How to select a suitable sort of downlead

The best thing to use is good quality 75 ohm coax. RG-59 is the standard, inexpensive choice in this area. Many folks prefer RG-6 because of better shielding. Oddly enough, 300 ohm twinlead under ideal conditions may have less loss than coax. The trouble is that conditions are almost never ideal and coax is far superior to twinlead under most conditions (except pinching). There is also another option called shielded twinlead. It's expensive, hard to find, and hard to work with, so it's not really worth it. It is mentioned here in the interest of completeness. Whatever you get, make sure you get good quality cable and connectors. The few dollars you save here are going to cause you lots of grief in the long run, especially if you get cheap connectors.

Below is a chart of attenuation for coax downlead. This chart is for Belden cable and represents typical loss from "good" cable. Inexpensive cable can easily be much worse. Notice that UHF frequencies are attenuated much more than VHF.

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