Transmit Ranges of VHF Marine Radio
What are the transmit ranges of our VHF radios? In other words, how far can I expect to be heard? What are the variables which affect the range?
These sound like easy straight forward questions. How far can I expect to be heard with my VHF radio? Easy; about 10 to 25 miles. What affects the range? Heights of the antennas (both transmit and receive) and the terrain between the two. These are the easy answers. But easy answers are often incorrect answers.
Let's look into it.
Because your VHF radio's signal travels in a straight line and because of the curvature of the earth, the height of the antenna is the most important factor to consider.
Here's the math
- sorry, there's always the math -The range in nautical miles - that is, how far your radio can 'see' to the horizon is equal to 1.23 times the square root of the height of your antenna in feet.
So if, for instance your antenna height is 60 feet, the distance it can transmit before it runs into the horizon is ~ 9.5 nautical miles. (for you math challenged - or those not having a calculator handy, the square root of 60 = 7.74. Multiply that times 1.23 to come up with approximately 9.5.)
Now, keep in mind, the vessel to which you are transmitting also has his antenna above the water. So, let's assume, for example, his antenna is 18 feet above the water. He can transmit about 5.2 miles. You can add your range to his to come up with 14.7 nautical miles.
A land based station is usually much higher. An antenna on a hill on a tower might be very high, but for argument's sake, let's say it's 400 feet. So theoretically, he can transmit 24.6 miles. Add that to your 9.5 miles and you have about 34.1 nautical miles.
But that seems to exceed the roughly 25 mile maximum range limit we spoke of above. Well, many other factors come in to play here. The first of which is the rapidly diminishing strength of the transmitted signal with distance. The power density of an electromagnetic wave is proportional to the inverse of the square of the distance from the source. So, for every 4 miles your signal travels, its strength is reduced by a factor of 16. It won't take too long for that 25 watt signal to reduce itself to next to nothing.
You may find other, slightly different math formulas for this, mostly having to do with the constant 1.23, which admittedly is a little arbitrary. We saw one which put the constant 1.42 inside the square root, to multiply by the height and then take the square root of the product. (Don't you just love it when we talk this way?) The square root of 1.42 is ~1.19, a little different from our 1.23. But they didn't mention whether the height was in feet, meters or light years; nor did they mention whether the range was in statute or nautical miles. What difference does all this make? Not much.
Several other factors including the type of antenna affect range. (The sensitivity of the receiving radio, background electromagnetic noise, land masses, weather, are a few others.)
Now, keep in mind, the vessel to which you are transmitting also has his antenna above the water. So, let's assume, for example, his antenna is 18 feet above the water. He can transmit about 5.2 miles. You can add your range to his to come up with 14.7 nautical miles.
A land based station is usually much higher. An antenna on a hill on a tower might be very high, but for argument's sake, let's say it's 400 feet. So theoretically, he can transmit 24.6 miles. Add that to your 9.5 miles and you have about 34.1 nautical miles.
But that seems to exceed the roughly 25 mile maximum range limit we spoke of above. Well, many other factors come in to play here. The first of which is the rapidly diminishing strength of the transmitted signal with distance. The power density of an electromagnetic wave is proportional to the inverse of the square of the distance from the source. So, for every 4 miles your signal travels, its strength is reduced by a factor of 16. It won't take too long for that 25 watt signal to reduce itself to next to nothing.
You may find other, slightly different math formulas for this, mostly having to do with the constant 1.23, which admittedly is a little arbitrary. We saw one which put the constant 1.42 inside the square root, to multiply by the height and then take the square root of the product. (Don't you just love it when we talk this way?) The square root of 1.42 is ~1.19, a little different from our 1.23. But they didn't mention whether the height was in feet, meters or light years; nor did they mention whether the range was in statute or nautical miles. What difference does all this make? Not much.
Antennas
Several other factors including the type of antenna affect range. (The sensitivity of the receiving radio, background electromagnetic noise, land masses, weather, are a few others.)
A sailboat would typically have use a 3dB (decibel) gain antenna. It's signal propagation might be depicted as above.
Typically, a power boat would use a 6dB gain antenna. Because it isn't mounted as high, it needs to be more efficient to make up for the lack of height.
Coax Cable
One thing to note is the loss of signal through the coaxial cable (coax) which leads from the radio to the antenna. A powerboat's cable is often only a few feet, so the loss is negligible. However, on a sailboat, the antenna cable is much longer. To make up for this, a heavier cable, with less loss is required.
