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