A reactance can not dissipate power.
The output of a transmitter is stated for a certain load, in radio most times 50 Ohm real. This power goes through a transmission line to an antenna. How much of that power really is transmitted depents on the rendement of the antenna. So you can put 100W in the antenna but it is possible only 1W isbtransmitted. The rest is lost in transmissionline losses, leakage, radiationnon the wrong places, coperloss, dielectric loss, ground loss, skin effect ect.
Vswr tells you nothing about the lenght of the antenna. It tells you id a load is matchted to the transmissionline, or tuner, transmissionline and load to a certain value like a 50 Ohm vswr meter. ( the transmitter likes to see 50 Ohm, it's outputvimpedance is not 50 Ohm. ( radio transmitters, a signal generator often has an internal termination.
There are two flavours, most known is VSWR voltage standing wave ratio. If a transmission line , lets say coax, is not matched in impedance to the load, for instance an antenna, the EM wave will partly or total reflect back to the source ( transmitter) .
If you do not terminate the coax with it's characteristic impedance ( 50 Ohm for instance) but short it, voltage at the end will be minimal, current maximal. A short does not dissipates the power so 100% returns to the transmitter. Or you leave it open, voltage is max, current is minimal, and all power is reflected back. Max swr again.
If you connect a load with the same impedance, in this case 50 Ohm, all power will be dissipated in the load and the swr is 1. (Not 1:1 like you often read)
But most times, in netwerk analyse, the reflection is measured in return loss and phase or as reflection cooficient. The latter tells you more about the load as VSWR. A vswr can be calculated to impedance but gives two solutions. A high and a low value. Like short and open both have the same vswr.
The reflection coofficient however, is 1 for open, 0 for a match and -1 for short. So this makes it possible to calculate |Z| the impedance magnitude. But to measure the rectangular complex impedance Z split in R and jX, you need phase too. That is measuered with a vector network analyser. From these to parameters you can calculate everythng else, like vswr, capacitance, inductance, Z= ( R+ jX ), Z= ( |Z|, phase), groupdelay, ect.
On my site I have a number of tutorials about netwerk analyse and one chapter is about antennes. There is also a separate tutorial about antenna basics.
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