Replacing Your Vhf Boat Antenna: A Step-By-Step Guide

Replacing a VHF boat antenna is a relatively straightforward process, but it's important to do it correctly to ensure optimal performance and safety while boating. The first step is to choose the right type of antenna for your boat, considering factors such as height, location, and cable length. The antenna should be mounted as high as possible to maximize range, with the cable routed away from the engine and any other electrical equipment to avoid interference. When connecting the antenna cable to the VHF radio, it's crucial to ensure secure and water-tight connections to prevent signal degradation and potential short circuits. While it is possible to splice the cable, it's generally recommended to replace it with a new one of sufficient length to avoid unnecessary connections that could lead to signal loss.

Choosing the right antenna

Height and Placement

The height of your antenna is one of the most important factors affecting its range. VHF waves travel in a line of sight, so a taller antenna will be able to transmit and receive signals over a greater distance. If you're mounting the antenna on a hardtop or another high point on the boat, this will also increase the range. However, it's important to note that the ideal height depends on the size and type of your boat. For powerboats under 24 feet, a 3- to 4-foot antenna is usually recommended, while larger vessels can accommodate taller antennas, with 12- to 18-foot options available. As a general rule, antenna height should be less than half the length of the boat.

Antenna Gain

Antenna gain, measured in decibels (dB), represents the transmission pattern of the antenna. A higher decibel rating will result in a greater VHF communication range. For example, a 4-foot antenna may have a gain of 3 dB, while a 12- to 23-foot antenna can offer up to 9 dB. However, higher-gain antennas broadcast at a narrower pattern, so they may be less effective on smaller boats that experience more roll and pitch. In such cases, a medium-gain antenna, like an 8-foot, 6 dB model, is a popular choice as it provides a good balance between range and susceptibility to the effects of boat motion.

Cable Considerations

When choosing an antenna, consider the type of cable connection it uses. The standard PL-259 plug is relatively large and usually needs to be soldered to the coaxial cable. Higher-end antennas may feature screw-on plugs that are easier to install and dispense with the need for soldering. Some antennas also have a connector at the base, allowing for easier removal or replacement without having to replace the cable.

Durability and Maintenance

The durability of your antenna will depend on the materials used. Higher-end antennas often feature more durable components, thicker fiberglass outer sheaths, and stainless steel mounts, resulting in a longer service life. To maintain your antenna, clean it regularly with soap and water, avoiding abrasive detergents that can damage the finish. Wax can also be applied without interfering with the antenna's function.

Brand and Price

Major brands such as AirWave, Digital, Glomex, and Shakespeare offer a range of VHF antenna models to suit different budgets. While it's important to choose the best antenna you can afford, keep in mind that the most expensive option may not always be necessary. Consider your specific needs and choose an antenna that offers the features and performance you require without overspending.

Antenna placement

When installing a new VHF antenna, it's important to prioritise height and minimal obstructions. Radio waves travel in a straight line, so the higher the antenna is placed, the greater the transmission range and reception clarity. For this reason, mounting the antenna on a boat's hardtop or another elevated structure is ideal.

For offshore boats, a bridge or helm installation is often preferred to ensure the antenna is well above the waterline. Sailboats frequently opt for mounting the antenna at the masthead, as this position offers optimal height and a clear line of sight.

Small powerboats and pontoon boats may benefit from mounting the antenna on the front deck or stern rail. This allows for the antenna to be laid down when trailering and provides a good balance of height and stability.

If you're mounting the antenna on a Bimini top, attach the antenna mount to the Bimini frame's highest point, ensuring minimal obstruction. This option provides better line-of-sight communication with other boats and shore stations.

When choosing a mounting location, it's crucial to consider potential interference from other devices like cell phones or GPS systems. To minimise this, mount the antenna as far away as possible from these devices.

Additionally, boating safety should be a priority when selecting a location. Ensure the antenna is securely mounted in a spot that won't obstruct the crew's movement or visibility when the boat is underway.

