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VHF Marine Radio Antenna Fundamentals

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VHF Marine Radio Antenna Fundamentals

Postby allingeneral » Sun Jun 01, 2008 9:16 pm

VHF Marine Antenna Fundamentals.

I am in the process of putting together a new VHF marine radio setup on my boat due to the current antenna being broken. While searching for information about selecting a marine radio antenna, I came across this article which is the clearest, most concise information I have found concerning VHF antenna principles and operation characteristics.

See author information at the end of article.

Selecting a VHF antenna for your boat is not difficult, but it is helpful to understand the basic concepts. While there are several types of antennas available for VHF Marine use, we will concentrate on the 8ft vertical whip, since it is by far the most common.

If there is one thing standard about VHF 8ft whip antennas; its the fact that there is no "standard" antenna. One brand of antenna may have a ground-plane built in, while another brand recommends a ground plane to help shape the antenna beam; some antennas include an interference filter built into the element, and some do not. Therefore, the discussion will cover a generic antenna, with the caveat that you will likely find an exception.

If you could visibly see electromagnetic radiation, a whip antenna’s radiation pattern would look like a donut. If we were to cut the radiation "donut" in half, we could get a good idea of the cross-section of the radiation pattern. The cross sectional area is known as the antenna "lobes", and provides a good basis for understanding the antenna's radiation characteristics.

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You should be aware that the antenna's radiation pattern is generally concentrated in the horizontal plane, with little in the vertical plane. For the horizontal plane, it can be said the radiation pattern is omni-directional or 360 degrees. The donut itself, especially on the horizontal plane can be miles in length, and any receiver antenna within the "donut" will receive the strongest transmitted signal. A receiver outside of the donut will not receive a very strong signal, or may not receive any signal at all.

Inside the donut is good: outside is bad.

Terminology

When you walk into a marine store or read manufacturer's literature, you will be bombarded with many specifications, some which are important, and some which are not. The following is an example of an actual antenna specification:

Antenna Specifications:
Length: 8ft
Type: Collinear 5/8 wave elements.
Gain: 6dB
DC Continuity: Yes
Bandwidth: 3Mhz within 2.0:1 VSWR

Antenna Length: There are actually two lengths of an antenna:

* The antenna's physical length.
* The antenna's electrical length.

The antenna's physical length is simply the length of the non-conducting fiberglass whip. It has little bearing on the electrical length of the antenna.

Electrical length is the "operating" length of the antenna, as the result of its electrical construction. Its the length the VHF radio "sees". To further explain electrical length, some discussion must be given to how electromagnetic signals propagate. Electromagnetic radiation travels through free space in invisible waves, not unlike waves upon the ocean. Electromagnetic waves have a specific wavelength, which varies with the transmitter's frequency. VHF Marine radios operate in the 156 to 157 MegaHertz range, which is often known as the "2 Meter Band". If you could measure the electromagnetic wave with a yardstick at this frequency, it would be about 1.92 Meters from "crest" to "crest".

For an antenna to generate (transmit) or capture (receive) these "waves" efficiently, it must have an electrical length that matches the length of the electromagnetic wave in some form. Turns out that the most efficient electrical lengths are somewhat shorter than the electromagnetic wave's physical crest-to-crest length. VHF Marine antennas typically use 1/4 wavelength, 1/2 wavelength, and 5/8th wavelength ratios. A 1/4 wave antenna then would be an antenna with an electrical length one-fourth of the actual signal's wavelength.

Since you will often see electrical lengths in antenna specification sheets, it deserves mentioning. However, the wavelength specification is not all that important to the typical boatowner wishing to install their own VHF antenna. There is generally a relationship between the antenna's wavelength and its gain where certain gain antennas typically have certain wavelengths. Wavelength is simply a by-product of the desired gain.

Gain: All antennas have a gain rating, and while wavelength is not particularly important, the antenna's gain is. The layman's definition of antenna gain is an "apparent" increase in power that is available from the antenna. Gain in a VHF Marine antenna is expressed in deciBels (dB), and is actually an exponential ratio between the power level entering the antenna and the power level exiting it. In electronics-speak, there is a base reference level known as Unity Gain. Unity gain, expressed as 0 dB, means that there is neither any increase or loss of the signal's power. It can be said that 0dB means both incoming and outgoing signal power levels are the same.

The typical VHF Marine fixed radio has a 25 Watt output. If applied to an antenna having a gain of 0dB, the power output from the antenna (assuming no other losses) will also be 25 watts. But if the antenna gain is +3dB, then the output of the antenna will appear to be higher - 50 watts in this case. This apparent increase in output is known as the antenna's "Effective Radiated Power", or ERP.

