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HOW TO RECOGNISE DIFFERENT TYPES OF BROADCASTING AERIALS FROM A QUITE A LONG WAY AWAY

By Mike Smith

LESSON 4: MF - MEDIUM FREQUENCY AM RADIO

To conclude this feature here are a handful of examples of Medium Wave broadcasting aerials. In simplest form many stations use an omni-directional monopole radiator, a plain mast in other words! Figure 31 is a typical example of a local radio mast.
Figure 31

Figure 32
If it is required to concentrate the signal in a particular direction then a sloping wire reflector can be added to a single monopole radiator to give it a little additional directivity, as in figure 32 (left). 

I have highlighted the sloping wire reflector in this photo as the original is a little indistinct.

A number of the early large ILR medium wave stations used very highly directional MF aerials to reduce interference to other co-channel ILR stations. In the example shown in figure 33 (below), Ashton Moss 1152 kHz, each of the four masts is driven at slightly different phase to produce a very directional beam to the west and very little power to the north west and south east where other large ILR stations operate on the same frequency. 

Figure 33


Figure 34

Figure 34 show a typical high power medium wave station at Droitwich.


Figure 35

Figure 35 shows a BBC medium wave station in the south-west, consisting of two masts with a wire aerial strung between the two. As with figure 32, I have highlighted the wires in this photo for extra clarity, as they are rather indistinct in the original. The vertical wires rising from the feed point between the two masts form the aerial itself, the main radiating part. The top horizontal wires, between the two masts, not only support the main vertical wire aerial, but also form the top part of the 'Inverted L' antenna.  This physically and electrically lengthens the aerial to make it more efficient, although the useful part of the signal is only radiated from the vertical section.

 

LESSON 1:

UHF Television
LESSON 2: VHF-FM Radio
LESSON 3: VHF-DAB - Digital Audio Broadcasting

LESSON 4:

MF-AM Radio

Section Index

If you can add some extra detail or provide corrections
to inaccurate information please e-mail the site - Mike Smith.

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