Procomm Medium Duty Stainless Steel Antenna Spring -   JBC403 4" Medium Duty CB Spring for CB & Amateur Radio Anrennas. Medium Duty Spring suitable for Stainless Steel & Fiberglass antennas up to 4 feet long. Spring helps protect the antenna, mount and vehicle. Springs are made from long lasting, top grade electro-polished stainless steel & have the standard 3/8-24 base thread.