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From: DD3LY / Andreé
Date: 17 Sep 2001
Time: 01:16:57
Remote Name: 195.145.17.185
Hi David, I do not know such a programm, but matching a halfwave vertical is not so difficult if you do not think of QRO. You can match it or I should better say you can feed it at one end by using a tuned or resonant circuit consisting of L and C in parallel. You feed the resonant circuit from the coaxial line: the outer connected to one end of the resonant circuit which is the "cold" end now and the inner to a tap at the coil which may be at about 10% of the total number of windings. The antenna which is halfwave resonant and may have a feedpoint impedance of about 8000Ohm is connected to the opposite end ot the resonant circuit which is the "hot" end and is the theoretical point of infenite (!!!)impedance (if L and C is ideal and the resonant circuit is not loaded). Let us rethink what values of L and C you should use. The antenna may be vertical or horizontal. This varies its capacitance. There is a rule of thumb (which I do NOT know actually) how much capacitance a straight conductor over a surface will have. With this rule (or an exact formula from a good book) you will be able to calculate the capacitance of the antennas conductor. If you choose a variable capacitor which is about 4 - 5 times larger than this value of capacitance, you will have no problems to tune your antenna independant from small variation due to wind, rain, or so on. Use a good coil to resonate the resonant circuit with a capacitor chosen as I said. Use thick copper wire, semiridgit cable or even copper tubing and try to make length and diameter the same for optimum Q of the coil. Of course use the doubled diameter of the conductor as "interwinding distance", no closewound please. Place the mentioned taps for the TRX coax line later by using a crocodile clip for finding the right tap points. Connect the cold end of the resonant circuit to ground or a cointerpoise (the antenna works even if you forget this because it is a halfwave radiator but it will work a bit better) which does not need to be resonant, use a rain gutter, a metal fence or whatever is there. If you do not want to use the outer of the coax feed line for this job which may be a possibility too, because it may cause TVI turn the coax feed line several times through a ferrite (NO iron powder!) toroid e.g. Amdon FT140-43. If you have a long run of coaxial line you may use the outer of the coax feed line for this job as it is a very unobstrusive antenna then. Place the ferrite toroid at a quarterwave or better a halfwave from the resonant circuit then. For further questions you are welcome: andree.knott@t-online.de