Hard line is a term used for RF transmission with hard and stiff materials used. Simple coaxial cable is found in most "hamshacks" which typically has a center, stranded conductor with a stranded outer, shield for transporting RF power. For repeater and mountain top use you need better shielding than simple coax can offer. LMR-400 and 9913 are improved versions of this coax and does help shield the RF transport to and from other stations at the site. Commercial and some Amateur repeater stations use hard line, which is far better than the mentioned coax types. It does come with a high price, mainly because more copper is used for such a line. If you happen to salvage some scrap pieces you can splice them together for that long run you may need for your repeater's antenna way up on a tower.
Hard line splice kits are expensive. They are like two connectors on the ends but specifically for just a splice. A cheaper way to do this is using some copper tubing and some physical effort. The usual hand tools, plus a high powered soldering iron will make this possible. strip back a clean portion each end, shinny up the copper that will be soldered. The outage "shield" is a piece of 1/2" copper tubing, while the center is some 3/8" or smaller such as use for refrigerant tubing. Slide the outer shield on first, then solder the centers, then solder the shield. They should come out like the pictures. You can greatly enlarge the pictures with a click on the center of any one.
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