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ContinuousWave: Small Boat Electrical
Effect of Power on Range
|Author||Topic: Effect of Power on Range|
posted 01-08-2009 01:38 PM ET (US)
Assume that an antenna with an isotropic radiation pattern is fed with power P. In free space the waves emitted from the antenna expand and at distance, r, create a field, f, of
f = P/4πr2
The term 4πr2 is the surface area of a sphere of radius r, and we have spread the available power over this area to create a particular field density.
Now we double the power (to 2P) and increase the distance to R, until we have the same field
P/4πr2 = 2P/4πR2
Now we solve for R to discover how much greater range we have created with a doubling of the power:
8Pπr2 = 4PπR2
In other words, doubling the power increases the distance at which an antenna can produce a particular field strength by a factor of 1.414.
We compare this with many anecdotal accounts of how changing to an antenna with a gain of 6-dB has improved the range obtained compared to an antenna of 3-dB gain. In one report it was cited that the range increased to "15 to 18 miles" from a range of only "4 to 6 miles." This is an increase in range of about three times.
It is very possible that such a difference might be observed, however there is no way to attribute the difference in range as being due to a difference in the gain of the antennas. A difference in gain of 3-dB is only a difference in effective radiated power of a factor of two, or the same as doubling the power. As shown above, doubling the power will only increase the distance to the same field strength by a factor of 1.414 times.
posted 01-09-2009 06:28 PM ET (US)
That's about the same gain as one would achieve in MPH when doubling the horsepower on a boat. Coincidental or is their some correlation?
posted 01-09-2009 07:40 PM ET (US)
At high speed, most of the power goes to overcome wind
With radio waves, the power required goes up as the surface
posted 01-10-2009 01:34 AM ET (US)
Interesting equations, but you lost me on context with the average (especially Whaler) boater. Are you trying to explain how a person could use these equations to extrapolate the difference of their radio signal when they flip between the 1w and 25w settings on most VHF radios?
Does this even apply the the VHF range? What about SSB? Or does it even matter and you are just talking about a specific antenna design?
posted 01-10-2009 01:39 AM ET (US)
Allow me to be dumb for a second. Does the same apply to AM broadcasts? I remember in college having to turn down our signal at night so that we wouldn't walk over other stations that could be on the other side of the country.
So, in reading all of this am I correct to assume that we are dealing with two distinct issues, antenna design, and spectrum location/type?
posted 01-10-2009 09:27 AM ET (US)
Brian--The change in signal level with distance and power is the same for all radio waves, all antennas, all modes, all frequencies. When the power of a transmitter is doubled, the range at which it produces a particular signal level is not doubled or tripled, but only increases by a factor of 1.414-times.
Broadcast stations in the AM band change power at night because the propagation characteristics of the frequencies in the AM Broadcast Band change at night. During daylight certain stations might not interfere with each other, but when darkness falls, the propagation changes, and signals may travel farther. This change in propagation requires some stations to modify their power level and (in some cases) antenna pattern. At the frequencies used in the VHF Marine Band radio wave propagation is not much affected by diurnal variations.
Antenna design does not affect the way a radio wave travels and decreases in strength. Wave propagation is independent of the antenna design. Once a radio wave leaves the antenna, it propagates the same way as other radio waves of that frequency.
posted 01-10-2009 09:50 AM ET (US)
My observations about transmitter power level and its affect on signal level at a distance from the transmitter should not be read as a suggestion that transmitter power level is not important in a communication circuit. Any time transmitter power can be increased it is helpful. Increasing effective transmitter power by using antenna gain is also doubly helpful.
Antennas behave the same way on receive as they do on transmit, so an antenna which provides gain on transmit also provides that gain on receive. In this sense, antenna gain is doubly useful, as it helps to improve both transmission and reception. An antenna which has 3-dB of gain will effectively double the power of the transmitter, and during receive it will effectively double the power of the received signal. Antenna gain is particularly useful in the VHF Marine Radio Service because stations are limited only by their actual transmitter power, not by their effective radiated power. Recreational vessels are all limited to 25-watts of transmitter power output. A recreational vessel can increase its effective radiated power using antenna gain. There is no penalty imposed on transmitter power by use of an antenna gain. A vessel using a 25-watt transmitter and an antenna with a certain gain will be able to transmit farther than another vessel using a 25-watt transmitter and an antenna with less gain.
