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What is the Range of an Radio Transmitter?
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.
In theory the range of a transmitted radio signal is infinite regardless of power used, basically it goes on and on forever until it encounters an obstruction. More power helps to penetrate any obstructions. Given this, when engineering and planning a station it is always better to consider the issues that limit the range.
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. 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, you can't have one without the other. You can have 30 miles of line of sight, but only 100mW, you will only transmit maybe 1/4 of mile. If you had 100W then because of the 30 miles line of sight the signal will be strong enough to overcome the background noise in the radio receiver all the way to the horizon. The 100W will reach the horizon without signal degradation. So the transmitter power ideally needs to be proportional to the distance of the horizon.
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.
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
