----- Original Message -----
From: "Gary Hoffman" <ghoffman@spacetech.com>
>I think the point being missed here by one of the posters is that if the
> feedline is really excellent....like the 1/2 inch Heliax, or perhaps 450
> ohm
> transmitting ladder line, or less radically, perhaps Times Microwave
> LMR-400, the losses in the feedline are trivial, no matter the mis-match.
>
> Then having the tuner in the shack is extremely handy, and results, as
> indicated, in only trivial losses.
>
Yes and no, Gary. Using good coax certainly helps, but as the
feedpoint impedance gets very high (like what you get when
operating a 1/2 wave dipole on an even harmonic) then losses
skyrocket even with very good 50 ohm coax, whereas the losses
stay relatively low with open wire line. The program TLW that
comes with the ARRL Antenna book is a handy tool for looking
at this. Consider a 3000 ohm load impedance (probably not
unreasonably for an 80 meter 1/2 wave dipole operated on 40
meters) feed with a 100' long feedline on 7 MHz. Using 600 ohm
open-wire line the total line loss would be 0.077 dB. If you switch
to 1/2" heliax, line-loss goes up to 3.1dB!! Line loss for 1 1/4"
diameter heliax is still 1.6dB. Using a tuner in the shack to match
a short run of coax is not a bad tradeoff if the antenna feedpoint
VSWR is modest (< 5:1) , but for very high antenna input
impedances you can really start to compromise system efficiency
even when using the best coax.
73 Mike W4EF.......................................
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