Shortened dipole. How does it work?

Recently I was asked about the shortened dipole. In a nutshell: the question was pretty simple - is this possible to create a small yet effective antenna for 80 meters band. Below is a result of my thoughts on this topic as well as some number of diagrams, which, as everybody knows, worse one thousand words.

Let's start from basics: the antenna is a wire conducting the RF current. As far as we are talking not about the conducting of the DC, the basic understanding of the RF transmission is a must, although this is out of the scope of this article.

How the height affects the VHF antenna

The previous article caused some amount of questions in respect to the VHF/UHF antennas and how the height of the mast affects the radiation pattern. This is to answer these questions and bring light on this very interesting topic.

We all know that VHF antennas are vulnerable. There is a lot of things which might destroy the antenna - wires, trees, birds, even its own mast. Therefore we try to place the antenna as higher as possible to avoid the impact of the mentioned above objects. For the birds, we have the scarecrow. The practice tells us: the higher antenna is, the farthest QSO you have. This is the true and the false simultaneously. The devil is in details. Two antennas for the 433 MHz GP located on the 20 meters mast and on the top of 20 meters building are very different antennas, even if they are twins. This is because the overall height of the antenna is calculated relative to the nearest more or less flat conductive plane: the ground or the roof. The radiation pattern of the roof-mounted VHF GP is the same as of ground-mounted one.

How the height affects the antenna

After the third article, I decided to perform more general work and calculate the same antenna on the different height. In a nutshell, the antenna parameters depend on the height the antenna is placed at. I stored some number of pictures, looking at which worse thousands of words.

The antenna is a quarter-wave GP with two radials. I know that the number of radials does matter, but this is very quick, thus not very thorough exercise. The antenna is made of aluminum pipe (R=10 mm, 3/8 inches), counterpoises are an aluminum wire (R=0.8 mm). The antenna was designed to be mounted on the 1 meter (3 feet) tall mast.

The twin-lead Marconi antenna

In general, the Marconi antenna is a well-known type of antennas widely used by hams, and, in fact, is a Ground Plane antenna. In comparison to the regular Marconi antenna, the twin-lead modification is a short-circuited one.