Hi Stefan / Group
Yes EZNEC can do this but knowing the actual relationship would be
nice to play with the math on a more primal level to 'understand'
the issues. So, if someone can point us toward a suitable reference
PLEASE!
For me a loop was the only viable option short of breaking out the
chainsaw and backhoe... Then I'd need a good divorce lawyer...
A loop made for a very nice compromise at my QTH give the
availability of tall trees and the fact it blended into the
surroundings (stealth) and it actually works the way it should.
Given we have a limited EIRP who cares that a vertical may be able
to do more for the given input power... Kinda a pointless debate.
What matters more is knowing your RF is radiating into the far field
and not warming the trees...
RE: Conductors:
RG213 is expensive and its dielectric and physical wear
characteristics seemed not as good as a common aluminum power
conductor. Copper conductors are 4 to 5 times more expensive then
AL and 4 times heavier for the same resistivity. Here, #2 AWG AL is
$1/meter new and this conductor can be found surplus for much less
as it is used for neutral support service drops to houses and
commercial buildings... The insulation is very robust and can deal
with rubbing on trees it is sunlight resistant and has a very high
dielectric strength. The peak voltage on my system exceeds 7KV and
no sign of breakdown even during heavy rains. The conductor is
also light and easy to work with. Another interesting item I
noticed as the measured RF resistance and the calculated where much
different. The conductor I used has seven strands (see attached).
The way these are arranged seems to create much more surface area
and a much lower Rac then calculated from a circular area which
RG213 would be closer to.
Food for thought. Although a really good quality coax with silver
shield conductors will certainly produce a low Rac too as Warren's
experiments can clearly attest but this comes at a cost in dollars
for such high quality cable. The compromise is the shield does not
have a very high dielectric strength material insulating it form
tree branches extra, unlike the center conductor...
The real advantage of a loop is it performs as modeled in pretty
much any installation site without much thought to ground, just keep
it away :-) A vertical is very environmentally sensitive and will
only perform at its best in very limited sites and requires the site
be tailored to it in most cases including much work on the ground
system which makes up half the antenna. The other advantage of the
loop is it makes a wonderful RX antenna even in the presence of
local QRM. I found rather empirically that the large TX loop
performs better at local QRM rejection then the smaller RX loops I
have on site and certainly better then any E-field antenna I have
tried here. Having said that the best RX antenna I have ever used
on LF was a tall vertical installed on the ocean in a QRM free spot.
But how many folks can do that at a convenient operating location?
In time I hope to 'cross' my loop with another near identical sized
one to allow for phasing the two to create a 'beam' of sorts. As I
have lots of convenient trees to support this so it is viable here
and will improve my RX situation as well...
My experiments with verticals have come to an end at this QTH. Not
to say that in the future I will not experiment with them at other
QTHs but they simply don't suit this QTH. That's the secret in my
opinion of getting a decent signal out is finding an antenna that
maximizes the potential of a particular QTH not following a dogma
certain mal-contents seem to preach...
73 Scott
VE7TIL
http://www3.telus.net/sthed/argo/
On 9/27/2010 8:38 PM, Stefan Schäfer wrote:
LF,
Am 27.09.2010 22:14, schrieb Scott Tilley:
If someone knows the relationships for how a loop behaves over
ground
I'd love to see them too!
Can't this be done in EZNEC?
Scott, this sound really impressive. I have thought your loop area
is
even higher. So, if i can find and climb some trees, i could try
to
test a closed loop as well. Sounds like an alternative, maybe.
What about RG213 as the loop conductor, i mean the shielding? If
the
supports are not spaced to much, like some trees, this could work.
Starting with a small wire, carrying a bigger one and so on. Oh
dear,
so many ideas, help!
73, Stefan
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