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Oops.............that should have been maximum return loss. Touch
typing...........you know! Pete "Pete KE9OA" wrote in message ... "Frank Gilliland" wrote in message ... On Sun, 10 Apr 2005 19:58:28 GMT, "Pete KE9OA" wrote in : Hey Vinnie............I can help you. The formula for a 1/4 wavelength radiator is (234 / F) whereby F is the frequency in MHz. This will give you the length in feet. For the radials, (237 / F) will give you that length. Always remember...multiply the radiator length by 1.05 and that is another way of calculating the 1/4 wavelength radials. It should be noted that these formulas are only appoximations. Actual sizes are dependent upon the conductivity and diameter of the elements, and the quantity and angle of the radials. When building a resonant antenna it's a good idea to make the elements a little long and trim to resonance. Very true..........I use a spectrum analyzer with a directional coupler and trim for maximum return loww. The feedpoint impedance of a 1/4 wave ground plane is 37 ohms when you have the radials at a 90 degree angle with respect to the radiator. If you have the radials drooped at a 45 degree angle, the impedance rises to approximately 50 ohms. If you have a single radial drooped at a 180 degree angle with respect to the radiator, the impedance rises to 75 ohms. These are impedances for antennas in free space, and are practical only if you can mount your antenna well above the ground and away from any tall objects. Also true, but a good starting point. There was an article in RF Design magazine a few years back, explaining why certain impedances are used in the RF industry. 72 ohms was the impedance that produced minimum cable losses...........50 ohms is a happy medium. Close. For lowest loss, the optimum characteristic impedance of coax is 76.9 ohms. 70-73 ohm coax is used as a compromise between low loss coax and coax optimized for minimization of flashover, the latter having an impedance of about 60 ohms. Good memory. And while many people have many different ideas as to why 50 ohm coax is made, it is just a compromise between low-loss/low-flashover coax of 72 ohms and coax optimized for handling power (about 30 ohms, which is too lossy for practical transmission lines). On a final note...........at 37 ohms, you will have a VSWR of 1.3 to 1.................at 75 ohms, you will have a VSWR of 1.5 to 1. What is the difference here? For a transmitter with a tube output and an internal matching network, you wouldn't really see much effect. For a typical solid state transmitter, there would be some difference between the two antenna impedances, because the broadband solid state transmitter would be called upon to deliver more current to the antenna. It probably wouldn't have any effect, unless the ALC circuit was aggressive in its operation. In this case, power foldback would occur into the 37 ohm load. Would it happen? Probably not. .....uh, what? Both tubes and transistors use matching networks, so I don't know what distinction you are trying to make there. Power will be reflected from an antenna/coax mismatch -regardless- of whether you have a tube or transistor final. And what does an ALC circuit have to do with transmission line propogation? This pertains to solid state amateur transceivers that don't have an adjustable output matching network.........most of today's units have fixed-tuned bandpass filters after the output stage. I am not referring to transmission line propagation; I am referring to the fact that, with a fixed-tuned output network that expects to see a 50 ohm characteristic impedance, the ALC can fold back the power. Usually, that doesn't occur until a VSWR of 2 to 1 is reached. My explanation is for illustrative purposes only. I hope this helps. Pete ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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