As previously stated, gain is an expotential ratio, and for every 3dB change, there will be a doubling in ERP. So, for a +3dB gain antenna, its ERP will be double of the transmitter power (50 Watts), a +6dB gain antenna will have doubling of its ERP again (100 watts), and a +9dB gain antenna will double its ERP again (200 Watts).

So what is going on here - how can more power be produced by the antenna when there is no amplifier within the antenna? Well, more power is not being produced, it just apparently seems that way, simply due to shaping the lobes emanating from the antenna. The same input power exists, but more power is concentrated in narrower lobes.

Here is an example that might be helpful. Consider a flashlight that has a reflector that when turned, focuses the light into either a spotlight or wide-angle beam of light. When changing the reflector, the batteries do not increase power, nor does the light bulb give off more light. The reflector simply shapes the light into a concentrated spot or wider angle, depending on the direction it is turned.

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If the spotlight setting resulted in light traveling twice the distance than the wide angle setting, then it could be said that there is a 3dB gain in the effective light transmitted. However, this is true ONLY along the horizontal plane. Objects vertically higher at the fringe of the wide angle beam would no longer be illuminated in the spotlight mode. The important concept here then is that the Effective Radiated Power is purely a function of the shape of the light beam, and any gain realized is only true due to the available light being concentrated in a narrow beam. The 3dB gain "effectively" allows you to see further distant objects that you are pointing the light at.

In a similar fashion, the difference between a 3dB, 6dB, or 9dB VHF Marine antenna is how the radiated power is focused and shaped. If we take a look at the cross-section of the donut again, we can compare the different gain radiation patterns.

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A 9dB antenna takes on the "spotlight" characteristic of the flashlight, while the 3dB gain antenna takes on the "wide angle" characteristic. It might be noted that while a 9dB antenna might provide longer distance along the horizon, a 3dB gain antenna might very well present a stronger signal to high-flying aircraft. The obvious question then is whether a 9dB gain antenna will allow conversation with objects high in the sky - say a rescue aircraft. Well, yes it will, but it may be at a reduced signal level due to the lower radiation intensity in that direction.

Another way to express the relationship of gain and ERP is that a 6dB gain antenna with a 25 watt transmitter would transmit along the horizontal plane as far as a 3dB antenna with a 50 watt transmitter. If you understand this point, you are doing good.

While the typical antenna produces a symmetrical radiation pattern (lobes), some antenna manufacturers design their antennas to work with a ground plane on the boat. The resultant effect is an asymmetrical radiation pattern. This can be of advantage, because the goal is to "fold" the wasted radiation on the underside of the lobes into longer lobes. This characteristic can vary so much beteen manufacturers that it is not appropriate here to show the differences. Just realize that alternatives exist.

So far, we have limited the conversation to transmission - but what about receive? Since the same antenna is used for both transmit and receive, the antenna gains are going to hold true for receivers as well. For instance, a 6dB gain antenna is going to receive a signal that is twice as weak as an antenna with a 3dB gain.

One not-so-obvious characteristic of antenna gain is that it is accumulative. So if the transmitting and receiving radios each have a 6dB gain antenna, there is a 4 times improvement in signal strength over radios using 3dB antennas (2x from the transmit side, and 2x from the receive side).

Mounting Techniques

Now that you know the electromagnetic radiation characteristics of the typical 8ft antenna, you should be able to determine which of the boats below will have a longer range when communicating with other boats.

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That’s right - the top boat with the completely vertical antenna will have the longest range to other boats. The boat on the bottom, while its raked back antenna might look cool, will have a diminished range in respect to other boats. However, in reality, from "there is always an exception" department; it would have a longer range if attempting to communicate with high altitude objects at long distances - such as the Moon.

Antenna Height

VHF electromagnetic propagation is known as Line-of-Sight (LOS). Like a flashlight, it can only transmit in straight lines. This differs from lower-frequency radios in the HF range, such as HF-SSB or Citizen’s Band (CB) radio that can rely on "skip" to transmit long distances. For this reason, the transmission range for VHF marine radios is effectively limited by the curvature of the earth and antenna height. In this regard, the higher you can mount your antenna on your boat, the further you will be able to communicate. Of course, this is simply because the line-of-sight for the higher antenna is greater. There is a mathematical formula that can help in determining the line-of-sight distance:

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To estimate the communications distance, you must perform this calculation for both the transmitting and receiving boat. Generally you would make the measurement from the top of the antenna to the waterline.

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In the above scenario, we can determine the likely communication distances between the two boats, as well as the distant boat and the tower.

* The boat with the 12ft antenna, the distance to horizon is approx. 4.2 miles.
* The boat with the 18ft antenna, the distance to horizon is approx. 5 miles.
* The antenna on the tower, the distance to horizon is approx. 12 miles.

