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Hi Charles and Dave and Sean,
The RATIO of the diameter of the INSIDE of the OUTER conductor to the OUTER diameter of the INSIDE conductor and the dielectric constant of the medium in between the conductors determines the impedance of a coaxial cable (or adapter). This is why, if you look inside the gold plated end, you will see the center conductor is a completely different size than the 50 ohm center conductor.

Tek gold plated the 125 ohm ends of these GR connectors to distinguish them from the 50 ohm end. ANY gold plated GR connector is a 125 ohm impedance and guaranteed to be part of the original set of 50 ohm to 125 ohm adapters and tools which came with every 519 in a beautiful mahogany box. I had one of those boxes many years ago. My friend Barrie Gilbert bought it from my friend who had most of the adapters that went with it when Barrie came to my house to pick up his 519.

A picture of the outside of the mahogany box is at

Unfortunately I no longer have a picture of the inside of the box with the adapters.

By the way, the price he is asking for these adapters ($12 each) is ridiculously cheap. DO NOT HESITATE to use his BUY IT NOW option. There are many, many others who will take the extra ones from you for twice what you pay for these four.

Dennis Tillman W7pF