Any impedance can be represented either in a series or parallel form.

Then analyzing a matching circuit or using a Smith chart, we often

end up converting back and forth a couple of times.

Either will tell you when the load on the analyzer is resonant.

If you are adjusting a series element (a base loading coil, for example)

then the series reactance tells you how much reactance you need.

If you are adjusting a shunt element (a beta match, for example) then

the parallel form is easier to use.

HOWEVER: when the SWR is not 1 : 1 (as it probably isn't if you are

still trying to match the antenna) then you can't count on X=0 as an

indicator of antenna resonance

*unless the analyzer is connected*

directly to the base of the antenna with zero coax length.This is one of the biggest problems that has been introduced with

SWR analyzers that measure reactance due to a mis-understanding

of transmission lines.

Let's take an example: assume you have a vertical antenna with

a base loading coil. The feedpoint impedance is 25 ohms, with adjustable

reactance in the form of a coil. We'll use 20m, and connect the

SWR analyzer through a 6' length of RG-213, then adjust the antenna

reactance until you get an X = 0 indication on the analyzer.

You can work out the results using VK1OD's transmission line calculator:

http://vk1od.net/calc/tl/tllc.phpIn this case, when we adjust the antenna for X = 0, we end up with

about 13 ohms, so the SWR is around 4 : 1. But if the antenna really

is 25 ohms, shouldn't it be closer to 2 : 1? Yes, it should: the actual

feedpoint impedance of the antenna in this case is 25-j45 ohms. It

is NOT resonant, even though you are measuring X = 0 at the analyzer,

because of the impedance transformation caused by the connecting

coax. If you tuned for minimum SWR instead (using the same coax length)

the impedance at the antenna feedpoint would be very close to 25+j0

ohms (basically resonant) even though the impedance measured by the

analyzer would be 42+j31 ohms (due to the impedance transformation

along the coax.) Even one foot of coax in this case is enough to add

about 5 ohms of reactance to the measurement.