Seas The Day Marine Ltd
Information About VHF Antennas
VHF radio range is line of sight, so height is a very important factor in achieving maximum range. On a yacht the most suitable antenna location is, therefore, at the top of the mast. Two boats, each with a masthead antenna 60′ above sea level can theoretically communicate at about 22 miles. A transmitter at a coastal station, perhaps 1000 feet above sea level, could communicate with these same boats at closer to 50 miles. On a power boat you have to do what you can to place the antenna as high as possible above sea level.
If the VHF antenna is to serve an AIS engine you have slightly different priorities; you could consider taking the opportunity to provide redundancy for your radio antenna. On a yacht it makes sense to put the AIS antenna on the rail so that if you are dismasted this antenna can take over the duties of the masthead antenna. Terminate the cable in a PL259 and use a SO239/BNC adapter to allow you to connect to the AIS engine’s BNC coupling. Then, if you need to switch this antenna to radio use, you just remove the adapter and connect into the back of the radio. Ships transmit their AIS information from antennas high above sea level, so even with a rail mounted antenna you are likely to enjoy a range of around 12 miles.
It is a simple fact that the potential performance of a marine VHF radio is limited by the quality of the antenna and its installation. It is important to select the right antenna and install it in such a way that it maximizes the performance of the radio or AIS engine to which it is attached.
Antenna performance, or gain is expressed in decibels – dBi. A 9dBi antenna increases the signal strength eightfold, a 6dBi antenna fourfold and a 3dBi antenna, the best selection for masthead applications, doubles signal strength.
Why choose a 3dBi antenna for the masthead when you could get all that extra power from a 9dBi antenna?
The increased signal strength of a high gain antenna is achieved by concentrating the signal radiating from it into a narrow disc – the transmitted power does not increase, just becomes more focused. The signal radiates at right angles to the antenna with very little radiation upwards or downwards. When the boat heels and the antenna rocks backwards and forwards the concentrated disc-like radiation pattern points the signal at the sky, or at the sea, instead of at the horizon. A 3dBi gain antenna has a radiation pattern that looks more like a fat doughnut than a disc, with a significant portion of the signal radiating upwards and downwards.
No matter how much the boat heels, such an antenna will still have some portion of its signal looking at the horizon and, because VHF transmissions are line-of-sight, this is vital to performance. Hence the universally accepted use of a 3 dBi gain antenna for boating applications, particularly for masthead mounting.
Another measure of antenna performance is the SWR – Standing Wave Ratio. (You may see this referred to as VSWR, Voltage Standing Wave Ratio, but it is the same thing). This is, in very simple terms, a measure of the amount of transmission power that is lost in the antenna system. If the antenna system was perfect and the entire signal power leaving the radio was transmitted by the antenna (and its cables and connections) the SWR would be 1:1. Sadly this is not achievable, but getting as close as possible is a worthy aim. A SWR of 2.0:1 over the whole system represents a ½ dB loss in signal strength and is barely acceptable.
To achieve or, preferably, better this performance the antenna itself should have the best possible SWR. A top-quality antenna such as the Metz Manta-6 has an SWR of less than 1.2:1. Others will have an SWR of 1.5:1 or higher.
Other criteria for selecting a marine antenna
Ability to be removed when the mast is taken down. This is when the antenna is particularly vulnerable and those with factory crimped connections cannot be removed without removing all the cable with them. The connection at the antenna should be a 1 3/4″ barrel connector which takes the standard PL259 connector.
The Metz antenna satisfies all these criteria. Its excellent durability as a result of stainless-steel construction combined with superb performance makes it the choice of the US Coastguard and UK Search & Rescue services.
Exclusive Features
- Metz Antennas are a one time buy. They are guaranteed for life and will be replaced free of charge should a defect in the unit develop.
- Stores Easily
- No Ground Plane Needed
- SWR Less than 2:1
- UV Resistant Cap
- Ideal for Racing
- Pretuned to Marine Band
- All Connections Soldered
- Stainless Steel Housing
- Power Rating 250 Watts
- Low Angle of Radiation
- Lightning Protected
- Total Weight (Including bracket) 248 grams
- Internally Grounded
- #16 AWG Windings
- Epoxy Filled Coil
- “O” Ring Weather Seal
- 1¼” Barrel Connector Stainless Steel Bracket