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Curious about the I.R.E. Standard Dummy Antenna
Brian,
This link seems to be broken. Pete AC7ZL Brian wrote: Pete, I forgot to mention the reason I believe the IRE dummy antenna tries to look like a 400-ohm resistive load at shortwave instead of a certain length of wire. You can see why he http://users.tns.net/~bb/antlen.gif This is an old RCA chart showing the behavior of an end-fed wire for lengths to 125 feet. It indicates the many resonances and antiresonances a short wire can exhibit. Since the antenna impedance and the resulting performance vary so markedly with wire length, and because it would be unrealistic to expect all listeners to use one particular length, I think the dummy antenna is just intended to roughly match the input impedance of a typical radio. I'm not sure about consumer radios, but old communications receivers all seem to have a specified input impedance of either 300 or 400 ohms. On the broadcast band, any wire shorter than about 150 feet will look capacitive. It won't exhibit the resonances you see in the chart at shortwave. So even if the wire is shorter than the 100-foot length the dummy antenna seems to model and exhibits a higher capacitive reactance, it won't affect the radio's RF tracking that much. On the broadcast band, most radios seem to use rather loose antenna coupling to minimize mistracking. This allows them to accomodate antennas of various length. There is an interesting discussion about antenna coupling strategies in the Radiotron Designer's Handbook. Brian |
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Curious about the I.R.E. Standard Dummy Antenna
This link seems to be broken.
Pete, I remove temporary items from my web site pretty quickly due to space restrictions. I tried to e-mail the chart to you but got the following reply: ----- The following addresses had permanent fatal errors ----- (reason: 591 your host [216.86.143.7] is blacklisted by bl.spamcop.net. No mail will be accepted) No more midnight spam for me! Anyway, the chart is back up for a couple more days. By the way, on the perfboard I used to build my dummy antenna I added a 0.1 uF coupling capacitor between the signal generator input and a second output lead with an alligator clip. This provides a 50-ohm DC-isolated signal for IF alignment. I usually align radio IFs by sweeping a signal at the front end, often just blasting through the RF and mixer stages at 455 kHz. But the alligator clip is handy when I want to drive the mixer grid directly. You can drive the grid with the dummy network, but the 50-ohm source impedance of the direct output ensures that the parallel impedance at the grid doesn't affect the sweep flatness. Brian |
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