RF propagation: What is Radio Propagation

RF propagation is how Radio Communication covers long range.

Here is a brief overview describing the range of wireless data communication coverage for various wireless technologies.

Raveon’s Data Radio Modems are long-range FCC certified data radios designed for telemetry, SCADA, AVL, wireless data, mobile-data, remote control applications and Low Latency Streaming. Its many competitive features make it an ideal choice for most data radio systems.

It describes the RF propagation of radios in the VHF (150MHz bands), UHF (450MHz bands), 800/900 license free bands, and 2.4GHz ISM bands.  It also compares conventional RF wireless technology to more advanced technologies such as LoRa. For more information about RF System design, see our application note AN119 on how to design a wireless data system.

VHF and UHF Bands

Many countries, use of VHF or UHF data radios in these bands requires a license from the government.  The radio channels in these bands are usually very narrow, like 6.25kHz, 12.5kHz, or 25kHz.

Some Nations allocate a few channels in these bands for license free-operation. In the USA the MURS channels are license free.

800/900MHz License Free bands

Many countries also allows 2.5GHS and certain 800MHz or 900MHz spectrum to be used without obtaining a license as long as the product abides by certain power, bandwidth, and operational restrictions.  In the USA, these include frequency hopping, spreading spectrum, data rate limits, and power limits.

2.4GHz License Free Band

Around the world, this band is used for Wi-Fi and many other license free wireless needs.

What is LoRa?

The LoRa (short for Long Range) modulation scheme is a modulation technique combined with a data encoding technique that gives a broad-band spread-spectrum radio the receive sensitivity of a very narrow-band long range radio. LoRa techniques give LoRa receivers unprecedented sensitivity levels. LoRa radios can receive signals 10 times weaker than most radios.

What Affects Communication Range?

  1. Antenna Gain
  2. Height of Antennas
  3. Transmit power
  4. Receiver Sensitivity
  5. Frequency of the System
  6. Over-The-Air Data Rate, (Energy Per Bit)
  7. The Terrain: plants, walls, buildings, and things blocking RF.
  8. Local Interference
  9. IF bandwidth of the Receiver.
  10. Over the Air protocol reliability. Modulation type, FEC, …

When a receiver’s sensitivity is increased by 10 times, that is the same communication range improvement as increasing the transmitter power 10 times. If the noise-floor increase 10dB that is just like turning the transmit power down 10dB. If the over-the air data rate is 10 times slower, then the slow radio acts like a radio with 10 timer more power. Wide-band radios work with faster data, but the wider the bandwidth, the more noise is in the receiver, so the RX sensitivity is impacted.

 

Typical Coverage Range

Here are some typical range coverage numbers Raveon and others use as typical range.
Few things in the world are actually typical.  Most all areas of the world are unique, but for discussion and comparison.

Terrain                                                         5W VHF             5W UHF           1W 800/900         2.4GHz

In open-terrain, 3m antenna                  15-75km             10-50km           5-30km                 0.1 – 1km

Wooded environment outdoors           5-16km               2-10km             1-5km                    0.05-0.1km

Urban, 1-3m antenna                               5-16km               2-10km             1-5km                    0.1-0.2km

Urban, roof-top antenna                         7-25km               5-15km             3-7km                    0.1-0.5km

Aircraft/drone line-of-site                       50-200km           40-150km         30-100km             0.2-0.5km

Golf Course, 10m antenna                     6-20km               4-15km             3-8km                    0.1-0.3km

Link Margin Comparison

When one compares a system to a traditional UHF or VHF radio modem add LoRa, we see that the communication range for LoRa is very similar to VHF and UHF, but it achieves this with less RF power.  The spreadsheet calculations below show the theoretical link margin for VHF and UHF radio systems and LoRa 900MHz.

VHF
5W		 UHF
2W		 VHF
2W		 LoRa 200mW LoRa 10mW
Operating Frequency (MHz) 160 460 160 915 915
Transmitter Output Power 33 dBm 33 dBm 33 dBm 23 dBm 10 dBm
Link Distance, km 675 km 150 km 450 km 180 km 40 km
Link Distance, miles 400 mi 93 mi 279 mi 111 mi 24 mi
Transmit Antenna Gain (dBi) 3 3 3 5 5
Antenna feed loss (both ends) 1 dB 1 dB 1 dB 1 dB 1 dB
Receive Antenna Gain (dBi) 3 3 3 5 5
Receiver Sensitivity -115dBm -115dBm -115dBm -128dBm -128dBm
System Gain  (dB) 148 148 148 151 138
Link Fade Margin  (dB) 26 22 22 22 22
Link Path Loss  (dB) -133 -129 -129 -136 -123
Effective Radiated Power 39 35 35 27 14

 

The best range is 2W and 5 W VHF data radios. The theoretical calculations show that a 200mW LoRa radio will work about as long-range as a 2watt UHF radio.

