Antennas and Amplifiers for Long Distance Wireless Communications

by on February 13, 2014

Hi all
This is just a quick post on how to increase the range of your wireless communication system. There are several ways to do it and they mostly apply no matter what frequency you are using. We will discuss several ways to do this below. We will also discuss some of the legal limits for how powerful your system can be.

You can use tools such as the Ubiquiti Airview or OmniWiFi to view how strong your signal is and what signals are in view. Or you can use less advanced software tools that use your computers wireless card. In Linux the command to check signal strength of networks in range is “sudo iwconfig wlan0 scan” where wlan0 is your wireless card device name.

Loss Reduction

The simplest way is to reduce the loss in your system. This is often from two sources; the cables and the connectors.
Cables – Try to use the lowest loss cable that you can use. Typically the lower the loss in the cable, the more it will cost, and the thicker and less flexible the cable will be.
Connectors – Minimize the number of connectors that you use and avoid using adapters to change connector types. Each connector and adapter interface represents a signal loss (called an insertion loss) in the final output. A general rule is that every connection of the same type (ex. N to N) is 0.25dB of loss and every adapter between connector types (ex. N to SMA) is 0.5dB of loss.
Clean your connectors – A common problem that most people ignore is cleaning the threads and inside of the connectors. When a connector is used several times a metal powder can form on the inside of the connector “shorting” the center pin to the housing. The best way to clean the connectors is with denatured alcohol (isopropyl alcohol can also be used but it leaves a film behind which is not ideal) and cotton swabs (often called Qtips). Be careful not to bend any pins!

Antenna

The next way to increase your range, is to use an antenna with an increased gain. You can get omnidirectional antennas with different gains that transmit in a torus pattern (they do not transmit directly above or below the antenna). As the gain increases the height of the torus transmission pattern decreases, however the range (diameter of the torus) increases.
You can also consider a directional antenna for increased range. These antennas focus the RF energy in a specific direction to extend the range. As with the omnidirectional antennas as the gain increases the directionality of the antenna increases for a tighter window. If the directional antenna is not pointed perfectly in the right direction communications will suffer.

The other thing that helps antennas a lot is increasing the height and avoiding obstacles in the line-of-site (LOS) path between the antennas. If you can increase the height and avoid breaking LOS you can get a much better signal. When you determine LOS you need to remember the Fresnel zone. The Fresnel zone is an area above and below the LOS line that should be clear for maximum radio transmission quality, however it is not always possible for that area to be completely free from obstruction. As a general rule you should make sure to have less then 20% blocked however communications should still work with up to 40% of the area blocked.

Fresnel zone: F is the diameter of the fresnel zone in meters, D is the distance between the transmitter and the receiver; r is the radius of the first Fresnel zone (n=1) at point P. P is d1 away from the transmitter and d2 away from the receiver, lambda is the signal wavelength in meters (from wikipedia).

$F = \sqrt{\frac{n\lambda d_{1} d_{2}}{d_{1} + d_{2}}}$

The easier way to figure out the center diameter of the fresnel zone for 2 aligned antennas is:

$r = 8.657 * \sqrt{\frac{D}{f}}$
r= radius in meters of the largest center point
D = distance in km between antennas
f = frequency of the signal in gigahertz

Signal Amplifiers

Signal amplifiers sit between the radio and the antenna and can inject power into your transmission to increase the output power of the signal. This is often one of the most effective ways to increase power. The downside is that you need to provide more power for the amplification as opposed to just using an antenna with a higher gain.

This might not always be possible but you can consider changing the radio you are using to get a higher output power model or to operate with a different frequency.

As a general rule as the operating frequency decreases, the range increases and you get better obstacle penetration. As the frequency increases you get a higher bandwidth (more data), and a smaller antenna.

Legal Limits

There are also legal limits for how much power you can output and what frequencies you can operate on. The information below is for the USA (While I think this is correct I make no guarantees).

Frequency Limits

The standard frequencies for open use are known as the industrial, scientific and medical (ISM) bands. These bands can be used freely as long as you do not interfere with other devices.

ISM Bands From: http://en.wikipedia.org/wiki/ISM_band

The 900MHz radios we typically use in the United States are not allowed in many parts of the world. Some other parts of the world have spectrum put aside with similar performance.

Location Bands
Australia 920 MHz
China 779-787 MHz
Africa & Europe 863-870 MHz
Japan Nothing
North America 902-928 MHz

Power

The maximum allowed transmitted power that gets fed into the antenna is 30dBm (which is 1W).
The maximum allowed radiated power is 36 dBm (4W).
To get the maximum radiated power you just add the gain of the transmit power to the gain of the antenna.
There are specific exemptions from this rule for the 2.4 and 5.8GHz ranges if the equipment are fixed point to point links (which robots are often not). You can get more details at www.afar.net/tutorials/fcc-rules/.
Remember that it is your responsibility to make sure these rules are followed and you dont make the FCC unhappy.

Here is a good video about antenna fundamentals:

Here is a good video explaining decibels (dB’s):

A good source for RF equipment (connectors, amplifiers, antennas, cables, etc..) is L-com.com or www.streakwave.com.