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What is the Range of an Radio Transmitter?

Paul Hollings Head Engineer At Aareff Systems

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.

It's always better and makes it easier to conceptualize in the mind if you think of the FM broadcast signal as going on and on forever even to the extremes of the universe. What determines the actual range are the obstructions that get in the way and block or limit the path of the signal. Think of it as 'What's going to block my signal?' and NOT 'How far will my signal go?'

Distance Is Limited By:

a) ANTENNA HEIGHT. The radio horizon, this is just a little further than the visible horizon, some rule of thumb figures are 20 to 30% further than the visible horizon. What this means is if you look out from where your antennas are mounted with good pair of binoculars, wherever you can see, you will broadcast to and about 20 to 30% further.

b) INTERFERENCE. Other FM broadcasts on the same frequency from other radio stations cause severe attenuation and blocking to the radio signal. Your antenna can have perfect 'line of sight' of 20, 30 even 40 miles if you have a large tower, but if there is another signal on the same frequency 40 miles away, the perfect 'line of sight' goes out the window and your signal is weakened and interfered with.

c) RADIO TRANSMITTER POWER and antenna height together determine range. With a line of sight of about 15 miles 1W ERP of power will probably send the signal only 1 mile or maybe 2 miles, this will be the limit because the signal is just not strong enough at more than 2 miles to overcome the background noise that's present in all radio receivers even though the line of sight is 15 miles. If 50W ERP is used in the same place, this may transmit up to 20 miles because it has sufficient strength to overcome the background noise and go 20 to 30% beyond the visible horizon. If 1000W ERP is used, it's probable that distance will still not be much more than 20 miles because the horizon starts to kick in and limit the signal.

A typical example is:

Our 100 watt radio transmitter using a high gain antenna like the Aareff 5/8's vertical which has a gain of 4.8 dBi, and using 20mt of LMR400 low loss coax would effectively more than double the power to 250 Watts EIRP. (up to 24 miles range)

With 25 years of customer reports, a lot of testing on our part, in addition to the science and math behind this, we have been able to get some rough average rule of thumb numbers on range, they are as follows:

This table is based on maximum reports from customers mostly using house roofs or small towers.

Watts EIRP	Range Miles	Range Km
1 2 4 8 16 32 64 128 256 512	1.5 2.1 3.0 4.2 6.0 8.6 12.0 17.0 24.0 34.0	2.2 3.1 4.5 6.3 9.0 12.9 18.0 25.5 36.0 51.0

Antenna Gain and EIRP

When you want to determine FM broadcast range you need to know the EIRP or ERP, this is essential. ERP stands for Effective Radiated Power and this is the power radiated from the antenna. Knowing the transmitter power is not enough because that is changed a lot by the antenna system. All antenna systems have gain usually measure in dBi or dBd. If you know the antenna gain and the coaxial cable loss you can use our online dB converter tool to ERP or EIRP. Wikipedia explain this here

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.

FM Broadcast range chart up to 10kW EIRP

FM Broadcast range chart up to 25kW EIRP

FM Broadcast range chart up to 50kW EIRP

FM Broadcast range chart up to 100kW EIRP

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