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ContinuousWave: Small Boat Electrical
Effect of Antenna Gain on Communication Range
|Author||Topic: Effect of Antenna Gain on Communication Range|
posted 06-07-2009 11:24 AM ET (US)
A frequent question regarding VHF Marine Band radio installation on small boats is the choice of an appropriate antenna, particularly the amount of antenna gain. The common choices are antennas with either 3-dB or 6-dB of gain. Is there much difference in range between them?
Increasing the antenna gain by 3-dB will increase the range. The amount of increase depends on the model you assume for propagation loss. In a free space model a 3-dB increase increases the range about 30-percent. In a real-world terrain model, a 3-dB increase increases the range about 18-percent.
If we use the real-world model, we could say that if a 3-dB antenna could only communicate to a maximum of ten miles, going to a 6-dB antenna would increase it to about 12-miles. Of course, this only applies when you reach the absolute limit of communication. If the circuit is not at the absolute limit, there is little difference between the two. The added gain only really makes a difference at the fringe of your range.
Antenna gain is very difficult to determine. Tests have shown that some antennas rated as "3-dB" gain antennas actually produce more gain than antennas rated as "6-dB" gain antennas.
Antenna gain seldom makes a huge difference in reception. Most receivers have plenty of reserve gain, so if a signal is low the gain can be made up in the receiver as opposed to in the antenna. Antenna gain only enhances signals at the very threshold of a receiver's sensitivity. Once signals are above the receiver noise floor, antenna gain is not needed for reception. This phenomenon is well known to many boaters whose antennas are just about non-functional. They still receive quite well.
Antenna gain on transmit is always a plus. It increases your effective radiated power. However, all antenna gain generally comes at the expense of the pattern. Gain antennas have narrower patterns and may not be suitable for use on small boats where the antenna orientation is often in rapid movement. For more detail on this, please see my article on antenna patterns at http://continuouswave.com/whaler/reference/antennaVertical.html .
The range of radio communication in the real world is influenced by many factors other than antenna gain. An important influence is the noise level of the receive site. The most sensitive receiver connected to a high gain antenna is of little value if located in a noisy environment. On small boats the spark ignition of the engine often produces radio frequency noise which can limit receiver sensitivity.
For communication to take place, two stations are required. The capabilities of the second station are influential. If the second station has a high gain antenna, a optimally located antenna site, a tall tower, and a low noise location free from interference, you will be able to communicate with this station at a long range. If the other station has a poor antenna located in a noisy location with lots of interference, and a receiver of reduced sensitivity, your communication range with this station will be limited.
posted 06-07-2009 04:39 PM ET (US)
Thanks Jim, this is very interesting to me (as you no-doubt have noticed over at THT).
Now how do we figure out where the 0-180° axis lies without a field strength analyzer?
posted 06-07-2009 11:41 PM ET (US)
Unless I'm missing something, the only axis is vertical. The graphical representations of radiation are equivalent to a view of a donut which has been cut in half. As the antenna gets longer the energy is preferentially radiated in a horizontal direction. This is great, but may be a limitation in rough seas.
posted 06-07-2009 11:50 PM ET (US)
My analysis of antenna gain on communication range assumes the antenna pattern is optimized to point the main lobe in the desired direction. With any antenna that has gain there will be a main lobe with maximum radiation as well as nulls and minor lobes with less radiation.
posted 06-08-2009 07:29 PM ET (US)
AZDave, what Jim referenced in his article are vertical (or polar) radiation patterns.
Some manufacturers (and text books) will also show an aziumthal pattern, which I believe is what you are referring to.
posted 06-08-2009 07:33 PM ET (US)
Excuse me, I meant elevation, not azimuthal.
posted 06-08-2009 07:50 PM ET (US)
Dave--You were right initially.
With a vertical monopole antenna the azimuthal pattern is assumed to be uniform. The patterns in my article are plots of the intensity of radiation from the antenna plotted against the vertical angle of radiation. The antenna is oriented orthogonal to the earth. The horizon is at the 0-degree or 180-degree angle.
For marine communication the most important angle is the 0-degree or 180-degree angle.
posted 06-08-2009 08:18 PM ET (US)
This stuff makes my head spin, sometimes.
I'm currently studying for my General, and while the material isn't particularly heavy on antenna theory, I've been fascinated my copy of the ARRL Antenna Book.
posted 06-09-2009 08:54 AM ET (US)
Suppose we have two boats that are ten miles apart. I have analyzed the communication path for this situation in an article at
Marine VHF Radio Communications
I found that a circuit like this will have almost 55-dB of fade margin. In other words, with the typical antennas, antenna height, 25-watt transmitters, and receivers of -107 dBm or 1-µVolt sensitivity, the signal level at the receiver input will be over 50-dB--almost 100,000-times--higher than the threshold level needed for communications.
In general we can say of radio communications that as the signal level improves, the signal-to-noise ratio of the recovered modulation improves: the more signal the better. In the FM modulation technique the detector exhibits a saturation effect. Once the signal level has risen above the detector threshold, a very significant improvement in signal-to-noise ratio occurs, but further increases in signal tend to only provide marginal improvement, and at some point no improvement. This is the saturation effect. For a communication circuit, once the signal input is about 10-dB above threshold, you are well into the saturation point of the FM detector.
In our ten mile circuit example, we have over 40-dB of saturation. That means that we can experience a loss in signal of 40-dB before we start to notice anything happening in the recovered modulation signal. It also means that if we increase the signal by 3-dB there will be no noticeable effect; the detector is already into saturation.
In this situation we see that improving the antenna gain by 3-dB is not going to make much difference. We compare this with the many anecdotal reports from boaters that when they changed to an antenna whose gain was advertised as 6-dB from another antenna with less advertised gain, or even another antenna whose gain was advertised as 6-dB but made by a different manufacturer, will swear it made a dramatic improvement in the range of their radio communications. No to call all of these many testimonies false, but the nature of FM communication means that the extra 3-dB of gain--if indeed it exists in the new antenna--comes into play only when you are on the fringes of your range, that is, only when the signal level is below the saturation level of the detector and a jump in signal level will produce some sort of noticeable difference.
posted 06-09-2009 10:48 AM ET (US)
This is slightly off topic, but I thought it may interest you. I have been a big fan of Digital Antennas and have both their 8 ft VHF and 4ft FM antenna on my 20 Outrage.
They are advertising a new wide-band VHF antenna on their website. I am not sure what that means. I love technology but am nowhere near the knowledge level of you guys. Your all on your own plane when it comes to radio technology understanding. My take is this antenna can be used for mutltiple applications at the same time. (i.e., VHF, AIS, FM Stereo) Am I correct or have I missed something. Here is a link.
posted 06-09-2009 12:08 PM ET (US)
Antenna bandwidth is generally defined in terms of two parameters:
The VHF Marine Band is allocated the spectrum from 156 to 162 Mhz (approximately). The antenna you mention has a claimed bandwith which is much wider, from 134 to 176-MHz. There is no intrinsic advantage to using an antenna whose gain or VSWR bandwidth is particularly wider than the range of frequencies in use. From the standpoint of a VHF Marine Band transmitter, the wider bandwidth is not particularly advantageous.
Exploiting a single antenna with wide VSWR and gain bandwidth can be difficult if you expect it to be used simultaneously with more than one transmitter. Connecting one antenna to two receivers is not particularly hard, but connecting two transmitters to one antenna is just about impossible to do for simultaneous transmission.
I would not pay extra for a wider bandwidth antenna for VHF Marine Band use.
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