I wrote most of this in response to a question on eham.net about what kind of coax to use for 2 meters.
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For UHF/VHF use, the best coax is the shortest run of the lowest loss you can afford that also meets your mechanical requirements - will you bury it? run it up a tower? Does it needs to be flexible, etc. For 2 meters (weak signal work especially) 9913 would be the highest loss I would put up with. LMR400 would be better, and LMR600 or a good commercial hardline would be better yet. Loss will be much less important if your run is short, say less than 20 feet, but at a hundred feet or more I would definitely be getting the good stuff. This will become more clear as you look at the loss characteristics of different types of coax.
A longer run means more loss. Take a look at cable specs at cablexperts.com or belden.com. RG8x has from 3 to 4.7 db loss per hundred feet at 150 MHz, depending on construction, while RG-213 has only 2.8 (still almost half your signal lost). Forget RG58 for 2 meters and above, with 4.3 to 6.75db/100' lost. LMR400, on the other hand, at 1.5db/100' will do a much better job.
Feedline quality will also affect SWR in that reflected energy will be lost on the way back to the SWR meter, so that a lossy feedline may make the SWR lower, even though the performance of the total system may be very poor. When any antenna shows a low SWR across a wide bandwidth I am suspicious - something is not right (loss is high).
Also, loss in the feedline is basically subtracted from antenna gain. As you can see, the feedline is a key part of your antenna system - no sense paying big bucks for a big gain antenna only to lose the gain saving smaller bucks on feedline (and you might have to pay extra for bigger rotor, tower, etc, as well as have a higher risk of weather damage). A decent (but not huge) antenna with top notch feedline may perform better and be significantly more economical.
For serious performance, or if for some reason you could only use a lossy feedline, a mast-mounted preamp would be optimal. If you must use lossy coax a preamp will compensate for your losses on receive, but won't stop a lot of your transmitter power from just melting the ice off the coax. An amplifier, in such a case, would help on the transmit side, but should be complemented with the preamp lest you become an "alligator" station - all mouth and no ears.
The bottom line is, don't neglect your feedline when designing your antenna system. Also remember that a good antenna system helps both transmit AND receive, while an amplifier may only turn you into an alligator.
(The author usually runs a "fruit bat" station - all ears and no voice.)
Feel free to contact me with your questions or comments.
73 de kt8k - Tim
