Roger,
Even in an area with very poor ground conductivity (such as mine),
environmental losses will be reduced by elevating the lower conductor. I
would suggest 2-3 meters as a minimum. In general, loop shapes having
the lowest proximity to ground are the best, with circular being very
desirable. The late Reg Edwards, G4FGQ, wrote several programs dealing
with loops for transmit use, and RJELOOP1 would apply the best to your
situation. You can download it from:
http://www.we0h.us/page3.html#S301%22
Your actual feedpoint impedance will probably differ, but the program
allows you to play with various options in advance, perhaps saving some
money, and certainly time.
The loop is an excellent option for a lot with so many trees that a
Marconi-style antenna would have excessive environmental losses at these
frequencies.
John, W1TAG
On 8/13/2010 11:08 AM, Roger Lapthorn wrote:
Having considered the antenna options for my small garden, I'm going to
erect a TX loop antenna rather than a Marconi vertical for 136kHz: the
earthing issues are removed and it is easier to build a capacitor
selection box (to tune and match the loop) than wind a huge loading coil
and match it. I've seen some of the webpages dealing with these and
understand basically what is needed (large loop area, thick wire,
capacitor match-box, dealing with high RF currents, etc). I understand
the loop will have directionality and nulls.
My question is this. *Is there any reason why the bottom of the loop
cannot be laid along the ground rather than elevated a few metres?* Most
designs show the bottom elevated a bit, but if laid on the soil I could
use thick coax or multiple wires for part of the loop so reducing the
loop resistance.
Views appreciated, although it may be a case of "suck it and see".
73s
Roger G3XBM
--
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
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G3XBM GQRP 1678 ISWL G11088
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