The cable for a power boat may be only 20 feet, and RG-58U cable could be used with only a -1.2dB loss. A sailboat using that same cable but running it 60 feet would experience a -3.6dB loss. Using a heaver cable, such as RG-213, a sailboat running the coax that same 60 feet would see only -1.6dB loss. (A -3dB loss halves the signal. )
Connectors
Another area of potentially high loss is in the connections. A sailboat needs to have a junction at the base of the mast so that the mast may be removed when necessary. The connectors at this and all junctions or terminals should be gold plated and installed carefully using the correct tools. It's usually less expensive to purchase the tools than hiring a pro, but use your own judgment. You know your own capabilities.
Weather-proofing the outside connections is important, too. Water intrusion can wick down the coax and cause considerable signal loss.
A typical gold-plated PL-259 Connector. The connector will have to match the coax size. The coax needs to be properly stripped, and you will need to crimp and / or solder to complete the connection.
A couple of step by step "how-to's' and a "how to" video to help you decide if you want to attempt your own installation of coax connectors. If you are feeding the coax down your mast, you're on your own. It's a lot of work and should probably be left to the pro's.
Three things to note: PL-259 instructions above are by HAM radio operators, and they are not using the gold plated connectors. On a boat, you should use the gold plated connectors as they will not rust or corrode.
Second, particularly on the connections which are outside, if it is possible use shrink wrap tubing over the connectors to protect them from the elements.
And third, none of the How-to's or videos mention that there are coax strippers that easily and quickly will strip the coax so that it fits into the connector. These can be expensive and probably not worth it if you are only doing one or two. However, ask around, you may be able to borrow one and save you some time, and make your connection more solid.
Further; once you have fastened the connectors to the coax, use an ohm meter to check for conductivity between the tip and the outside of the connector. If it does not read infinite, you have a short. Don't use it until this is corrected. Some radios, when you transmit and there is a short will send all the transmit power back into the radio and fry it. Maybe modern technology has corrected this problem, but even if the radio is designed to protect itself from this, your transmission will be seriously degraded.
Second, particularly on the connections which are outside, if it is possible use shrink wrap tubing over the connectors to protect them from the elements.
And third, none of the How-to's or videos mention that there are coax strippers that easily and quickly will strip the coax so that it fits into the connector. These can be expensive and probably not worth it if you are only doing one or two. However, ask around, you may be able to borrow one and save you some time, and make your connection more solid.
Further; once you have fastened the connectors to the coax, use an ohm meter to check for conductivity between the tip and the outside of the connector. If it does not read infinite, you have a short. Don't use it until this is corrected. Some radios, when you transmit and there is a short will send all the transmit power back into the radio and fry it. Maybe modern technology has corrected this problem, but even if the radio is designed to protect itself from this, your transmission will be seriously degraded.
Ground Plane
Some VHF antennas do not require a ground plane. These antennas have an element built into the antenna that simulates a ground plane. If your boat does not have a bonding system, make sure you purchase this type of antenna. If you have an antenna which requires a ground plane, make sure you have connected to that ground per the antennas instructions.A final word, or three regarding range of your VHF radio
There are many other things which can adversely affect the transmission range: weather, land masses such as islands or peninsulas, over which you have little or no control.
Sometimes you'll hear a radio transmission quite clearly which is many, many miles beyond normal transmission capabilities of your normal VHF radio. This can be caused by skip, or in the case of Coast Guard, possibly repeater stations. Don't bother trying to talk back. Such transmissions are anomalous and usually only in one direction.
Most radios have a high (25 watt) and a low (1 watt) setting. If you are transmitting to your buddy boat which is close by, use the low setting to avoid cluttering the airwaves for 25 miles around you, Also, on that same subject, usually if you are hailing a bridge you'll want to use the low power setting. Normally, you would not contact a bridge until you are within a quarter mile and the low power setting is quite adequate. This is particularly true if you are hailing bridges where there are many close together such as in southern Florida between Miami and Fort Lauderdale on the ICW. There are so many people hailing so many bridges that it is sometimes very difficult to get through.
Also, be aware that on some radios there are certain channels which ONLY transmit on low power. Read you manual. If you contact a boat on channel 16 which is 10 or 15 miles away, and agree to switch to another channel, and that channel is on low power on one or both radios, you'll not be able to converse.
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