The reciprocal nature of antenna gain is very useful. When antenna gain increases the range of a station on transmission, it also tends to increase the range of that station's receiver. The antenna amplifies the incoming signal by the same factor it amplified the outgoing signal. In this way the range of communication should be increased on both transmit and receive.
posted 01-10-2009 10:20 AM ET (US)
In one sense, all transmitters are alike, and especially so in the VHF Marine Band radio service where they all produce 25-watts of power. They send their 25-watts of radio frequency power to the antenna, and the antenna launches a signal into the surrounding space. When this takes place, it happens without much influence from the surrounding electromagnetic environment. For example, there might be another transmitter located 500-feet away, transmitting on a nearby frequency. This does not affect the transmission from our transmitter. Our transmitter generates its 25-watts, sends it down the coaxial transmission line to the antenna, and off it goes into space. The presence of radio waves from other local sources do not effect the transmission of our radio waves.
On the other hand, reception of radio waves is very much influenced by the surrounding electromagnetic environment. The receiving antenna picks up all signals in its environment, some of which are desired and some which are not. Because of this, the environment in which the receiver is located has a profound effect on the receiver's ability to detect a desired signal and amplify it to a useful level. The receiver environment may also vary greatly from one vessel to another. For example, one vessel may be a sailboat on which there is absolutely no other electrical equipment operating. That environment may have no other local sources of electromagnetic radiation which can interfere with reception. In contrast, another vessel may be using an engine with a spark ignition system that generates a great deal of radio frequency noise and interference. It may have several electronic devices which also emit some radio frequency interference in the VHF Marine Band. And the vessel could be located near another vessel or land where there was a significant source of interference being generated. As a result, the receive environment can be significantly different between two vessels. On one the receiver may be able to operate with its maximum sensitivity, while on the other the receiver may have its sensitivity reduced by interference from local sources.
Because incoming signals from distance stations arrive at very low power levels, a local signal of even a very low level has the potential to create interference. Local sources of radio signals might not even be noticeable until they reveal themselves by interfering with weak signal reception. Because the local environment can be so influential, it is possible for very significant variation to exist between vessels with regard to their ultimate receiver sensitivity and performance.
posted 01-11-2009 10:00 AM ET (US)
Unfortunately, the gain in an antenna on receive amplifies all signals, including any local noise sources. If an antenna is located in a noisy environment, its gain will not improve reception because it will amplify the noise along with the distance signal.
A receiver located in a noisy environment will have its effective sensitivity reduced by the local noise floor. When we make predictions about the range of communication between two stations, there is an assumption that there is no local noise or interference at either receiver. We assume the receivers will operate at their maximum sensitivity, which is typically about -107 dBm or the equivalent of one micro-volt of signal in a 50-ohm antenna. Unfortunately, this situation is not always obtained in a small recreational vessel where the antenna may be located close to the spark ignition outboard motor and other vessel electronic devices. If any of those sources create interference which is stronger than -107 dBm, the effect is to decrease the sensitivity of the receiver.
In practical antennas with gain, there is always some concentration of the radiation pattern of the antenna into a main lobe and a corresponding decrease in radiation in nulls or minor lobes. In vertically polarized omnidirectional antennas, the concentration of the radiation pattern is in the vertical angle and the radiation in the azimuthal angles is assumed to be omnidirectional. The direction of the vertical main lobe is assumed to be at the horizon (although this is not always the case nor is it guaranteed in all antennas). In general we can say that the deepest nulls in the vertical pattern occur at straight up or straight down.
If we consider the orientation of a vertical antenna on a small boat with respect to other sources of local noise (like the outboard motor or other electronic devices), we find that antennas which are mounted at low elevations and close to the helm tend to have their main lobe oriented directly at these other sources. Raising an antenna above the level of the outboard motor and other electronic devices tends to move those sources into a sector of the radiation pattern where they will be in a null or minor lobe of the pattern. In this way it is possible to reduce the influence of these local noise sources on reception. We know that raising an antenna will improve the distance to the radio horizon, but raising the antenna also provides the additional benefit of removing local noise sources from pointing directly into the main lobe of the antenna on receive.
posted 01-11-2009 01:24 PM ET (US)
When we investigate practical antennas that have gain we discover that the gain comes through concentration of the radiation pattern into a main lobe and at the expense of radiation in other directions. Indeed, when an antenna is described as having a particular gain, the gain measurement is in reference to the radiation in the most favorable direction. The gain measurement is also in reference to some standard antenna or other reference antenna. In some cases the gain measurement is inflated by reference to a theoretical antenna which radiates equally in all directions and has no concentrated main lobe. Such an antenna is called an isotropic radiator. In reality, all practical antennas can be described as having gain over an isotropic radiator, since all practical antennas have a tendency to focus radiation into one or more lobes and to suppress radiation in other minor lobes or nulls.
