Firestik SS-3H Heavy Duty Stainless Steel CB Radio Antenna Mount Spring - Durable Truck & Off-Road Vehicle Accessory for Clear Communication

I have this attached to a 2-foot Fire-Stik, on the mirror of a class-8 truck. It does fine. There is some small amount of movement at highway speeds, but it doesn't flop around. More like a wiggle. It would probably handle a 3-foot Fire-stik, but I don't know about the longer ones. I tried the "medium duty" with the same antenna, the spring was not stout enough for the fire-stik - the antenna was flopping all over at highway speeds. I do have the medium duty spring attached to a lighter, smaller diameter 2-foot antenna, on the other side of the truck, it does fine.