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![]() "Reg Edwards" wrote It is equally correct to say that the radiation resistance of a dipole is 2*73 = 146 ohms, since this is the value when the radiation resistance is assumed to be uniformly distributed along its length as is the conductor resistance. For calculating purposes the radiation and conductor resistances can simply be added together. ===================================== Hints and tips - By the same arithmetical reasoning - The centre feedpoint input resistance of a 1/2-wave dipole is 73 ohms plus HALF of the conductor end-to-end HF resistance, which to be precise sometimes matters. This is exact insofar as 73 is exact - not an approximation. The HF resistance of a copper wire at 20 degrees C is - Rhf = Sqrt( F ) / 12 / d ohms per metre, Where F is the frequency in MHz and d is the wire diameter in millimetres. For example, the end-to-end loss resistance of a 1/2-wave dipole at 1.9 MHz using 16-gauge copper wire is 7.07 ohms, which increases the feedpoint resistance to 73 + 3.53 = 76.53 ohms, to give a radiating efficiency of 95.4 percent. Some people would consider that's enough to lose a contest! Why not start a little notebook to record useful, simple, little formulae such as above which don't appear in Terman et al? Or if they do appear then you can never find the right page in the right volume. ---- Reg. |
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