Everyone seems to have the right idea, therefore, another reply probably isn't necessary.. However..
If we're going into the testimonials, I am using 200 feet of 1/2 inch 75 ohm aluminum on a 440 MHz repeater with no means of matching. You can effectively build a matching stub with 60 whatever ohm coax but unless you have the materials readily available, it's not worth the trouble (tried it once myself).. Another fellow mentioned something about the impedance repeating itself if the length of the feedline was just so. This is true. If the electrical length of the hardline was a multiple of an EVEN half wavelength, then you will see a mirror impedance of the antenna (50 ohms). However, if the length of your feedline just happens to be an odd quarter wavelength multiple, you have inadvertedly created a matching transformer which changed your 50 ohm antenna into a 100 ohm impedance at the other end of the feedline. One says to put a watt meter in line, then "trim" the coax until you get a good match. But take the watt meter out of line and you have altered the length of the cable once more and without careful calculation, your efforts are trashed. The worse thing that could happen is you have a mismatch with an impedance of 100 ohms at the end of the feedline going into your duplexer. If your VSWR was 2:1 then you're just over 10 % reflected power. Not that big of a deal. In fact, since 75 ohm feedline has a smaller center conductor, the loss is actually less than 50 ohm of comparable size due to less capacitance to the shield, and less surface area actually touching the dielectric. The point is, the few tenth's of a dB you may lose by running mismatched cable is made up in the end because the 75 ohm cable has a few tenth's less loss. You'll never know the difference. Don't change a thing. In fact, most of the CATV hardline has a sticky dielectric which repels moisture. 73, KC4FWC http://www.w4dex.com/kc4fwc/443825.htm --- In [email protected], "k9bf" <[EMAIL PROTECTED]> wrote: > I have a repeater attached to 300 feet of 75 ohm CATV hard line. I > believe one can match using a length of 62.5 ohm coax, but how long > and where to get it? The repeater is on 443.450 > thanks > Ben K9BF Yahoo! Groups Links To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ To unsubscribe from this group, send an email to: [EMAIL PROTECTED] Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
