At 08:18 AM 2007-08-29, Dave Arruzza W1CTN wrote:
>I am planning on erecting a 40 meter 4 square. Standard 1/4 wave spacing.
>All elements are to be elevated at 20 feet with 4 elevated radials
per element.
>Question: If I used inverted-L elements, 22' vertical, 11' horizontal, would
>this enable me to have a good signal on short hop.
I thought I knew the answer to this, but it turned out that modeling
showed some surprises (to me anyway).
A single 40m inverted-L (up 22 ft, out 13 ft) with four 33 foot long
horizontal radials at 20 feet above ground doesn't have much high
angle radiation, since the current in the 13 foot horizontal wire is
pretty small. It's down about 6 dB at 45 degree takeoff and down
about 11 dB at 60 degrees. (I lengthened the top wire to 13 feet to
get closer to resonance with bare #12 copper wire in EZNEC.)
However, when you use four of them in a 4-Square array, it really
depends on what you do with the radials. If you connect wires
between the bases of the 4 elements (34.4 feet ~ 1/4 wl) and have two
open-ended 33 foot radials per element, there is a considerable
imbalance between the currents in the elevated radials that
introduces lots of high angle radiation, much of it horizontally polarized.
If you orient the radials so that there are 4 open-ended radials per
element with NO interconnections between the elements, the high angle
signals are significantly lower. I ran models with both horizontal
and drooping (-20 degree) radials. Note that the "front" and "rear"
elements of the 4 square have their radials rotated 45 degrees to
minimize radial overlap. (The sloping radials raise the feedpoint
impedance of a single inverted-L to about 33 ohms. It is about 18
ohms with four horizontal radials at 0.15 wl height.)
In all cases, the top horizontal wires for the inverted-L elements
point toward the center of the array.
Using EZNEC+ 5.0.9 and "Real/High Accuracy" "average" ground and
ideal current phases and magnitudes, here are how they compare:
Takeoff Bonded Rdls Open Rdls Drooping Open Rdls
======= =========== ========= ==================
19 deg 3.4 dBi 5.4 dBi 4.5 dBi
------- -------- --------- ---------
45 deg -5.3 dB -6.5 dB -6.7 dB
60 deg -7.7 dB -10.8 dB -15.4 dB
90 deg -3.7 dB -23.9 dB -24.8 dB
(NB: For the main lobe at 19 degrees, the absolute gain is shown,
while the gain relative to this is specified for the higher angles.)
The issues of elevated radial interconnection in vertical arrays was
discussed recently on the list, but I don't recall seeing any
modeling results. As usual, caveats about NEC-2 models with elevated
radials relatively low to the ground apply, so take the absolute
gains as upper bounds. Of course, in a real array suitable current
baluns/chokes are needed at the element feedpoints to minimize
feedline radiation and reduce ground losses.
73, Terry N6RY
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