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A little bit about radio propagation

14 Dec

waves_prop

This post is a follow-up to a previous post “a little bit about antennas”.

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prop·a·ga·tion  

n.

1. Multiplication or increase, as by natural reproduction.
2. The process of spreading to a larger area or greater number; dissemination.
3. Physics The act or process of propagating, especially the process by which a disturbance, such as the motion of electromagnetic or sound waves, is transmitted through a medium such as air or water.
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In this post, I am going to speak about radio propagation and its effects on the most common frequencies used for data transfer in the oil & gas E&P industry, excluding cellular and satellite.

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Radio propagation is study of the behavior of radio waves as they are transmitted from one location to another, either on earth or through the atmosphere (example: ground station to satellite). Since radio waves are in essence electromagnetic radiation, they are affected by things such as refraction, reflection, polarization, scattering, and absorption, much like light waves.

Radio waves propagate differently at different frequencies. There are many modes of propagation affecting different frequencies.

This article only deals with direct wave propagation, also known as direct mode.  Although other propagation modes can affect the frequencies I am going to discuss, such as tropospheric ducting, it is a rare occurrence and will not be discussed in this particular article. If you would like to learn more about the different ways radio waves can propagate, I encourage you to click HERE, but do come back.

It is a general rule of thumb that as frequency increases the propagation losses increase, therefore the distance a receiver can “hear” a transmitter decreases (with of course some exceptions).  Also as  frequency increases its sinusoidal wavelength decreases.

To put this in a visual perspective, I have modeled a radio link using the most common frequencies used in the E&P sector. I have modeled a host radio with an antenna height of 20 meters and a remote radio with an antenna height of 5 meters. All frequency models are at the same geographical location representing the normal terrain found in the Appalachian basin. The distance between the host radio and the remote in the models is 5.5 Km. All models are identical, with all having an EIRP of 3.98 dBm, and all having the same receiver characteristics, line loss, antenna gain, etc. Below are the frequencies modeled.

Licensed

VHF – 150 MHz

VHF – 220 MHz

UHF – 450 MHz

ISM (non-licensed)

902-928 MHz

2.4 GHz

5 GHz

The radio path models:

(Note: Click on the images to enlarge them in a new window)

150150 MHz

220

220 MHz

450

450 MHz

915

900 MHz

2400

2.4 GHz

5gig5 GHz

As you can see by the link models, as the frequency increases the signal level or RSSI at the remote site decreases:

Frequency Band

RSSI

150 MHz

-68.2 dBm

220 MHz

-72.5 dBm

450 MHz

-82.3 dBm

900 MHz

-93.8 dBm

2.45 GHz

-112.1 dBm

5.8 GHz

-139.7 dBm

As you can see there is significant signal degradation as frequency increases.

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As frequency decreases, its wavelength increases.

Frequency

Wavelength

155 MHz

1.94 meters

220 MHz

1.36 meters

450 MHz

0.67 meters

915 MHz

327.86 millimeters

2.45 GHz

122.44 millimeters

5.8 GHz

51. 72 millimeters

The lower frequency antennas can be quite large. Yagi antennas almost always incorporate a 1/2 wave dipole as the primary element. The reflecting element is slightly longer and the driving elements are slightly shorter, so a Yagi at 150 MHz can be quite a cumbersome beast to handle.

144mhz yagi____________________________________________________________________________________________

The counterpoint to the distance at lower frequencies is higher frequency bands usually have larger channels(with exceptions). Larger channels means that there is more bandwidth, so data rates can be increased. Another factor favoring higher frequencies is the fact they happen to be in the ISM band, so there is no licensing to worry about, and that can be a positive attribute as spectrum becomes more scarce, and therefore more expensive.

It is always a good idea to try to optimize your data such as using an optimized protocol that can operate at lower data transfer rates, because as you increase your data speed you increase your bandwidth. Increasing bandwidth decreases the distance you can communicate. It is very important to understand your protocol. How will it deal with being buffered? How will it deal with latency? What kind of error checking does it perform? How will it perform with a substantially higher bit error rate versus wire? How will it cope with data collisions? Will it work with your network topography, such as report by exception or point to multi-point? All important things to consider.

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Some topics about propagation and some terms to know:

Signal To Noise Ratio

The Signal To Noise Ratio or SNR or S/N is a measurement comparing your received signal to the background noise you are receiving. You can receive a pretty strong signal and still have poor performing communications if the spectrum you are utilizing is polluted with other signals. You want as much separation between your desired signal and the noise floor as possible. If having problems, considering another antenna such as a high gain Yagi (don’t forget to reduce you transmit power to stay legal!,) may help you deaden the noise. Expect problems with poor SNR in the 2.4 GHz ISM band. It is a very polluted band. Also pay attention to the sensitivity and selectivity in your radios specifications. These values indicate the quality of your radio. For more information on these terms, click HERE & HERE.

signal_noiseSpectral depiction of signal versus noise.

Refraction and Reflection

Since radio waves are electromagnetic, they are susceptible to refraction and reflection just like light. Both of these phenomenon cause a condition called multipath fade. Reflection is, well, reflection. Radio waves reflect off of surfaces such as metallic objects and bodies of water. Refraction is where the radio wave passes through a medium that changes the velocity (slows it down), but the frequency mains unchanged.

Add MediawatercupRefraction – The pencil appears to bend because the light travels 3/4 as fast in water compared to a vacuum.

Since refraction can slow down the radio waves, and reflection can bounce the radio waves changing the angle and time it gets to the antenna, multiple signals of the same source can be received by the antenna at slightly different times or “out of phase” with the original signal. This is multipath fade. Those that can remember the days of yore when analog television was around have probably seen multipath fade. Remember “ghosting” on your television? That is multipath fade.

tv_ghostingVisual example of multipath fade.

There are radio systems with antenna schemes such as MIMO that take advantage of multipath, but when dealing with a simple data radio, multipath fade is bad. Remember that we are dealing with zeros and ones, and when they get unintentionally mixed with other zeros and ones that are slighty out of phase, you get garbage for data.

Bit Error Rate 

Most modern data radios include provisions for checking the validity of the data packets through cyclic redundancy checking or some similar error checking scheme. This usually happens automatically with maybe just a few settings to optimize this. If you are having latency issues, you may want to check you Bit Error Rate. A high value (usually expressed as a percentage) indicates a good bit of the data is being corrupted which is causing the packet to be resubmitted, which will cause latency. The thing about digital communications, is that it either works or it doesn’t. Your radio is usually working pretty hard to fill in the voids.

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In summary, I have just touched on a few of the things that can cause issues with your radio signal. There are many more things that can help or hinder your signal. Remember that all of the radio bands I have mentioned are considered line of sight. For instance, an antenna at a height of about 6 feet only has a 3 mile line of sight due to the earths curvature. And that is if the ground is perfectly flat, and obstacle free, so obviously antenna elevation is a big deal. Another thing to mention, it would benefit you to study the topography to see if you have an option to use the lay of the land in your favor, such as shooting a signal down a river valley. Another topic would be weather. Weather also affects radio propagation,(especially at the higher frequencies in the ISM band). A marginal link on a calm day, can be a dead link on a foggy, rainy, or snowy day.

One last point I would like to make, is picking a radio system on upfront cost alone, can be a very costly decision later when it doesn’t perform to you standards. Never hesitate to consult someone, propagation studies and professional proposals can be a bargain compared to a failed installed system.

 
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Posted by on December 14, 2013 in 2013, Current Month

 

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