Sunday, 7 December 2014

What's inside a old GPA 27 1/2


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! 


4 comments:

  1. Hello Bas,

    I know alot of people that used these antennas, although I was a fan of the 5/8 models myself. That transformer matches 50ohm to about 1000ohm, but the impedance for a resonant endfed is over 3000ohm. I am guessing the vertical element is not really resonant but a bit short, so the impedance is closer to 1000ohm and the reactive part of it's impedance (capacitive, because it's shorter than resonance) cancels out the reactive part of the transformer's (wich is inductive, being a coil - and airwound). Probably the cheapest way to match such an antenna.

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    1. Hello Razvan, your explanation makes sense. But how do they calculate this kind of transformer. There seems to be a program for it but till now I can't find it. So to make it work you need to keep in mind to use a shorter then resonance radiator. 73, Bas

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  2. Hallo Bas, het blijft een interessante antenne. Ik heb interessante info gevonden met de publicaties van G8JNJ over een breedband vertical. Intussen wacht ik op droog weer om mijn eerste brouwsel van de 5m vertical te gaan uittesten. 73 Hans, PE1BVQ

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    1. Succes Hans, het weer zit vandaag mee. Vanavond hebben ze het weer over storm. 73, Bas

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Thanks for your comment. Bedankt voor je reactie. 73, Bas