Topband: Delta Loop

[Bill Tippett]

KN4LF wrote:

I put up what I call a squashed delta loop in Colorado in 1994 and it
worked just fantastic. I put the base of the antenna 10 feet above ground,
and the apex at 100 feet above ground. Using the formula 1005 divided by
frequency the wire length used was 539.4 feet, 199.4 feet for the bottom,
170 feet on each angled side. I fed the antenna 35.1 feet (0.08 wavelength)
up from a bottom corner for vertical polarization, with an EZENEC 3.0
calculated takeoff angle of 23 degrees.

        I agree with Thomas' comments.  I used a "squashed" Delta for
awhile at both of my NC locations.  Here are a few additional observations:

1.  You can "squash" it even more than Thomas did which slightly improves
the gain by separating the two vertical sides more.  I recall my horizontal
base was about 250'.

2.  I fed mine in the corner even though that is not exactly 1/4 wave from
the apex.  That is easier to do mecahanically and electrically it does not
make much difference in the impedance.  Input impedance of mine was about
110 ohms which can be matched to 50 ohms by using 1/4 wave 75 ohm cable
as a transformer.  I also used 12" worth of ferrite beads at the input to
minimize current on the feedline.

3.  You can open the apex and move the vertical wires apart if you have a
very tall support which slightly increases gain.

4.  You can tilt a Delta if your support is not tall enough.  I believe
W3BGN uses a Delta this way since his apex is only 90-100'.  This will
also slightly increase the gain in the direction the base is moved.

5.  I recall that a vertically polarized Delta works best around 6' above
ground.  If you raised one so that the top were at 250', performance would
be much worse than near the ground because vertically polarized Deltas need 
a good ground to work against.  

        All of the above points were verified by EZNEC modeling. I was 
generally very happy with mine but gave it up in favor of shunt-feeding 
my tower because it was very difficult to work stations to the side of the 
Delta.  Of course you could put up two Deltas to solve that problem if you 
had room.

                                                73,  Bill  W4ZV