What is VHF Marine Radio?

Marine VHF (Very High Frequency) Radio includes the radio frequency range from 156.000 MHz to 162.025 MHz. The International Telecommunications Union (ITU) labels this as the "VHF Maritime Mobile Band."

Physically, the marine VHF radio is a combined transmitter and receiver, often called a transceiver. The PTT (Push to Talk) button located on the microphone or handset, controls which mode the radio is in at that moment. When you depress the PTT the radio becomes a transmitter when you release the PTT the radio returns to the receiver mode.

All marine VHF radios are designed and built to operate on either a specific frequency or pair of frequencies within the VHF maritime mobile band. These frequencies or matching frequency pairs are then stored in the radio as channels. This allows for rapid and accurate tuning of the VHF and saves the radio operator from having to look up or memorize each individual frequency or even more complicated, knowing which transmit frequency is paired to which receive frequency.

Today’s VHF’s are nothing like those of 20 years ago. Today the VHF is capable of transmitting digital distress messages or calling specific stations that you can program into your radio (Digital Selective Calling - DSC), they are able to interface with numerous other electronic systems including Global Positioning System (GPS) and the Automatic Identification System (AIS.) Data transmission (think email among other things) is coming in the near future.

How Far Can I Communicate?

While Very High Frequency (VHF) communications offers plenty of positives: such as sound quality, smaller and low cost antennas, and is less subject to atmospheric interference. It does have one large drawback and that is its relatively short range.

The range of VHF radios is dependent on many factors, the most limiting of these is that VHF is basically line of sight radio. Simply put, the transmitting and receiving antennas must be able to see each other in order to establish communications.

Below we will discuss a few of the most common factors found to affect VHF communications: Transmitter Power, Intervening Topographical Features, and what can be considered to be the most important factor affecting VHF range, antenna height.

Transmitter Power

All marine VHF’s available to the average boater offer a maximum transmitter power of 25 watts. They are also equipped with an operator controlled switch allowing the transmitter output power to be changed to 1 watt for short range communications. Newer VHF’s also have software installed that limits the output power to 1 watt automatically on certain frequencies such as 156.650 MHz (Channel 13) the Bridge to Bridge navigation frequency. Reducing the power of your radio to 1 watt for short distance communications is a simple courtesy. When you consider that at 25 watts you may have a transmission radius of 20 NM. This equates to an area of almost 1300 NM² blanketed by your signal and preventing any one else the use of that frequency. By reducing your power to 1 watt you may reduce your transmission radius allowing other users access to the frequency.

To give you some perspective of how much wattage is required to communicate, the Voyager I space craft at the end of 2016 was almost 13 billion (13,000,000,000) miles from earth and it is still transmitting and we are still receiving, scientific data using it’s 23 watt radio. So as you can see, 25 watts is more than enough output power for a line of sight radio to overcome any normal interference and provide an effective communications platform.

All else being equal, the more power you are able to send to the antenna the further your signal will reach.

Intervening Topographical Features

As mentioned earlier, under many circumstances some bending of VHF radio waves occur, however slight. Regardless, VHF is still considered to be line of sight radio. What this means is that if your antenna cannot see the antenna of the vessel you want to communicate with, chances are your signal will not reach them.

It also means that if there are any intervening obstructions, your signal will also fail to reach its destination. VHF radio waves are very similar to light waves. If you were viewing a candle at night and someone put a curtain between you and the candle you would no longer be able to see it. Likewise, if you were listening to a VHF broadcast and someone moved a mountain between you and the transmitting antenna you would no longer here the transmission.

So to communicate effectively using VHF, you must have a clear line of sight to the station you are communicating with.

Antenna Height

When considering marine VHF radio communications, more important than all other factors is the height of your antenna. The higher you can place your VHF antenna the further you will be able to communicate.

While we are talking about antennas, keep in mind that the typical marine VHF antenna is designed with vertical polarization. This means that the best performance from your antenna will be had when your antenna is mounted perfectly vertical. Any angle beyond vertical will lessen the performance of your radio signal.

So How Far?

There are many formulas in use for computing the theoretical range of your VHF radio installation. Some are based on the distance to the horizon and some for the radio horizon. Remember, VHF is considered to be line of sight radio, although in most cases there is some bending of the radio waves however slight. I will offer up only 2 that should provide you with a reasonable guide of what theoretical range you can expect from your particular installation. The fact is that the differences in the answers derived from either method are so small that it is unlikely to make much difference in the scheme of things.

Visible Horizon

For the conservatives among us this first formula does not take into account any bending of the radio waves and equates to the distance to the visible horizon.

Where D is the distance to the horizon in NM; ro is the mean radius of the earth (3440.1 NM); hf is the height of eye in feet (in this case the height of your antenna); and βo (0.8279) accounts for terrestrial refraction.

Formula for distance to horizon.

Luckily for all of us, this formula can be simplified to: (d = 1.169 ∗ √h) and since we are working in large units like nautical miles we can lose at least one of the decimal places and simplify it further to:

d = 1.17 ∗ √hf

Where d = range in nautical miles and hf = the height of your antenna in feet.

If you are looking to solve the distance in statute miles or kilometers you can use these formulas:

dsm = 1.345 ∗ √hf

dkm = 3.57 ∗ √hm

Radio Horizon

The radio horizon is probably a truer representation of the maximum range of your VHF radio. It takes into account the slight bending of the radio waves along the earth’s surface.

d = 1.2246 ∗ √hf

Where d = range in nautical miles and hf = the height of your antenna in feet.

If you are looking to solve the distance in statute miles or kilometers you can use these formulas:

dsm = 1.415 ∗ √hf

dkm = 4.124 ∗ √hm

Here is an example of the range you could expect for a typical sailboat with an antenna height of 64’.

d = 1.2246 ∗ √hf
d = 1.2246 ∗ √64
d = 1.2246 ∗ 8
d = 9.796 NM

But wait a minute, I talk regularly with other vessels at much longer distances than 10 NM, how is that?

You have to keep in mind whenever you are communicating with another station; you must factor in their antenna height as well and add this to your vhf radio range. You will then have the distance that you can expect to be able to initiate communications.

So if you are looking to make radio contact with another vessel, after factoring in their antenna height, let’s say 49’. Their radio range would calculate out to 8.57 NM’s. Combine this with yours and you would likely be able to establish communications at almost 18.4 NM.

So, what does all of this mean? All other things being equal, the higher you can place your antenna will result in increased range of your VHF radio.

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