[scooby214]

That is a nice antenna - good work!

The problem is the need to adjust the variable capacitor on the antenna, especially if you want to sell the radio or give it as a gift.

Does anyone have any ideas on how to make an antenna which either does not need any such adjustments, or which adjusts automatically with some sort of circuit (does not have to be tubes)?

There are ways to make one without the variable capacitor, but the tunable antenna will provide better performance. That is why the AM antennas for sale, such as the Select-A-Tenna have tuning controls. If you are looking to make the antenna for yourself, you should go with the tunable one. If you decide to later sell or give away the radio, you can keep the tunable antenna and make a similar loop without the variable capacitor.