There is almost no interaction when the two antennas are perpendicular and stacked, however they do interact as soon as they are moved out of the perpendicular position. Infact this setup shows the lowest interaction between the two antennas in any configuration short of them being more than 50m apart.

Bellow is the plot of 2 Inverted V's for 40m stacked 50cm apart at 20m in height. Take off angle is 32 deg (maximum gain), however the gain difference between the nulls and lobes increases further at lower angles up to around 10 db.

As you can see from the image there is 6bd+ difference between the null of one antenna and the peak of the antenna perpendicular to it. The gain difference increases as the height above ground increases and also as at lower angles.

Here is an inverted V at 1/2 wavelength high at 16 Deg takeoff. It shows 4dbi of Gain, plus the bonus a couple more db at higher, but still useful, takeoff angles.

4db more than doubles the effective radiated power and 16 deg take off is a good DX take off angle.

Ok, but what about really low take offs ? Most propagation prediction software predicts that take offs around 10 deg - 15 deg work the best for the longest DX circuits

The vertical at 10deg shows -0.72 dbi gain

At bellow 10 deg takeoff, the vertical exhibits a slight advantage over the inverted V dipole at 1/2 wavelength high, however you can increase the gain of the inverted V at the lower angles by adding some more height to it. Here is an inverted V at one wavelength high compared to the Vertical Ground plane (at any height). If you live on a hill with decent drop offs then the height the inverted V dipole 'sees' might be higher than support height. I recommend using YTAB for take off angle prediction from hills (for horizontal antennas only). I have done some work modeling my QTH with YTAB and will make a webpage with the graphs when I get a chance.

If you can get your dipole up 1 wavelength either through mast height or possibly hill height then you would see the follow radiation pattern. Black being inverted V dipole at 1 wavelength in height and blue being the vertical ground plane.

Of course the above results are simulated in the direction of highest gain of the inverted V (i.e. broadside) The inverted V is directional at height, while the Vertical is omnidirectional,
however by using 2 inverted V's the worst case scenario would be a 2 - 3db reduction in gain in smalls nulls, so the inverted V still wins over the vertical most of the time.

This page is still in progress.

Feel free to contact me at steve(at)mansfieldweather.com
73's
Steve VK3LMR

Here are a few extra pictures of the inverted V dipole. This is the top section showing a 1 metre section of fiberglass to which are attached 2 x 1:1 commercial baluns (to ensure radiation pattern isn't distorted from feedline radiation). The telescopic mast is also guyed using plastic coated, non conductive poly rope at 2 points.

As the top of the mast is high the side of the inverted V need to be raised to stop the apex angle of the V been too small, which results in inefficiencies. This support is 3m of fiberglass with a pully on the top. The inverted V is held in place with kevlar rope.