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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.

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 Is Limited By:

a) ANTENNA HEIGHT. How far the transmitting antenna can effectively see. If you stand where the antenna is mounted and look out with a pair of binoculars, wherever you can see it is possible to transmit to. This can sometimes be up to 20, 30 even 40 miles if you are looking out from a mountain top.

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 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

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.

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