Shortened dipole. How to design one for 40 meters band.

Sometimes you don't have an opportunity to choose an antenna. You can be either limited in finances or space, but the only antenna you may build is something small. Just imagine you live in a country house and have two 10-meters-tall trees with a distance of 15 meters between them, leaving you only 11-12 meters of free space for your antenna. You want to be present on 40 meters band, where the propagation does live the whole day. Since you don't want to build a ground plane, you have chosen to build the dipole. Good choice: the dipole is a pretty good and easy to build antenna requiring not so many materials. But you have realized you have no space for this antenna: diagrams tell you that the antenna must be 20 meters long. So sad...

But, don't panic! A shortened dipole is your choice. Let's try to design the shortened dipole that could fit the space between your trees!

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.

Ground plane height and overall efectiveness

In my previous post, I have shown the Ground Plane antenna for 80 meters. It was designed to use the 16.5 meters tall mast. There on the market higher masts do exist. For example, it's not a problem to buy 19.2 meters semi-telescopic aluminum mast. I changed the model to see how big is the difference between the two antennas built on 16.5 and 19.2 meters tall masts.

Ground Plane for 80 meters without tuner.

Being a HAM for years I always thought of an efficient antenna. My current goal is to make the effective antenna for 80 meters so that I could spend long winter evenings on listening to DXes. I'm living in a private house in the heart of the big city and not able to hang out wire antennas at enough height. Although my experiments with ground standing masts and wire antennas have shown me some positive result I decided to build the one-band GP.