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Michael wrote in
: On Jul 2, 7:05*pm, Owen Duffy wrote: Michael wrote innews:7c2889df-092d-4471-bfb2-6db : ... * So how do I go about feeding a Lazy H with ladder line back to the tuner for high power without over heating the balun? *The tuner is a Dentron MT-3000a. *The built in balun is not balanced at all at 10 I will respond in a couple of hours, but in the meantime, please clarify that everything between the ATU and LazyH is up for consideration. What impedance line are you using? Do you feed the antenna midway between the elements of at one element (branch or distributed feed)? What do you think the feed point impedance is? What band(s)? Owen I am using 450 ladder line. The Lazy H is center fed with 1/2 wave spacing between the top and bottom elements and the elements are 1/2 wave elements. 450 ladder line is connecting the top and bottom elements. I am not using the expanded Lazy H version. This is the classic center fed Lazy H design as shown in the ARRL antenna handbook. I only intend to use the antenna on 10 meters. I don't know what the exact impedance is (sorry). The Lazy antenna is about 50 feet off the ground at the top wire. The amplifier I want to use with the Lazy H is a Drake L4B with two Eimac 3-500z tubes. Ok, if we define the feed point to be half way between the dipoles, you ought expect that the feed point impedance is relatively low, some where of the order of 50 ohms, give or take some reactance and it is a fairly balanced / symmetric load. Note that the feed system to each dipole is a tuned length of transmission line, that makes this a narrowband feed system. So, your challenge is to deliver the transmitter a nominal 50+j0 load. Re the ATU's integral ATU, always regard them as likely to be unsuitable. Let's regard that, integral balun aside, your ATU is probably capable of matching a fairly wide range of impedances at 10m with reasonable efficiency. If you objective in using a balun is to minimise feedline direct contribution to radiation, your objective is to try to force equal but opposite currents in each conductor. An ideal voltage balun will approach that objective ONLY with a perfectly symmetric load. If you think you have a perfectly symmetric load, it is probably because you haven't measured it. An ideal current balun will approach the curent balance objective irrespective of load symmetry, so it is a better choice for your application. Why do you need a 4:1 transformation? Depending on your feedline length, the impedance presented to the ATU may be as low as somewhere round 50 ohms, and perhaps as high as several thousand ohms. It might seems good to transform several thousand ohms by a factor of 4... but transforming the lower impedances by a factor of 4 exacerbates ATU losses. Since wavelength is short, I would be inclined to try to use a feedline length of an integral number of half waves, and an effective 1:1 current balun at the ATU. This minimises working voltages within the balun and ATU (reducing risk of flashovers), without driving high losses in the ATU are very low impedances. If you think about your own requirements, and finding a solution that fits, you will do better than Googling up HamUniverse articles and the like. Owen |
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