Hi there.

I'm Paul Hollings (LinkedIn profile) Head Engineer at Aareff Systems and I'm going try and explain how range and power works with radio transmitters.

We always think about the FM broadcast signal in terms of, how far will it go?, what is the distance? etc. Well the truth is the FM signal will go on and on to the extremes of the universe, obviously this in practice does not happen as the signal is blocked or limited by objects. So it is always better to think about what is going to limit the signal as opposed to how far the signal will go

Range and Distance are Attenuated By:

a) ANTENNA HEIGHT. The FM broadcast signal behaves a lot like visible light. Imagine a huge bright light in the same place your antennas are mounted, I mean a really bright light, like a football stadium flood light, beaming out to the horizon. In the dark you would be able to see this for tens of miles until it reached the limits of the horizon. This is exactly what the radio signal does, but the radio signal will generally go about 20 to 30% further than the light bending over the horizon a little. This is caused by a little bit of diffraction that occurs at VHF FM band frequencies.

b) INTERFERENCE. A serious suppressor and blocker to signal range is another FM broadcast in the distance using the same frequency. With good band planning from the authorities, this does not usually happen, but it can occasionally. If another operator has a powerful FM broadcast, even 100 miles away, it can interfere and limit the fringes of your service area and signal.

c) RADIO TRANSMITTER POWER. If the antenna can see 15 miles and say 1 Watt ERP of power used, it's very likely that about 1 or 2 miles of range will result. This is quite simply because there is not enough power to propagate the signal 20 miles. If 50 Watt ERP is used, it's very likely that 20 miles of range will be achieved. This is because 50 Watt ERP is ample power to propagate a strong signal 20 miles. If a 1000 Watts of power is used, it is very likely that signal will only propagate just over 20 miles. This is because the range is limited as described in point a) above.

Power Radiated From Antenna

This is the most important parameter, can be ERP or EIRP, either will work. Knowing the transmitter is not enough as depending on the type of coax used and the type of antenna, this can change the radiated power from unity. The power radiated by the antenna is generally at least the transmitter power or more and is determined by the dB gain of the antenna and the dB loss of the coaxial. Check Wikipedia for ERP/EIRP and our online dB / dBi converter.

The four graphs below are based on average customer reports in some directions, but not all directions. Basically they show the maximum ranges really ever recorded for our 10kW, 25kW, 50kW and 100kW EIRP systems. Most of the reports were radio station listeners that phoned, texted, Whatsapp etc into the radio station programs operated by our customers.

Better Accuracy

If you need more precise results, well we can't get 100% accuracy, but we can get about 85%, which is much better than the 'rule of thumb' numbers shown above. We get 85% accuracy by using radar height data collected by NASAs space shuttle during it's operations about 15 years ago.

If you go to my personal site you can see some spectacular plots from all angles of the iHeartMedia 100kW ERP FM Services in Miami compared with the FCCs 60dBuV/m contour line.

For a small fee we can make plots like this for you, we just need to know the GPS coordinates of the antenna site, the the tower or structure height and your level of EIRP or ERP. If you like more details on this, message us at info@aareff.com or WS +1 829 698 0733

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