john doe wrote:
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

Sure. In the example I gave, where the antenna has a 75 - j300 ohm
feedpoint impedance, you'd just need an inductor with 300 ohms of
reactance. Then you'd have a feedpoint impedance of 75 ohms resistive.
Of course, this would be useful in feeding an antenna only if the
resistive part of the feedpoint resistance is close to 50 ohms or some
other value your system can conveniently feed. Unlike the tapped coil
method, it doesn't give you any way of transforming the resistance.

If you need to match any arbitrary impedance, say 250 - j85 or
something, to 50 ohms or some other impedance, you need two things you
can adjust or choose, since you have two things (R and X or magnitude
and phase) which you need to transform. We could get 50 ohms resistive
using the tapped inductor scheme because we could choose the capacitor
and the tap position. Another very common solution is an L network,
which of course has two components. In theory, we can match (or
transform) anything to anything with an L network of some sort. In some
special cases, one of the elements of the L network is zero or infinite,
such as if we're transforming 50 - j200 to 50 + j0, where all we need is
a single inductor. But this isn't generally so.

A 5/8 wave radiator conveniently has a feedpoint resistance in the
neighborhood of 50 ohms, so it can often be matched well enough by
simply adding a series inductor as you suggest.

Roy Lewallen, W7EL