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## [TowerTalk] >LONG< Pse help calculate my feedline's length, it is 1/2"

 To: [TowerTalk] >LONG< Pse help calculate my feedline's length, it is 1/2" CATV Alum Hardline K4OJ@aol.com (K4OJ@aol.com) Tue, 23 Sep 1997 23:32:54 -0400 (EDT)
 ...OK, I have some nice brand new, manufactured in 1997, CATV hardline that I want to use to feed the new 20 meter beams....my question....what should I anticipate as a velocity factor? I will try and cut it so that it is (a 75 ohm) half wave multiple on 14.025.....making its 75 ohmedness transparent. In order to know roughly how long to cut the coax, I need a starting point to cut a little long, so that I can cut a little back....then a little more.......then a little more....and then OH SH** TOO MUCH! I will aim for 14 MHz as my target when cutting so that a piece a little too short will hopefully be at about 14.025, eh? The CATV line will actually be barreled into some RG11 at the shack end so it is flex-ie for attachment to either the ground panel or the coax switch 20 meter position. The coax is only going to be used for 20....so it is ok for it to be one length only... The ARRL Antenna Book says VF of 1/2" Aluminum Jacket Foam Dielectric coax is .81.....my question to you all is: Have you recently "started" where I am starting, and did you find that the .81 was a good number? If you have recently gone through this exercise I would appreciate your input....especially if you found that what really happened was it ended up 10 foot too short, etc. Taking it another step....please correct me if I am wrong The equation (I believe) for converting a frequency to its electrical length is: One w/l in Feet = 983.6/F(MHz) * VF so, for me: One w/l = 983.6/14.025*.81 ....or.... 56.8 feet for one w/l of the hardline.... I think..... THEREFORE, If I am shooting for a multiple of a half wavelength to make the line's impedance a non-issue I should shoot for a multiple of half the above, a multiple of 28.4 ft.... Am I on the right wavelength? (couldn't resist) Since it is about 140 feet from the shack wall to the switchbox for the 20 metre antennae, I could follow this logic, but it may be flawed (this is where you come in).... I will aim for overall run of hardline that is four times 28.4, or 113.6 feet.....then NEXT QUESTION Since it will then barrel into some flexable 75 Ohm coax, RG11, which has a different velocity factor.....should I aim to make the tail of RG11 simply a multiple of 1/2 wavelength.....or 983.6/14.025*.66 (RG11 has a .66VF) = 46.287 feet........ This means that physically I would have 113.6 feet of hardline plus 46.287 feet of RG11 or 159.887 feet of overall feedline length...... ----> OR <------ Do I need to worry about each of the types of feedline having their own lengths in terms of multiples of 1/2 wavelength at their own velocity factors, OR Should I just make a hard line run that goes from the switch box at the tower to the wall-through, no matter what that length is (roughly 140) and just make my flex piece of RG11 my tuning adjustment for the line as a whole......accepting it as simply the next 1/2 wavelength longer than the hardline was..... I would like to be able to simply mechanically cut the hardline to the length it takes to go from the switchbox to the shack wall, without looking at v/f as an issue. I do not know if the RG11 tail on it, which will have a different velocity factor, mandates the hardline be a multiple of 1/2 wavelngth.....do you get my drift? If I can simply cut the hardline to a physical length, I can then use the flex tail as a tuning element inside the shack.....it will be the right length to make the whole thang look like a half wave at 20 meters....but, does that flex piece change the behaviour of the .81 v/f hardline....or should I indeed look for it to be around what the v/f calculations say? I want this feedline to seem like it was 50 ohms, like the rest of the system will be.... Thanking you all very much, I will summarize your input to the towertalk reflector. de - an appreciative QCAO Jim, K4OJ k4oj@aol.com -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search
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