The new radio station being built is in Porto Novo, the official capital of Benin, West Africa. The exact coordinates are (coordinates 6°28'25.08"N, 2°36'17.93"E), just off the bit of Porto Novo that is crystal clear on Google Earth. Click on the image to the left and you'll see exactly where we are. This area is very flat, with a high water table from the surrounding marshes. So coverage of the station is for all practical purposes line of sight.
APM has been using a 500 watt transmiter and a simple dipole until now, and no audio processing on the transmitter. Because power is a big problem in Africa, we've decided to see whether a lower power (300 watts) will actually go further than the old transmitter with more power. We'll be using a more efficient antenna, placed slightly higher, and processing the audio to "punch" through the static. Remember that doubling the power doesn't double the distance covered. Under normal conditions, FM signals only travel line of sight. From the top of the mast in Porto Novo you can see the commercial centre of Contonou in the distance (looking south-west) and well into Nigeria if you look East. As a guideline, here's the range you can expect.
1 WATT TRANSMITTER
1 watt usually gives a coverage area of 2-3 kilometres, more if a high mast or a mast on a tall building or hill is used. You could cover western Porto Novo with 1 watt, but not much else.
20 WATTS
- 20 watts will give a strong signal for 8-20 km when operated in a typical environment, (flat land, 20 metre mast, no hills or built-up buildings). With a poorly chosen transmitter site (bottom of a hill) you may only get 1-3 kilometres. With a 20 metre mast located on a hill overlooking the intended coverage zone, you could expect the signal to be reasonable at distances exceeding 35 kms.
300 WATTS
When a stronger signal is required, 300 watts is better than 20. This is the power we've chosen because the coverage area is to be the whole of the capital and into the commercial centre, Cotonou. The extra power will allow the signal to penetrate more effectively and reach around 30 kms radius from the transmitter. Once again 300 watts on a 5 metre mast above the ground (flat land scenario) may only be effective for few kilometres. Putting the antenna on the mast above the building (4 stories) at APM means we expect the coverage to get well into the western part of Cotonou. If you're designing a station remember this:-
- Put the antenna as high up as possible.
- Make sure the antenna tower itself is properly earthed to the ground. It may be struck by lightning (in Porto Novo in the monsoon period in May-July, almost certainly)!
- Choose a sensible power for the transmitter. Remember, the power will be one of the biggest ongoing bills for the station.
- Check the cost of a more efficient audio processor and antenna with a lower power transmitter (say 300 watts) against a higher power transmitter (600 watts) with a simpler antenna and no audio processing. In the long term, you may be better off with the lower power transmitter.
We're using a car radio with a standard whip antenna as a guideline for a typical receiver.
