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Curious about the I.R.E. Standard Dummy Antenna
Well put, John.
I have understood the purpose of the 'matching network' for years, but I have been puzzled by the source impedance expected on the generator side. I have several RF generators of various vintages in my collection and most of the older tube ones have most peculiar output attenuators that would have impedances that vary all over the place depending on the setting of the output attenuator, frequency and, to some degree, output cable length. On 'modern' generators, once you cut in 10dB or more of attenuator, the output inpedance it pretty close to the specified, but even many of them are quite a low impedance at 0 dB. Perhaps someone with spice can characterize the output impedance of the IRE network when fed from a generator of 0 ohm, 50 ohm, 75 ohm and, say, 200 ohm output impedance. Neil S. John Byrns wrote: In article , wrote: Michael A. Terrell wrote: "H. P. Friedrichs" wrote: I am interested in the origins and purpose of the "IRE (Institute of Radio Engineers) Standard Dummy Antenna," as used during the alignment of radio receivers. Its to match the receiver to the generator. Or more specifically, it's to present a uniform high impedance from the generator to the radio. At the time, with the exception of a few lab grade instruments, signal generator outputs were all over the map with regards to output impedance. The "dummy antenna" presented a fairly uniform, and high, impedance to the radio and kept from detuning the front end while doing an alignment while connected to a signal generator. This doesn't sound right, why would it be desirable "to present a uniform high impedance from the generator to the radio"? The impedance of the "IRE Standard Dummy Antenna" is not exactly what I would call "high" especially on some of the short wave bands, exactly what is a high impedance in this context? It was my understanding that the purpose of the "IRE Standard Dummy Antenna" was to provide a standard antenna to use for making performance measurements, as well as for alignment at the factory and on the service bench. As to the exact design of the of the "IRE Standard Dummy Antenna", I understood that it was meant to represent the impedance characteristics a typical antenna that might be used by a radio set owner. This topic has been discussed here before and I seem to vaguely remember that someone may have even dug up the exact nature of the antenna the "IRE Standard Dummy Antenna" was designed to simulate. IIRC if you are only doing MW broadcast band alignments a single capacitor will provide a very good approximation of the "IRE Standard Dummy Antenna" across the MW broadcast band. The coil, resistor and second capacitor are there mainly to provide the desired impedance on the short wave bands where the prototype antenna has resonant effects There are at least two problems that the "IRE Standard Dummy Antenna" solves, the first being standardized measurements for obtaining performance specifications, as it is possible for a manufacturer to "improve" his products specifications by using his own specially designed "Dummy Antenna". The second problem is how to align sets in the factory, or on the service bench, when the final users actual antenna system is not available. The problem is that the alignment of the antenna stage of a radio is greatly affected by the impedance of the actual antenna that is used with the radio. The "IRE Standard Dummy Antenna" provides an approximation of what was assumed to be a typical consumer antenna. Of course what you really want to do is align the set with the actual antenna attached that it is used with. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
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