[TowerTalk] 40 meter 4 Square recommendations/observations

[Terry Conboy]

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