[RodB]

You can think of the antenna loop as a rf transformer with a tuned secondary. One end of the primary can be connected to an external antenna to feed a stronger signal to the primary. The other end is grounded through a capacitor. One end of the secondary is usually connected to the large section of the tuning capacitor and also the control grid of the first tube. The other end of the secondary is usually connected to the AVC line in order to feed AVC to the grid. The variable tuning capacitor will set up a resonance at the desired radio frequency. You can determine the primary by finding the connection to the capacitor and measure continuity with one of the other connections. Should be less than one ohm. The other two connections are the secondary. This is very important to get correct because it affects signal strength.