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A little bit about antennas

06 Dec

antennaDuring my time spent within the oil & gas E&P sector in various functions and settings, I have always in some capacity been involved with telecommunications. I have seen a few things and thought I might share a few things about antenna usage and optimization.

The most common thing I see is a misunderstanding about antenna gain.

Advertised antenna gain is almost always stated in dBi (decibel – isotropic). An isotropic antenna is a theoretical antenna in which the antenna radiates in all directions equally. An isotropic antenna is also referred to as a unity-gain antenna.

sunThe Sun is an isotropic radiator, radiating in all directions.

It seems it’s the belief of many that a high gain antenna increases the output power of a transmitter. This is a misconception. It’s not that the antenna is “increasing” the wattage, it’s how the antenna radiates the available wattage. To explain this further, I want to give a little information on the two most common antennas used in the oil & gas E&P sector.

Omnidirectional:

The omnidirectional antenna is usually a monopole, and sometimes a dipole antenna, that when mounted vertically radiates its power in a horizontal pattern. The radiation pattern of these types of antennas is shaped like a doughnut.

dipole_patternExample of an omindirectional radiation pattern.

In the typical point to multi-point radio layout, the omnidirectional antenna is usually used for the “host” or “master” radio, and sometimes with repeaters or “store-and-forward” radios to extend radio network coverage.

Yagi:

The Yagi antenna is a directional antenna with a driven element (usually a dipole) and additional parasitic elements with usually one serving as a reflector, and one or more elements serving as directors. Yagis are sometimes referred to as beam antennas. Yagi antennas, with their gain expressed in dBi, will also have a stated beamdwidth expressed in degrees. With a Yagi antenna, as the stated gain increases, the length of the antenna increases, and the stated beamdwidth decreases.

yagi_pattern3D Yagi antenna radiation pattern

yagi_pattern2d

2D Yagi antenna radiation pattern

In the typical point to multi-point radio layout, the Yagi antenna is usually used with a remote or “slave” radio.

Now as I stated before, an antenna cannot increase the output power of a transmitter. What it can do is effectively direct the output power of a transmitter.

This is my analogy to understand this concept:

Imagine you are in a pitch black room, and in the center of the room on a table are two light sources. A battery operated lantern and a battery operated flashlight. Both light sources have the same amount of batteries, the same voltage, and the same light bulb.

You first turn on the lantern. Notice how the light radiates, in a 360 degree  pattern, on a horizontal plane. You can see all four walls of this room, and you notice a map on one of the walls, but you really can’t make it out in detail. You cannot make out details about the floor or ceiling nearly as well as the walls. This would be analogous to a omnidirectional antenna.

coleman_lanternNow imagine that you turn off the lantern, and turn on the flashlight. As it’s laying on the table, a beam of light shines on the wall with the map. Now you can make out the map in detail and can make out the text and graphics with ease, but the rest of the room is indistinguishable. This is analogous to a Yagi antenna.

flashlightDisclaimer: Yagi antennas are more complicated than the analogy with side lobes and back lobe radiation, but I felt that this is a good basic analogy.

This is where the common misconception comes into play. Lets say you are transmitting with 4 watts at 450 MHz. Not taking into account line loss, if you have a 6 dBi gain omnidirectional antenna, you do not magically get 16 watts output power, rather since more power is being radiated horizontally, and very little vertically (remember the doughnut?), your radiation in the horizontal plane would be equivalent to a unity gain isotropic antenna being fed with 16 watts of power. 4 Watts would be your radiated power, 16 Watts would be your ERP or effective radiated power. By using a 6 dBi gain omnidirectional antenna, you are effectively radiating 16 watts equivalent to a theoretical isotropic antenna. The FCC limits both power out and effective radiated power out.

With a licensed radio you will have a power limit and an ERP limit, so antenna selection and proper line/connector loss calculations are important.

licenseLicense example (source FCC ULS Database)

In regards to unlicensed radios, the FCC limits equipment in the ISM band (902-928 MHz) to 30 dBm (dBm = power output measured in reference to 1 milliwatt) and 36 dBm ERP.

(30 dBm = 1 Watt & 36 dBm = 4 watts)

The next important factor to consider is using properly sized feed line (coax).

I have seen a few installations using about 20 feet of RG-58 (or RG-58 equivalent) coax with a 902-928 unlicensed spread spectrum radio. 20 feet of RG-58 coax at 915 MHz is a loss of about 3 dB. Using a 6 dBi Yagi, we now have 33 dBm ERP instead of the maximum 36 dBm.  So now we have 2 watts ERP instead of the allowed max of 4 watts, all because of a bad choice in coax.

As frequency increases, coax becomes more and more critical. This is due to a phenomenon called the skin effect. As frequency increases, electrons tend to travel on the surface of the conductor. Therefore, the smaller the surface area of a conductor, the more resistance as frequency increases.

Take the same installation above, and replace the RG-58 coax with LMR-400 coax. Loss decreases to 0.783 dB. So now with a 6 dBi Yagi we have a ERP of 3.32 Watts. Much improved.

And last but not least, when using lightning arrestors such as a polyphaser, grounding is very important.

polyphaser

Polyphasers and other lightning arrestors such as gas tubes, shunt high voltages from the center conductor to the shield, and the shield to ground. That is their main purpose. So a good healthy low ohm ground wire, as short and straight as possible, is critical for proper operation. An anemic jacketed 14 gauge wire to a ground bus is not a good install.

It is my hope that this brief and basic post may be of use to someone.

To continue on with this theme, read “A little bit about radio propagation”

 
1 Comment

Posted by on December 6, 2013 in 2013, Previous Month

 

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One response to “A little bit about antennas

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