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BOATING Magazine Antenna "Tests"
|Author||Topic: BOATING Magazine Antenna "Tests"|
posted 02-23-2013 11:54 AM ET (US)
I received an email recently suggesting I could learn about VHF Marine Band radio antennas from a recent "test" of several done by BOATING magazine. Having a great interest in antennas in general and in VHF Marine Band antennas in particular, I immediately followed the link to the article and read it with great interest. It is mostly hogwash, and contains a great deal of misleading information. The article contains some astonishingly misinformed statements and some dubious findings in the "test" results.
In describing antenna "gain", Boating says:
The first sentence contains three errors:
The gain of an antenna is always expressed as the maximum gain in the main lobe compared to some reference antenna. In the case of marine antennas, the comparison is usually set up to be unrealistically favorable by choosing a comparison antenna that is theoretical and an environment for the actual antenna which is inherently favorable. As a result of the unfair or misleading comparison of gain, most of VHF Marine Band antennas have 3-dB to 4-dB less gain than their manufacturers advertise and as these "tests" claim to measure. All antenna gain comes from concentrating energy into a main lobe at the expense of losing it in other minor lobes. The "average" gain of an antenna is, at best, 0-dB, if you consider that you are averaging the gain over all directions.
Gain is not a "degree of polarity." The polarity or polarization of an antenna is determined by the relationship of the E-field and H-field orientation to Earth, and in the case of VHF Marine Band, the polarization should always be for vertical polarization. There is no polarity in a sense of DC voltages. Antenna polarization is usually described as being linear, elliptical, or circular. All the antennas tested by BOATING were linear vertical polarization antennas. This feature is completely separate from their gain.
All the antennas tested have no horizontal focus or directivity; they are all omni-directional antennas in the horizontal plane. All of the directivity or gain comes from changes in the vertical plane, by concentrating radiation at the most useful vertical angles.
The second sentence repeats the error in confusing the plane in which the signal concentration occurs. All the antennas tested are omni-directional in the horizontal plane, but have some varying degree of directivity in the vertical plane.
The third sentence continues the embarrassing parade of errors. An antenna with 6-dB gain has twice the power gain of a 3-dB antenna, not quadruple. Using an antenna with 3-dB more gain is generally considered to be the equivalent of doubling the transmitter power. These relationships are really not that hard to understand, but apparently BOATING does not understand them.
The fourth sentence continues the error, claiming a increase in gain by 6-db from a +3-dB to a+9-dB antenna constitutes an eight-times increase in power. Sorry, it represents a four-times power gain. A simple way to assess decibel relationships of antenna gain is to remember that a change of 3-dB is a doubling of power (if a positive 3-dB or a gain of 3-dB) or a halving of power (if a negative 3-dB or loss of 3-db). BOATING seems to have not learned this.
Next BOATING says:
This is badly stated. SWR is a ratio, but they fail to describe what the ratio is between. Antennas have an AC impedance, which consists of both a resistive and reactive component, which is then modified by internal matching networks so that impedance becomes close to the characteristic impedance of the transmission line that will be used to deliver power to the antenna. The SWR is a measurement of how well the antenna impedance matches the transmission line impedance. A SWR of 1:1 is hard to obtain, but it is often done. Radio specification do not require a specific VSWR. In general, the closer to 1:1 the better, but anything under 2:1 is going to work. The notion that SWR can be read by inexpensive meters is a topic for its own discussion. Perhaps a better definition would be that SWR is a figure of merit that describes how well the antenna is matched to its transmission line; lower is better.
Now to the antenna ranges. BOATING says:
Sorry, but all loss in propagation of radio signals is the same for all antennas. Radio waves do not know that they came from three-foot antennas and are therefore supposed to lose intensity faster.
BOATING computes the radio horizon for the antennas. All eight-foot antennas are said to have a radio horizon of 5.3-miles, and all three-foot antennas are said to have a radio horizon of 4.5-miles.
The usually accepted theory for radio horizon says the distance to the radio horizon in miles is equal to the square-root of the antenna height in feet. Let's derive the actual heights needed to get these two figures
Radio Horizon = 5.3 miles
Radio Horizon = 4.5-miles
This is a difference in height of 4-feet, which is about the same as the difference in length between the eight-foot antennas and the three-foot antennas. However, the radio horizon has nothing to do with the antenna length, but everything to do with the height of the antenna above ground. The radio horizon increases at the same rate for any antenna as it is raised.
In the article it is mentioned that the eight-foot antennas were mounted seven-feet above sea level. This would put the top of the antenna at about 15-feet. I assume this means the base was seven-feet above sea-level. If the three-foot antennas were similarly mounted, their tips would be 10-feet above sea level.