Splicing the cable

If you are not confident in your ability to splice the cable, it is recommended to buy a new antenna with the cable of the required length. This will save you the trouble of having to snake the cable through the boat. You can also contact a local radio shop and see if they can splice the cable for you.

If you decide to splice the cable, make sure to use a light coat of Boeing T9 and then wrap it with silicone self-sealing tape. You can also use waterproof shrink tubing to make the connection as waterproof as possible.

It is important to note that improper shortening of the cable can degrade the performance of the radio. When cutting the cable, make sure to leave at least 5 feet from where it exits the antenna. Do not coil any extra cable, as this can decrease performance.

Connecting the antenna

When connecting the antenna, you should first decide on the antenna's location. The higher the antenna is placed, the greater its range. On a yacht, the antenna can be placed on the pushpit or the masthead. For a powerboat, an 8-foot, 6-decibel-rated antenna is recommended. The antenna should be placed as high as possible and at least a metre away from other antennas.

The next step is to connect the antenna to the VHF radio. The antenna should be connected to the radio's antenna port. This can be done by soldering or using a mechanical connection. If you are not confident in your soldering skills, it is best to opt for a mechanical connection.

It is important to ensure that the antenna cable is properly installed. The cable should be free of kinks, nicks, and abrasions. It should also be routed away from the engine and any power cabling carrying high currents to avoid interference. If the cable needs to be extended, use #12-14 AWG wire. For long coax runs, in excess of six metres, low-loss coax such as Benelec RG58 foam/foil or RG213 should be used. Any excess cable should be coiled and stowed away from other electrical cabling.

Finally, test the antenna to ensure it is working properly. You can do this by transmitting a signal and checking the strength of the signal received. If you have access to one, a VSWR meter can be used to fine-tune the antenna for optimal performance.

Testing the radio

Testing your radio after replacing a VHF antenna is a crucial step to ensure optimal performance and safety. Here are some detailed instructions on how to test your radio:

A marine electronics dealer can test the radio by measuring its power output, modulation (speech) levels, and transmission quality using specialised workshop test gear. They can also assess the radio's performance by connecting it to another radio. This professional testing ensures that the radio's internal components are functioning correctly.

Testing the Antenna

To test the antenna, a marine electronics expert can visit your boat and employ an in-line SWR (standing wave ratio) meter designed specifically for VHF radios. This meter measures the ratio of forward and reflected power, indicating how efficiently your antenna is transmitting your radio signals. A new aerial typically has an SWR of 1.2:1, so a significantly higher reading suggests an issue with the aerial. Additionally, resistance measurements can be taken at the set end of the aerial cable and the connector at the mast base. For older Vtronix masthead aerials, the reading should be approximately 2-3 ohms, while newer Vtronix aerials will read around 9.8 kohms due to added components for lightning protection.

Evaluating the Coaxial Cable

The condition of the coaxial cable is vital for clear transmissions. Check for any signs of weathering, sun exposure, kinks, nicks, or abrasions on the cable. Ensure that the PL-259 connector is securely fastened or soldered to the end of the coaxial cable and is free from corrosion. A loose or damaged connector can degrade VHF performance and even damage the radio.

Using an SWR Meter

An SWR meter can provide valuable insights into your radio and antenna system's performance. Connect the meter between the radio and the antenna, then switch the radio to an unused channel. Adjust the knob on the meter to set the deflection to the 'set' line on the right side of the meter scale. Next, switch the meter to 'reflected' to get your SWR reading. An SWR of less than 2:1 is considered acceptable, but you can further optimise your antenna by adjusting its length in small increments to lower the SWR.

Professional Assessment

While the above tests can be performed by boat owners or with the help of ham radio operator friends, it is always advisable to seek the expertise of a marine electronics professional. They can provide a comprehensive assessment of your radio, antenna, and cable, ensuring that your VHF setup meets safety standards and performs effectively when you need it the most.

Frequently asked questions