HOW TO RECOGNISE DIFFERENT TYPES OF BROADCASTING AERIALS FROM A QUITE A LONG WAY AWAY
By Mike Smith
LESSON 2: VHF-FM RADIO
|FM radio from the main stations is transmitted
using some large and very impressive aerials, especially since the BBC
re-engineered all their main stations for mixed polarization. Before the
1980s the main stations tended to use cylindrical slot aerials which,
while technically very impressive, were rather uninteresting to look at
from a distance. The new mixed polarization arrays are much bulkier affairs
which meant that the BBC had to construct new masts at some sites since
the old masts could not carry the additional weight of these larger antennas.
Compared with these impressive main stations small local stations and relays often have very humble aerials. Many domestic hi-fi systems are fed with a simple three or four element yagi aerial in the loft or on the roof and figure 15 shows that similar aerials are used at broadcasting stations. In the example below there are four crossed yagis, two vertical and two horizontal, in a stacked arrangement providing a highly directional and mixed polarized signal to Wolverhampton and the Black Country.
Another type of mixed polarization sometimes used by local stations is 'slant' polarization and figure 16 provides an example of this type of aerial.
Mixed polarization was introduced by local FM radio in the 1970s to give good reception for listeners both at home with fixed horizontally mounted roof aerials and with vertically positioned portable radio and car radio aerials. Figure 17 shows an example used by some ILR stations consisting of two stacked folded vertical dipole aerials and two horizontally mounted folded dipole aerials. The horizontal aerials are bent into a circular shape to provide a more omni-directional signal in the horizontal plane. The overall radiation pattern will be roughly omni-directional, but since there is only one array there could be a mast null, in this case reducing the signal to the west.
Figures 18 (left) and 19 (below) show two different ILR masts that simply have four or eight slanted (at 45°) dipoles to provide an omni-directional and mixed polarized signal. Mixed polarization generally involves transmitting an equal amount of power in both the horizontal and vertical planes, although a small number of recent ILR stations transmit the horizontal component at 6dB below the vertical (i.e. one-quarter power). These two examples, however, transmit equal power in both the vertical and horizontal planes. Incidentally both of these examples are mounted on towers not specially constructed for local radio broadcasting, but on structures intended originally for other forms of communication. It's always worth looking out for other masts, such as those used by mobile phone providers, for a new local radio aerial!
|LESSON 2:||VHF-FM Radio|
|LESSON 3:||VHF-DAB - Digital Audio Broadcasting|
If you can add some extra detail or provide corrections
to inaccurate information please e-mail the site - Mike Smith.