At 2/17/2008 14:24, you wrote: >I have access to a spare run of about 150' 1-1/2" heliax at a repeater >site. It goes from the "Radio Room" to the roof. There is no antenna >connected to it. I would like to verify if it still any good. >Obviously, hooking it up to a TDR would be a preferred method of >verification. Next best method probably would be hooking it to a >transmitter with a Bird thruline at the transmitter and a Bird >termaline at the end on the roof. I could then verify SWR and verify >actual transmit loss. Lacking that, I wonder if a megger, which I do >have, might be of use? If I were to suspect if the heliax is bad due >to moisture incursion, one might be able to see a low resistance path >between the center conductor and outer shielding. Has anyone tried this? >Is there a spec, or reasonable approximation for insulation resistance >center to shield per 100 ft or the like?
The 2 tests I'd perform are: 1. Simple loss at your TX frequency: measure the TX power going into & out of the feedline. At 450 MHz I'd say it should be no more than 1 dB. 2. Connect the antenna port of your repeater's duplexer to one end & put a dummy load at the other. Make sure there is no desense. We have a couple of antenna systems at work that "look" fine VSWR-wise, but generate lots of noise if we try to put RF into them. One of them started doing this after it got hit by lightning. It's a Sinclair 4-dipole array, so the problem is very likely with damaged coax, either the 1/2" hardline feed or the coaxial phasing harness. Bob NO6B