Now to range of communication. BOATING defines a parameter, "Tested Range."
All radio signals begin to deteriorate the moment they leave the antenna. They all decrease by the inverse square law relationship. To define a range of communication requires there be a definition of a certain level of signal needed. When the signal decreases to less than the required level, a range can be declared.
BOATING says that the eight-foot (6-dB) antennas mounted seven-feet above the water at their base have a "tested range" of 5-miles, and all of the three-foot antennas (3-dB) (presumed to also have a mounting height of seven feet at their base) have a "tested range" of 2-miles. This result is hard to understand. The range increase for a 3-dB gain in the test is an increase of 5/2 or 2.5-times. But BOATING has already said that the difference in 3-dB and 6-dB is a factor of 4-times. Their test results have no correlation to their definitions. Not surprising, as the definition was wrong and the test results badly defined.
If we use a free-space model for propagation, we can deduce the difference in signal level that occurs between a distance of 5-miles compared to a distance of 2-miles. The distance ratio is 2.5. Signal level due to propagation loss would tend to decrease (at the most favorable instance) by 20 x LOG(d) or -8dB. The implication of this is very hard to reconcile. BOATING is saying, more or less, that the signal at 2-miles from the shorter antenna was the same as the signal at 5-miles from the longer antenna. We know that in the most favorable case, the signal propagating the extra three miles is going to decrease by 8-dB. Somehow the longer antenna, whose gain is only said to be 3-dB, makes up an amazing 5-dB of extra loss. This is not really understandable based on propagation loss.
The most important concept to understand about radio horizon and communication range is the outcome depends on the capabilities of the two stations involved. The range of communication of a boat with an antenna at 7-feet above sea level is not a fixed distance; it varies with the range of the other station in the communication circuit.
Concerning the "Average Gain" there were also some amazing results. Every antenna tested produced exactly 3-dB or exactly 6-dB of gain. This sort of result is extremely hard to imagine. By using the term "average" one makes the inference that many measurements were made and the results were averaged. The amazing congruence of the stated results suggests that no tests of gain were made at all, and the presentation of the "Gain Average" is just something read off the packing slip that came with the various antennas.
It is a good thing that my 3-foot-long GAM SS-II antenna (which coincidentally has its base mounted about 7-feet above the water) has not heard about its shortcomings as described by BOATING. The GAM receives NOAA weather stations from ranges of 75-miles or more, and talks to other boats with excellent signals at ranges of 15-miles. According to BOATING, anything past two miles would be unlikely.
BOATING also expresses concern about antennas that do not come with a transmission line made with RG-8X cable, and implies that use of RG-58/U cable represents a failing.
There really is no standard defining RG-8X cable, and manufacturers use this designator for all sorts of cable. Typically cable marked as RG-8X employs a outer diameter that is slightly larger than RG-58/U but much smaller than RG-8/U, a cable designator that was once a military standard. Cables marked RG-8X also tend to use a dielectric made of foam instead of a solid polyethylene dielectric material. There is some concern for the tendency of foam-dielectric cables to absorb moisture in the foam. (Does that sound familiar to Boston Whaler boats owners?) RG-8X cables also tend to have outer jackets that are not resistant to UV deterioration. A cable marked RG-8X has whatever characteristics its manufacturer chooses for it to have. There is no real standard
There is a strict definition for RG-58C/U. It has to comply with military standards. It uses a solid dielectric and a UV-resistant non-contaminating outer vinyl jacket. Cable marked RG-58C/U is made to meet or exceed a defined standard. It often costs more than RG-8X cable.
The difference in loss in transmission line between RG-8X and RG-58C/U depends on the particular sample of RG-8X being compared. In the best case, the RG-8X cable will have marginally better (that is lower) transmission line loss compared to RG-58C/U. Because most installations on a small boat will use a transmission line of 15-foot or less, the difference in loss characteristics amounts to about 0.2-dB difference or less in the practical installation.
Transmission line loss tends to increase as the cable ages, particularly if the outer vinyl conductor deteriorates in sunlight due to UV. Chemicals in the vinyl begin to leach out of the outer covering and begin to contaminate the shield and dielectric insulation, causing transmission line loss to increase.
RG-58C/U cable has an outer covering of non-contaminating vinyl. There should be much less increase in loss with cable aging with this type of cable.
It might be interesting to actually test two samples of transmission line, one RG-8X and one RG-58C/U , after they had been exposed to sunlight for five years, and to see if their transmission line loss characteristics were much changed from new.
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