The maximum theoretical distance that the two boats can communicate is 9.1 miles (4.2mi + 5mi)

The maximum theoretical distance the distant boat can communicate with the tower is 17 miles (12mi + 5mi)

It must be noted that this is a theoretical distance due to the line-of-sight limitation. The gain of the antennas will not increase this distance; however, they will make a marginal signal stronger - within the limitation of the line-of-sight distance. In reality, environmental conditions, such as islands that obstruct the line-of-sight path, weather conditions such as rain or fog, or atmospheric ducting all affect the distance the signal travels.

OK, so you have been on the East side of Lake Michigan, and have heard the USCG from the other side of the lake - more than 80 miles, what gives? There is weather phenomena such as atmospheric ducting that act like a conduit for longer distance communications, but this is not reliable, since it relies on changing weather conditions. And, it is not likely that even though you may hear the transmission, you will be able to engage in a 2-way communication.

Installation Practices.

Antenna Location. As already discussed, the antenna should be mounted as high as possible. Also ensure the antenna's radiating element (the fiberglass part) is not close to any metal objects. Metal objects directly under the antenna are OK, and may actually improve the antenna's performance, but vertical objects along the path of the radiating element can change the antenna's characteristics and/or radiation pattern.

Safety. There is a safety concern - at least from this boat owner's point of view - about electromagnetic radiation from the antenna. Most radio manufacturers will recommend one to three meters separation between occupants and the antenna. This may be difficult to achieve on a small boat., While you might minimize this problem by using a lower gain antenna to reduce the ERP, this is no assurance that you are safe from excessive electromagnetic radiation. The only safe practice is to read and heed the VHF radio manufacturer's safety instructions.

Ground Plane. If your antenna recommends a ground plane, then your boat must have a good ground bonding system. You would then attach your VHF radio's ground to the boat's ground bonding system to provide the necessary ground plane. The ground plane provides a "reference" for the antenna to operate. Antennas that do not require a ground plane actually have an element that represents ground built into the antenna. Since many boats do not have a bonding system adequate for use as a ground plane, your choice of antenna will be determined by the suitability of a ground. For example, some VHF antennas from name-brand manufacturers do not require a ground plane, and some do. There could be a lot of debate whether or not one type has higher performance than the other, but for most boaters, the choice is going to be determined by the characteristics of their boat.

West Marine and other marine suppliers often carry copper strips or other bonding devices that can be placed in the bottom of your boat to construct a ground plane.

Routing Coax. Proper installation of the antenna requires careful routing of the coax cable so that it doesn't become damaged, deformed, or pinched. Refrain from excessively tugging on the coax, as deforming the shape could result in trouble. Try to minimize any sharp bends in the cable as well.

Coax Cable length. It is permissible to cut coax if it is too long, however, you must make sure the antenna is at least 3ft from the radio. This is an almost universal requirement from most radio manufacturers. At any rate, you probably won't want the antenna that close to you when you transmit. If you decide not to cut the coax, you can loosely wrap up any excess into a large coil if you wish. Coax cable does present some signal loss, and that loss is greater the longer the coax is, so you can improve the performance of your system by ensuring you do not have excessive cable. This loss is highly dependant on the coax type being used, but a good rule-of-thumb for the RG-58 coax that is commonly used by antenna manufacturers is about -3dB loss for every 50ft.

If you have not figured it out by now, a -3dB loss halves the signal. Therefore, 50ft of coax would present a halving of the signal, and if connected to a +3dB gain antenna - doubling the signal's ERP, and the result would be 0dB, or unity gain. However, the -3dB loss along the coax is a true loss in signal strength, while the +3dB gain is only an increase in ERP. Therefore, you must pay some attention to the interconnection of the antenna to the radio. Some antennas come with lower loss coax, such as RG-8X, so if you have a long cable run, such as in a sailboat mast, you may wish to consider low-loss cable.

Connector. VHF Marine antennas typically come without the connector installed. This is so the coax can be routed through the boat easily. When installing the connector, there are two opinions. Some folks believe the coax connector should always be soldered, and others find that crimped connectors are OK. I personally have used and like the solderless Shakespeare brand "CenterPin" connectors, and have had no corrosion or reliability issues with them. Regardless of the connector you use, you should not splice the coax if possible. Each splice can introduce a 1dB loss or more - depending on how good the connector and its installation is.

Testing

Simple Test. It may be possible to perform a simple test on the antenna after you have installed the connector by measuring the DC resistance between the connector's center pin and outer shield (obviously, the radio must be disconnected from the antenna to do this). Oddly enough, you may measure either an open (high resistance) or a short (low resistance). This characteristic is dependent on the antenna's design, and unfortunately, there is no way to know which indication is correct. However, some antenna manufacturers will indicate whether or not their antennas will measure open or short, as shown in the sample specification above (DC Continuity). While this test will give you a basic go/no-go indication, it is a rudimentary test. A far better indication is to measure the VSWR of the system.

