On Thu, 18 Sep 2008 09:44:42 -0700, "Ron D'Eau Claire" <[EMAIL PROTECTED]> wrote:
>That's true. Indeed, if you have an "antenna tuner" (built-in automatic or >external manual) you don't even need to do that. That's what made them so >popular in recent years; they "fix" impedance problems at the rig end >without fiddling around with the antenna at all. > >The issue then becomes feed line loss. It can get quite high, especially at >the higher frequencies and with longer lengths of coax. It's not unusual to >throw away 50%, 75% or more of your RF as heat along a coaxial line that >way. > >But, if your antenna is designed to provide a decent match, or if you've >done some adjustments 'on the ground' that suggest you're at least >approximately correct, you're not likely to experience such drastic losses >even though things will change when the antenna is raised to its final >position. > >Doing measurements up on a tower is one of the major reasons for the >popularity of the modern "antenna analyzers". They're self contained and >small enough you can carry one up to the feed point. Most Hams only need to >do that at rare intervals, which is why many Ham clubs have a "club >analyzer" everyone's contributed to buying so members can borrow it on >occasion. > >A much cheaper approach is, as Bill says, to cut some coax to a multiple of >1/2 wave, electrically. Then the impedance you see at the end on the ground >will be the impedance at the antenna. The issue there is "how many times do >you want to climb the tower"? Perhaps a buddy at the rig on the ground and >you in the air with some HT's is the answer to that. > >Ron AC7AC > >-----Original Message----- [snip] I bought an AIM 4170 analyzer at the Ft Worth Hamfest this year and love it. When measuring antennas with it I calibrate the coax before making the antenna measurement with a piece of 100 ft RG213 on a ten foot mast. After I have done that I can measure the antenna as though the analyzer is attached to the terminals at the antenna. Then I put the antenna on the tower and measure it again with the same calibrated coax to find out what changed at the additional height. If it checks ok I connect the LMR600 to the antenna and call it good. Tom, N5GE - SWOT 3537 - Grid EM12jq "Those who would give up Essential Liberty to purchase a little Temporary Safety deserve neither Liberty nor Safety" An excerpt from a letter written in 1755 from the Assembly to the Governor of Pennsylvania. Support the entire Constitution, not just the parts you like. http://www.n5ge.com http://www.eQSL.cc/Member.cfm?N5GE _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
