On Wed, Nov 11, 2009 at 5:45 PM, Phil Smeaton <Phil.Smeaton@mipac.com.au>wrote:
An inverted 'L' has been strung up a tree at my QTH, with a 65 foot
> vertical section and the remainder tied-off to another tree. The top
> section slopes downwards to it's fixing point somewhat (at about 50 foot
> or so high) and is parallel to the single radial beneath it at 10 foot
> off the ground - like a big 'C' almost.
>
**** questions below - please answer:
1) what are the dimensions of the top and the bottom of the C.
2) what is the compass orientation of the open part of the C.
3) is your soil rich and wet (farming), city poor, or dry (desert)?
4) are you using a "balun" type or blocking device at the feed point.
5) describe your coax run from antenna to shack.
A single all-by-itself "radial" is not a counterpoise. It is a low
horizontal radiator. In an elevated radial system, there is an opposite
radial with out-of-phase current which cancels horizontal radiation.
I lack specifics, the feedpoint, length and orientation(s) of servicing
coax, whether any isolation is at use at the feedpoint, the length of the
bottom of the C, and the specific number you mean by the "remainder" in the
top of the C.
Since you are in VK, I set the ground as dry, and constructed your antenna
presuming that "remainder" meant 125' less the 55' from 10 up to 65, or 75'
strung as the top of the C down to ~50' I set the length of the "radial" to
125'. Please correct any differences with your actual installation.
The model made from these assumptions does not model close to your stated
match leading me to wonder if the feedline shield is one of your major
radiators, or if you have varied a dimension to achieve a match.
EZNEC will compute the sum of radiated energy in the 3D mode. In commercial
quality vertical systems, a figure of -3 dB overall gain is good, improved
near or over salt water.
Depending on assumptions your "C" antenna models out at -8.5 to -9.5 dB
three dimensional loss. Not so good for an antenna you probably want to be
omnidirectional. At a single bearing, in the direction of the open C
elevation 30 degrees, the gain looks to be -3 dBi or so. The backside at
that elevation is -7 or -8 dBi.
What is most likely going on is the presumption of omni-directional with the
one "radial". It would most likely perform "passably" in the direction of
the open "C" and substantially worse away from that quadrant.
73, Guy.
_______________________________________________
160 meters is a serious band, it should be treated with respect. - TF4M
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