I recently discussed some antennas with Hans PE1BVQ. Hans is looking for something he can
use for 21-30 MHz with a antennatuner that can be placed vertically and is doing better as his sloping 3 band endfed. We came across a "original" GPA 27 1/2 that was sold on the internet for a reasonable price. This kind of antenna was the most sold antenna early eighties of last century when CB became legal here. I think the original was made by several compagnies, all looking almost the same. I name HMP and Hygain from what I remember...
Of course we know this antenna now as the populair endfed. A half wave antenne fed with the use of a transformer. For my endfed test back in 2012 I disassembled the transformer and kept it for further investigation. I'm still curious why it is designed like that and on the internet the only thing I could find is that it is a wideband way of adjustment. The transformer wire is insulated and the core is about 0,8mm2 twisted copper wire wind on a 30mm long plastic coil holder with a diameter of 32mm. There are 2 1/4 windings from "earth" to "center" of the PL plug and 7 3/4 windings from "center" PL plug to radiator.
I have no idea what else I could tell about this. I do know a little how they build a endfed and there are several ways to get a 50 Ohm match. But you don't find this type of simple matching often. Though it was commonly used in the old GPAs. Probabely one of my readers with more technical knowledge can tell why this kind of matching transformer was used? How do you get to the amount of windings or is that just a matter of trying? Don't think so as this type of antenna was produced by hundreds!