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Top Band Antennae?
"Jeff Liebermann" wrote in message
... On Sat, 21 Feb 2015 11:09:31 -0000, "gareth" wrote: "Jeff Liebermann" wrote in message . .. On Fri, 20 Feb 2015 21:38:30 -0000, "gareth" wrote: Many years ago (50?) it was to adapt a mobile area for fised station use. Huh? What's a fised station? typo - one key lateralised - fixed Is that because everything in the station needs fixing? Only the photographic negatives |
Top Band Antennae?
On Fri, 20 Feb 2015 15:58:32 -0700, Irv Finkleman VE6BP
wrote: Magnetic loop antennas require very little space. If you Google VK3YE you will find he has a design for a homebrew loop that covers 1.8 to 21 mhz. It will not be very efficient, but you can always build a loop of greater diameter which will raise the efficiency. Nope. The efficiency is in the diameter of the conductor used to build the loop, not in the diameter of the loop itself. I bigger diameter loop just allows it to tune to a lower frequency. A "fatter" or larger conductor, reduces IR losses and therefore improves efficiency. There are a number of loop designs on the net. There are an increasing number of loops being manufactured as well. Size-wise, a magloop will outperform many full sized antennas. Yep, and there's a subtle reason why a loop will outperform other antennas. A loop has a very high-Q and therefore a very narrow operating bandwidth. This does nothing for the actual signal being received, but does wonders for reducing the QRM and QRN (mostly atmospheric noise) on nearby frequencies, thus improving the SNR (signal to noise ratio). If there's more noise than signal, you're not going to hear much. One might guess that such noise, which is out of the receivers IF bandpass, would not have an effect on the receive signal. Not true. The atmospheric noise mixes with other noise sources (and other signals) in the band and eventually produces noise that lands in the receiver bandpass. The loops high-Q eliminates much of this noise before the mixing occurs, thus improving the receive SNR and letting you hear signals that would buried in the noise on a wider band antenna. In transmit, the loop isn't any better than a similar size conventional HF antenna. Transmit antennas rely on their directivity to obtain usable performance. Unless you're doing spread spectrum or frequency hopping, a high-Q antenna doesn't do anything useful. Note that these are receive only loop antennas in the 300-500 KHz range: http://radiomarine.org/idbfiles/0000/0469/kgh-02a.jpg http://radiomarine.org/idbfiles/0000/1509/pioneer6.jpeg -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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