VSWR. A concept that is useful to know is SWR, or more accurately, VSWR (Voltage Standing Wave Ratio). Fixed mount VHF Marine radios interconnect to their antennas by coax cable. The cable is known as a "transmission line" that transfers power from the radio to the antenna. To work properly, the transmission line must be compatible with both radio and antenna. The cable's job is to "couple" the radio to antenna while minimizing any losses. Like many things in electronics, SWR is a ratio. A ratio of 1:1 is theoretically perfect, and means that 100% of the energy from the transmitter is coupled to the antenna. However, in actual practice, even if you take a high quality VHF Marine antenna out of the box and attach the connector to the coax properly (most antennas come with the connector unassembled), and every thing is perfect, about the best you can hope for is a 1.5:1 ratio.

There are several issues that will prevent you from obtaining a perfect 1:1 ratio:

* One influence are the manufacturing variances in the antenna that may reduce the VSWR ratio. In our sample antenna specification above, you can see that the antenna is only capable of a VSWR of 1.5:1 at best.
* If you remember the discussion on wavelength, you know that wavelength varies with frequency. Therefore, as you change frequency, the required electrical length for maximum performance of the antenna will change. This then would require a different length antenna for each frequency you wish to operate at. This is obviously not practical, so antenna manufacturers compromise by typically "tuning" their antennas to be the proper length at a favorite frequency, and only that frequency will result in the best ratio. All other frequencies that use that antenna will have a slight length error, and result in a higher VSWR ratio. Again, the sample specification shows that over the entire VHF radio band, the VSWR can be as high as 2.0:1 simply due to this mismatch problem.
* During installation, if the coax cable was deformed, pinched, or abused, or the connector was installed improperly, the result can be an impedance variance, which can also reduce the SWR ratio.

Measuring VSWR is quite easy to do. Shakespeare markets a VSWR meter that is good enough to accurately determine if you have a problem. This is not an instrument you will likely use a lot, so maybe you can get together with a bunch of boating-club buddies to share the cost. Whatever meter you use, make sure it is designed for VHF Marine frequencies. The marketplace is flooded with inexpensive VSWR meters for CB use - but for the most part, they do not operate at VHF frequencies.

You will need a short piece of coax with connectors on it to connect the VSWR meter to the radio; then simply connect the VSWR meter to the antenna's coax and follow the directions from the meter to measure VSWR. Typically you will also have to perform a simple procedure to calibrate the meter to obtain an accurate reading. The VSWR reading is taken while transmitting, so be sure you do not do this test on Channel 16 or other restricted channels.

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During the manufacturing process, antennas are typically "tuned" for a single frequency. Hopefully this will be near the center of the frequency range, so that the error introduced by changing to a channel at each end of the frequency range is minimized. Therefore, some channels will be closer to the antenna's tuned frequency. For this reason, the VSWR is not constant, but varies with the channel being used. The antenna manufacturer will sometimes specify a minimum and maximum VSWR for their product. A range of 1.5:1 to 2.0:1 is not untypical for a VHF antenna.

If your reading is 1.5:1 or less then consider yourself quite lucky, as many antennas themselves can introduce a VSWR up to 2.0:1. You can also try to change the transmit channel (be sure you are on a legal transmit channel) to see if you can get a lower reading. Generally, anything less than 2.5:1 to 3:1 is acceptable, but anything more than that and you are going to have a poorly operating system. If you have a high SWR, double-check the coax cable and connector. If the cable has been pinched, crushed, or bent at too sharp of an angle, the VSWR can raise. As well, a corroded connector or cold-solder joint will also contribute to a poor VSWR reading.

One technique used here was a stainless steel extension tube to get the antenna above the metal radar arch. This is one possible solution should you have a VSWR that is excessively high. A tower such as this, especially if it were bonded to the boat's bonding ground, would provide an excellent ground plane - should the antenna require one. But the antenna base should be higher than the vertical tower tubes.

Conclusion.

There is a wide range of antenna prices, beginning at around $35 to well over $100. Some cost may be justified by the different electrical characteristics of the antenna such as its gain and whether special items such as filters are used. However, a major factor is due to the construction and quality of the components used. An antenna on the expensive side will last longer than its cheaper sibling.

You should now have a fundamental understanding of different antenna specifications, how to select, and install a VHF Marine antenna on your boat. If you look at other antenna installations on other boats, you should easily be able to pick out those that are done right and those that are not.

This article was written by Al Corkins. The original article can be found at members.toast.net/boatguy/vhfant.htm - along with many other very informative topics concerning boat repairs, upgrades, etc.
--
Rick
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