A 2watt VHF will work a bit longer range but it is using 2 watts. Raveon’s field tests support these theoretical calculations.

We’ve seen VHF and UHF reliably go 50-100 miles line of site, and LoRa can easily go 20-50 miles (at 1/10th the power consumption).

Increasing Communication Range

There are many ways to increase the range of a communication system, and the LoRa protocol has implemented many of them. This list is something everyone building a wireless data system should consider.

The range of a communication system is determined by:

  1. Antenna gains and antenna heights.
  2. Transmit power
  3. Receive sensitivity
  4. Frequency of the system
  5. The energy per bit (data rate)

Antenna gains and antenna heights can make hug improvements to the range of a communication system.  Antenna performance is some of the most economical ways to improve a system.

Antenna gains are sometimes limited by regulatory rules or practical considerations.

The more transmit power a device radiates the further away it can be received. In the real world, you must increase power by 4-10 times to double the communication range. Sometimes even 20 times increase is needed to double the range.

A receiver with good receive sensitivity can pick up a signal from much further away than a poor receiver.  A receiver that is 4 times (6dB) more sensitive than another receiver will greatly increase the communication range. 6dB receive sensitivity improvement is the same as transmitting 4X more power.  But a 6dB better receiver will consume very little additional power.   It’s always better, cheaper, and more efficient to use a more sensitive receiver than a more powerful transmitter.

Every RF engineer knows the equation for RF communication that shows the higher the frequency of the system, the more the loss.

L = C + 20 * log(D) + 20 * log(F)

C is 36.6 if D is measured in miles. L is the free space path loss.

You can see that the loss goes up as the distance (D) goes up and as the frequency (F) goes up. If you use a frequency like 900MHz instead of 150MHz, the path-loss is 15dB more. This common formula is often cited as proof that the atmosphere attenuates high frequencies more than lower.

We like to note that the atmosphere doesn’t favor 150MHz over 900MHz. The reason the 900MHz system has more loss is that the 900MHz antennas are shorter, so they collect less energy than a 150MHz antenna that is almost 2 meters tall.

The energy per bit (Eb/N0) is the transmit power divided by the over-the-air data rate. In the 1940s, Claude Shannon determined the communication performance limits for channel coding. He determined that the noise floor, channel bandwidth, and the energy per bit were a fundamental limit to the ability to communicate. To this day, his theorem still stands true. If by reducing the data rate in half you get the same range improvement as doubling your power or doubling your receive sensitivity.  Slow data rate radios go much farther than fast radios.

Over the Air Baud Rate

Low data rates GREATLY increase your communication range.

Receiver sensitivity is directly related to the; “RF power per Bit”.  So if the data is 2X faster, the bits are ½ as long so the RF power per bit is ½ the RF power.   Below is a table showing how the data rate affects the range of a radio that at 4800 baud, the radio is working over a terrain of 20 miles.  It you slow down the baud rate, the system will go over 40 miles, and if you speed up to 19200 the range will only be 100miles.

Baud Rate  1,200  2,400  4,800  9,600   19,200   57,600   100,000   1,000,000
Power per Bit Change 400% 200% 0 50% 25% 8% 5% 0.5%
Power per bit in dB <> 6 3 0 -3 -6 -11 -13 -23
Communication Range % 200% 142% 100% 71% 50% 29% 22% 7%
Typical Range change compared to 4800 Baud Radio  (miles) 40mi 28mi 20mi 14mi 10mi 6mi 4mi 1mi
Range in km 65 46 32 23 16 9 7 2

 

This table also shows that going to 1megabit data rate with the same power on the same frequency will reduce the range 93%.  If you want long-range RF communications, use slow data rates.  Back in the day when Paging was popular, one paging transmitter would often work over 50 to 100 miles, almost 1000 square miles running at 500 baud.

Impact Related to Things that Affect Range

The IF Bandwidth

The IF Bandwidth changes the Sensitivity of the Receiver. When the IF bandwidth is 2X wider than one radio, that wide radio receives 2X more noise, so RX sensitivity is impacted. Different data rates require different signal/noise ratios. Fast data needs less noise. And the wider the bandwidth of the IF, the more noise there is on the channel, so receiver sensitivity is worse for fast data rates and wide bandwidth. That is why narrow-band slow data radio radios have better sensitivity. The chart below shows typical RX sensitivity based on data rates and IF bandwidth. Colored lines are different IF Bandwidths, and the rows are various data rates.

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