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!
9 comments:
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.
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
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
Succes Hans, het weer zit vandaag mee. Vanavond hebben ze het weer over storm. 73, Bas
The large diameter of the half wave Antenna tubing gives a lower impedance than a wire. Therefore the the estimated 1000 Ohm output of the transformer can be used for this perpose.
73 John pd7maa
Tnx for your explanation John, 73, Bas
Ik heb hier mijn antenne "opgeblazen". De transformer was doorgebrand. Ik wil een nieuwe transformer maken met dikkere koperdraad.
Maar ik kan na lang peinzen niet begrijpen wat wordt bedoeld met "There are 2 1/4 windings... and 7 3/4 windings..."
Leg aub uit wat met winding bedoeld wordt. Je kunt toch alleen maar helemaal rond of half, maar 2 halve windings is toch gewoon 1 winding? Je leest het al ik begrijp het niet. 73 PC1MH
Hallo Marc, het schema zegt het al. Het zijn 10 windingen in totaal echter op 2,25 winding vanaf de onderkant waar de massa op komt word het onderbroken, daar word de kern van de coax aangesloten )men noemt dat ook wel een tap. Daarna komen er nog 7,75 windingen waarna de antenne straler word aangesloten aan de bovenkant. Of dit met dikker draad net zo gaat werken kan ik je niet met zekerheid zeggen. Je kan eens gaan experimenteren. Succes. 73, Bas
Als ik het goed lees dan is dat 10 meter ???
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