This is a good article by Dave Lawrence. From what I have read about variometers, they are not ideal in a electrical sense , the rotary inductor or roller tuner is a better method of electrically adding length to the antenna.
https://www.qsl.net/ve3bdb/antmeas.html
These days, you can buy a nanoNVA device for hobby use. The VNA has the ability to analyze the complex impedance of the 19 set variometer and antenna and give a direct result in various forms. The nanoVNA are easy to buy on most of the popular retail platforms . And small spectrum analyzers are available for hobby use as well.
https://www.aliexpress.com/item/1005...d=gkAQdhUhNQmF
https://www.aliexpress.com/item/1005...170019037756_1