This is a single dipole.
Some aerials are designed to be highly directional. So they pick up information from one particular place and are less likely to pick up anything else that might cause interference. Your TV aerial on the roof is highly directional. So are proper FM aerials sometimes. If you are in a poor reception area you need a directional aerial.
Omni directional areas, like your rod aerial, pick up information from all over. Some FM aerials are shaped round to do this and pick up the maximum amount of stations but also may pick up more interference.
Maplin > OUTDOOR FM AERIALS
With DAB, DAB actually takes advantage of things FM doesn't. For instance imagine an aerial near a brick wall. The aerial may pick up the original signal and a reflected signal from the wall arriving a split second later. With an analogue TV you get a double image and it's know as 'ghosting'. You see the main image and a second weaker image together. A directional aerial will solve this. Now with DAB it takes the information from both and combines them. Some information arrives late but DAB has some fast processors and there's a slight delay in DAB anyway. This might mean that sometimes you get the original signal, sometimes a reflection. DAB will do its best with either or both. Interference isn't an issue as it's removed by being digital. The problem is if too little of the original signal is there and too many errors come in then you get the 'bubbling mud' sound.
Anyway for DAB you can use a single vertical aerial, ideally around 30cm high, to pick up information from all around or you can get directional aerials like this:
Digital Radio (DAB) 6 Element Aerial
Which are for poor signal areas when you are trying to reach a distant transmitter.This is the one I have at home and can get Coventrys transmitter. it also gets a good signal from all the Birmingham stations too giving me something like 60 stations. (Though some are duplicated).
