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Curious about the I.R.E. Standard Dummy Antenna
Brian--
Your spice simulation looked familiar...VERY familiar. As it turns out, I had plotted similar curves and happened to have used Microcap as well! I was just not sure I understood the relationship between the transfer function I saw and some kind of physical antenna. Your comments make sense, though I do have one question: How did you calculate the impedance of the antenna with the shape and dimensions that you described? 73 Pete AC7ZL Brian wrote: Pete, I was curious about the dummy antenna, too. Some time ago I modeled it and checked the impedance looking back into the output leads with a 50-ohm generator at the source. The results are he http://users.tns.net/~bb/dummy.htm Before I did the model I figured that the dummy antenna was trying to simulate a certain length of wire on shortwave. But you can see that the magnitude of the output impedance (upper plot) is around 400 ohms throughout the shortwave part of the spectrum. Down in the broadcast band the network becomes almost purely capacitive, which is what a short wire will do. In fact, at 500 and 1000 kHz (the two spots I checked), the dummy impedance is almost exactly that of a wire that runs 15 feet vertically and then 90 feet horizontally over perfectly conducting ground. I imagine they had a 100-foot wire in mind, a figure I think I've seen recommended for BC antenna length in the old days. So I think the network is intended to be 400 ohms resistive at SW and at BC behave as a 100-foot wire would. I align radios with the network and then at installation I tweak the antenna capacitors, if they are easily accessible, on the actual antenna. Brian |
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