up, the impedance we'll see at
that tap will be very nearly 1275 * k^2, and it'll be purely resistive
(no reactance) because the impedance across the whole coil is purely
resistive. For example, half way up the coil we'll see 1275 * (.5)^2 =
1275 / 4 = 319 ohms. So to get 50 ohms, we tap up sqrt(50/1275) = 20% of
the way up the coil.

Roy Lewallen, W7EL

john doe wrote:
I'm trying to understand the shunt tapped inductor as a circuit.

How does it work??? As long as the inductor cancels out the reactance of
the radiator you just need to find the 50 ohm point on the coil ... or
for
that matter any feedline impedance???

Pardon my lack of knowledge.

de ka2pbt