On Sat, 15 Mar 2008 17:51:47 -0700 (PDT), AndyS
wrote:

Consider an airborne LORAN antenna, used on aircraft, to receive
100 khz signals. It works out that around 22 inches is the length
where
the atmospheric noise exceeds the general receiver noise for most
receivers. Making the antenna longer will pick up more desired
signal,
but also more atmospheric noise, in the same ratio.....so the SNR
doesn't
get much better.

Agreed. However, the short 18" antenna is commonly used for handheld
and aircraft Loran receivers. However marine Loran antennas are
typically 8ft long.
http://shakespeare-marine.com/antennas.asp?antenna=5220

That's not the only reason that Loran antennas are rather short. If
the antenna were longer, the impedance would increase, causing it to
pickup more percipitation static, atmospheric noise, and 60Hz
harmonics. A longer antenna would also not be as narrow band and low
impedance as a short (loaded) antenna. The relatively narrow
bandwidth is helpful for eliminating broadcast, beacon band, and other
forms of interference.

Incidentally, that's also one reason why some remote Loran systems
have a pre-amp that really burns some watts. It needs to handle the
out of band overload and stay linear. If the antenna were made
longer, the amplifier would need to handle proportionately more power
(and probably melt). Some details in the patent at:
http://www.google.com/patents?id=ONUrAAAAEBAJ&dq=4875019
The "background" section is worth reading. The other reason for the
amplifier is to give the antenna system a 50 ohm output impedance so
that cheap coax can be used.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558