An small air gap is generally capacitive and will pass the higher frequencies while rejecting the lower ones. In the old days (when we only had a few channels

An small air gap is generally capacitive and will pass the higher frequencies while rejecting the lower ones. In the old days (when we only had a few channels on cable television), the CATV systems had a lot more loss and ingress (60Hz from the AC and other signals induced onto the center conductor). To support the higher frequencies now used, the cable has to be carefully installed and maintained. In the '80's we could get full cable TV by pointing a large Yagi antenna at a particularly strong "leak".