Peter,
When we started on 73kHz some years ago G2AJV was the first to put out a
reasonably
strong signal and he was using a large loop. G4JNT and G3XDV also put out
reasonable
signals from very restricted sites and later found that a Marconi from the
same site
radiated a stronger signal for the same transmitter power. As a result it
has become part of UK LF collective wisdom that the Marconi is superior to
a
loop of the same size.
That was also the 'collective wisdom' amongst Lowfers in the US, until 3
years ago. The Wisdom was based on 'cut and try' and "how am I coming in?"
types of scientific study. The net result in the US was ZERO loop antennas.
It is also the 'collective wisdom' of most Low frequency textbooks in the
MIT Engineering library dating back to the 1920s that loops have no
practical use for transmitting at low frequencies.
I have spent 100s of hours experimenting with different configurations
making field strength measurements, comparing them with the classic
radiation equations, determining the effect of soil loss, and conductor
loss, determining if a ground screen is needed. My data and mathematical
studies indicate that a simple 50'x50' loop with a .37" conductor is ~6db
inferior to a 50' tall vertical, having a 50' diameter top hat and with a
elaborate ground system located in an open field. On my property, because of
the canopy of trees, there is absolutely no hope for a vertical antenna and
I spent 2 years with the same diligence with verticals as I have with my
loop antenna and it ended in failure - as is the case for, no doubt, for
many other vertical users that we don't hear from any more.
I'm not surprised at all about the mediocre reputation of loop antennas when
I read about other loop antenna descriptions. I often find conductor Rac
inadequate and loop-to-ground spacing too small, poor matching, and a
general lack of knowing what math to apply to these variables. There is one
'expert' that doesn't even know how to match a coax to a large loop and
boasts how poorly his 'best effort' loop compares to his vertical on
receive.
Regards,
Bill Ashlock
PS: I would give my eye teeth to be able to set up a loop and vertical in
that large field for a shootout. Perhaps the opportunity will arrive before
long!
_____________________________________________________________________
This e-mail has been scanned for viruses by the WorldCom Internet Managed
Scanning Service - powered by MessageLabs. For further information visit
http://www.worldcom.com
| 