The location of an antenna in relationship to other electrical conductors can also influence the radiation pattern. Theoretical antennas are considered in free space, a void in which there are no other electrical conductors or anything else to modify the behavior of the antenna.
A more useful standard for comparing the gain of a practical antenna is to compare it to a standard halfwave dipole antenna which is located in the same environment. Stating antenna gain in this manner removes about 3-dB of gain from all marine antenna gain claims. That is, most marine antenna manufacturers state the gain of their antenna by comparing it to an isotropic radiator in free space. If you compare the gain to a standard halfwave dipole at the same location, you will have a better assessment of the practical antenna gain you are achieving.
When an antenna creates gain we would like it to create a radiation pattern that was optimal. Typically an optimal radiation pattern is one in which there is a single main lobe that is relatively broad. Unfortunately, this is not always realized in practice. Creation of gain by formation of a main lobe depends on the waves emitted from different parts of the antenna coming into an alignment in such a way as to be additive in the direction of the main lobe and subtractive in the direction of the null or minor lobes. As a general rule, to get more gain requires more radiating elements in the antenna. The more elements, the more precision needed to create the alignment to produce gain. In particular, the phase and current in the antenna elements must be precisely controlled to produce an antenna with a clean single-main-lobe pattern.
It is not particularly difficult to create antenna gain by addition of more radiators to the antenna, but it can be very difficult to produce an antenna pattern which is free from minor lobes and deep nulls.
Practical antennas on small boats are often installed in close proximity to other electrical conductors such as windshield frames, canvas top frames, radar arch supports, and even other antennas tuned to precisely the same frequency. The radiation pattern of an antenna will be affected by the presence of these other conductors. The affect will be particularly significant if the relationship of the other conductor is not completely symmetrical to the antenna. For example, if an antenna is 7-feet long and is in close proximity to an electrical conductor only at one end, this is a highly asymmetrical influence, and we can expect the effect on the radiation patter to be greater than if the conductor were oriented in a way that was symmetrical to all parts of the antenna.
There is an assumption with the gain expressed for a VHF Marine Band antenna that the main lobe is oriented in the most useful direction, that is, precisely at 90-degrees to the antenna or at the horizon. There is no guarantee that this is obtained. Due to the very prevalent use of end-feed antenna, there is an inherent asymmetry in the antenna current distribution. This may result in the main lobe being created at a slight angle to the horizon in these antennas. The antenna may produce the stated gain, but the main lobe may not be aligned in the most useful direction.
A further consideration of practical antennas is the width of the main lobe. It is common to describe the main lobe of an antenna in terms of the the angle off axis needed to reach the point where radiation decreases by 3-dB. In practical antennas there is a tendency for the width of the main lobe to decrease as its gain increases. This creates a situation where alignment of the main lobe become more critical. An ideal antenna would have high gain and simultaneously a very broad main lobe devoid of any nulls. Such an antenna would be most useful and easily aimed.
There is a general misconception that a vertical antenna that is operated with the radiating element at a slight angle to the earth produces a radiation pattern in which the main lobe is correspondingly changed by the same angle. For example, consider a vertical antenna that was precisely orthogonal to the earth and produced a main lobe at an elevation angle of 0.5°. If the vertical radiator were tilted 10-degrees from vertical there is an assumption that the main lobe would now be tilted at 0.5 + 10 or 10.5° from the horizon. This is not true. There may be some influence from a tilted vertical radiator, but the effect is not precisely one-to-one with the angle of the tilt.
Further to the above misconception, the main lobe is not a pencil thin beam, but rather more likely a rather broad main lobe. If the radiator is tilted slightly, even if there is some change in orientation of the main lobe, it does not mean that the radiation at the horizon falls to zero or into a deep null. Radiation at the horizon may decline somewhat, but precisely how much depends on the pattern characteristics of the antenna. If the antenna is tilted wildly, say to 45-degrees, then there is more chance the radiation at the horizon is affected.
posted 01-14-2009 08:48 AM ET (US)
The effect of physically tilting a vertical antenna on its radiation pattern at the horizon was studied by L. B. Cebik in
Cebik used computer modeling to investigate the behavior. He found that tilting a vertical dipole antenna toward a distance station reduced the signal, but the amount was not particularly significant, and it varied with the height of the antenna. On a small boat the typical antenna height is less than 15-feet, which at 156-MHz is less than three wavelengths. The model showed that a tilt of 20-degrees for an antenna at 3 wavelength height only reduced the distant signal strength by 0.9-dB.
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