> Probably true for something I put up on my own, especially if > I needed > to get it up and 'making noise' quickly, but I would want to work > towards better, 100% shield feedline as fast as posssible. > And I would > expect, and be the first to tell users, that it's not going > to work as > well as it could with better coax, that there will probably be some > desense or noise on windy days, etc. > > -- > Jim
There has been plenty of back and forth here regarding shielding quality for feedline runs, and the reason for using braid+foil shielded cables and solid-shield (Heliax) cables. The other issue, which, in my opinion, is more critical is the cable's tendancy to produce noise in a duplex installation, or even in a half-duplex/simplex installation if it is in a high-RF field. Cables with braided *copper* shields will make noise, lots of noise, when flexed as they age. It doesn't take much time (nor air, nor moisture) for surface oxidation to form on bare copper. Copper (cupric? cuprous?) oxide is a poor conductor. Slight flexing or movement will cause microscopic connections to be made and broken as the braids rub against each other. The higher the RF environment the cable is in, the more noise will be created. Movements as slight as vibration from cabinet fans can cause an increase in the noise floor; the cable doesn't have to be flopping around in the wind to make noise. Braid-over-foil cables (9913, LMR, etc.) have a simliar problem as copper-braided cables because the braid and foil are dissimilar metals (usually aluminum foil tape and tinned-copper braid). Cables with braided, silver-plated shields don't suffer the majority of the effects that copper and foil+braid cables do. Unlike copper oxides, silver oxide is conductive. The noise problems created by cable movement is much lower with silver braided cables. RG-142B/U, RG-400, RG-393/U, RG-214/U, RG-223/U, etc. are all make very good patch cables, and for the most part, their double-braided construction provides adequate shielding. Braided-shield cables can't compare in shielding effectiveness to solid-shield cable. Andrew has a somewhat useful document regarding this: http://www.andrew.com/search/docviewer.aspx?docid=737. An important thing to note, which is discussed in the Andrew doc, is the shielding effectiveness at the *connector* as well. BTW, Times spec's LMR-series cables for >90 dB shielding effectiveness in their literature, though I've never seen any actual plots or other analyses like Andrew did. Bottom line: Heliax and other solid-shield cables give you the best of both worlds when it comes to shielding effectiveness and low-noise performance. Between Ebay, hamfests, local tower crews, and good old fashioned ham scavaging, you can find Heliax for very, very reasonable prices, or sometimes free if you know where to look. --- Jeff --------- Jeff DePolo WN3A Broadcast and Communications Consultant Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
