A question I often have to deal with is just how important the geometry is to the antenna radiation pattern. Most people with some experience know that it IS important, but they don’t know just how important. Let’s consider a very simple model.

In this case, I’m going to model two wires connected together and fed at the connection point. However, rather than model it as one wire with a source in the center, I plan to rotate a bit around that center point, so I’ll model it with a split source between the two wires.

In actual fact, I’m not very concerned here with the accuracy of the calculation because the point of the exercise is to show that even a simple reorientation of the wires can change the antenna pattern in noticeable ways.

The Simple Dipole over perfect ground

Let’s start with a simple dipole. I’m going to make each leg out of #14 wire, 5 meters for each leg, and 10 meters above a perfect ground. I’ll run my tests at 14.2 Mhz (20 meter band) just for fun.

Here is the initial antenna:

1/2-wavelength Dipole Antenna

The detailed setup and the radiation pattern can be seen by clicking on the image below:

Now it starts to get interesting.

Simple dipole with one leg rotated up 45°

In order to rotate a leg up, I need to compute the end point coordinates for the leg. Right now, I have leg 1 going from (0,0,10) to (0,5,10) … in other words from X=0,Y=0, Z=10 (all in meters) to X=0,Y=5, Z=10. Leg 2 is the same except that we take it in the negative Y direction, so it goes from (0,0,10) to (0,-5,10). You can see that in the wire table above.

To rotate the leg up 90°, I can do this pretty easily since I’ll take the 5 meters off the Y coordinate and add it to the Z coordinate, so Leg 2 will then go from (0,0,10) to (0,0,15). This makes it look like this:

Dipole Leg rotated up 90°

The results from re-running the model with these coordinates can be found by clicking on the image below:

Pretty Simple really. SO how about 45°?

Simple dipole rotated 45°

Rotating 45°requires a bit more calculation.It’s simple enough if you know a little trigonometry or a little geometry.Let’sdo it geometrically.

If the three sides of aright triangle are SIDE1, SIDE2, and HYPOTENEUSE(the long side), then we know that

Since we’re rotating the leg up by 45°.SIDE1 and SIDE2 will be equal and HYPOTENEUSE will be 5 meters.So we have the simple equation to solve where we’ll let SIDE = SIDE1 = SIDE2:

So the new end point of the second leg will be 3.54 in the negative direction and 3.54 upwards, or (0,-3.54,13.54). The antenna looks like this now:

And the model results in this output:

Categories: Antenna Modeling