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!

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.

LiIon 12V battery. Schematic and model of housing.

Sometimes you go to the flea market and spend some amount of money on some scrap. After you get back home you start thinking of what to do with all these things you have bought. The same happened to me: I have bought batteries for some unknown portable device. There were four 3.7V 3100 mAh cells. Unfortunately one of them was completely dead. Fortunately, three cells is enough for building the pretty powerful yet small rechargeable battery providing 12 volts.

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.