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On Mon, 23 Aug 2004 04:24:52 GMT, Richard Clark
wrote: Workmanship and quality materials tests those reputations vastly more for smaller antennas than standard sized ones. Those 1 meter loops used for HF are not rated for the lower bands for very good reasons, and they claim (and I believe them) high standards for their product. However, if you could resonate them in the 160M band, you'd be lucky to see 1% efficiency. You would be lucky if you could get 10 % efficiency at 40 m for these 1 m loops. Since the radiation resistance is inversely proportional of the fourth power of frequency and the skin effect losses proportional to the square root frequency, one could expect to get nearly 1 % efficiency at 80 m and well below 0.1 % efficiency at 160 m. On the European 135 kHz LF band, the practical vertical antennas are usually less than 0.01 .. 0.02 WL, the estimated efficiency is less than 0.1 %, so more than 1 kW has to be driven into the antenna to even get 1 W of ERP. This 1 W ERP limit is used by many countries and still narrow band contacts of several thousand kilometers are made. Unfortunately, trying to compensate the low efficiency in a small magnetic loop with a high transmitter power is not very practical, since the voltages would be huge. Thus, if some exotic small antenna with inevitably low efficiency is to be used, I would first check that it can constantly handle the full legal limit power, so that it would be possible to compensate for the lower efficiency. The low antenna efficiency is not much of a problem in receiving on LF, MF and lower HF frequencies, since the band noise is still well above the receiver front end noise. However, on upper HF and above, a low efficiency will degrade the reception, especially if the receiver noise figure is high (which it often is in HF receivers that try to maximise the intermodulation performance). Paul OH3LWR |
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