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| To: | <[email protected]> |
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| Subject: | LF: Re: Loop RX Antennas |
| From: | "James Moritz" <[email protected]> |
| Date: | Thu, 5 Jan 2012 21:33:36 -0000 |
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Dear Clemens, LF Group, So a RX noise figure of about 15dB would be necessary to have a SNR margin of 5-10dB.Good MW/LF receivers have such noise figures (at least I know one). 4x2m seems to turn out as the lowest reasonable limit with a sensitive RX. I was wondering why your estimate is so pessimistic on the required size of loop - in practice it is fairly easy to hear the band noise with a 1m^2 loop at 136k. Part of the reason is assuming a 15dB RX noise figure; of course, if using a suitable preamp, achieving < 1dB noise figure is usually possible, which reduces the loop size requirement. 15dB NF is a fairly typical number for a receiver with "good" sensitivity at LF - it amounts to sensitivity of about 0.4uV for 10dB SNR in an SSB bandwidth. Unfortunately, many receivers, particularly amateur HF transceivers with general coverage RX are much worse than this. My IC-718 is closer to 35dB noise figure at 136k, which seems quite normal. I think poor RX sensitivity is frequently part of the reason for disappointing results with small antennas, with or without preamps. Another aspect is what external noise level is it realistic to expect? Measurements here under quiet, daylight wintertime conditions indicate a field strength as low as 60nV/m per sqrt(Hz) could be reached when the man-made noise level is also low. The noise level rises typically 10 - 20dB at night. According to the formula in ITU-R "Radio noise" document P.372-10, this equates to an antenna "external noise figure" Fa of 88dB. The ITU noise curves allow for rather wide variations in possible noise levels at 136k. The ITU-R curves suggest about 80dB median Fa on a winter morning, rising to over 100dB late at night. Since most long-distance propagation occurs after dark when noise levels are higher, the noise performance of an antenna for long-distance DX is probably less critical than for receiving intermediate-range signals via ground wave during the day. Cheers, Jim Moritz 73 de M0BMU |
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