For leisure vessels with masts often, when it comes to type of HF antenna, the choice is often weighted in favour of using the backstay. However, whip antennas also have a lot to offer. On fishing boats and other commercial vessels there is a third choice: a broadband wire system...

At HF frequencies, one of the most important consideration for performance is antenna length. For an antenna operating in the 1/4 wave grounded mode to perform satisfactorily at 2 MHz, it really needs to be 336m (119ft) in length. This length must be doubled for a half wave antenna. Both backstay and whip antennas operate in the 1/4 wave grounded mode using an automatic coupler (ATU) to tune throughout the band. Radiation characteristics are similar.

Required lengths for frequency in 1/4 grounded mode

The length of a backstay antenna can easily be 15m (50ft), which is close to a 1/4 wave at 4MHz and will be quite efficient, although longer is better for operation at 2 MHz where the emergency frequency is (2.182 MHz).

While backstay antennas have the advantage of length, there is a down side. This type of antenna is often considered a weak link in the chain for racing yachts and thus considered not desirable. Moreover, many modern yachts, especial racing types, have carbon fibre or Kevlar backstays, which are not suitable for fastening a wire to them. This is because, contrary to what is often believed, they are in fact conductive and any such antenna would give very erratic and unreliable performance, particularly with wet rigging.

Whip antennas are constructed from a number of materials. Metal rods offer a large, low loss radiating surface that promotes good performance. Fibreglass types only rely on a small thin wire radiator embedded within. The fibreglass sheath is subject to damage from the marine environment , so it is important that the surface does not develop cracks where the salt spray can get in. As salt is an excellent conductor of electricity, performance will be adversely affected by damage to the fibreglass, and the life of the antenna considerably shortened.

Whips have the advantage of requiring much less space. This can be an important consideration, as obviously space is at a minimum on vessels. Racing yachts, in particular, may prefer a removable type whip that can be stowed safely below deck when it is not wanted, leaving the deck area free, especially for shorter races.

Although whip antennas are of necessity much shorter physically, on larger vessel it is possible to put up larger types that will give very good performance. Because of the losses that occur across insulators with backstay antennas, when you get into the longer whip antennas there is not much difference in performance. Even a 9m (30ft) vessel can support a 5.5m (18ft) whip, at 10.5m (34.5ft) a 6.7m (22ft) can be used and at 18m (60ft) a 8.8m (29ft) type.

An HF whip antenna with a frequency range of 2-30 MHz (via an automatic antenna tuning unit) operates in different modes at different frequencies due to the physical length of the antenna. At 5/8 wave and below, a symmetrical pattern is achieved across the range. After this the pattern starts to break down into lobes, losing the desirable lower angle radiation.

9.5metre vertical whip

Obviously for 2 MHz, it is desirable to have as much physical length as possible, as this is the most effective way of achieving gain. However, a 9m (30ft) antenna is quarter wave at 8 MHz, half wave at 16 MHz, 5/8 wave at 20 MHz, 3/4 wave at 24 MHz and 1 wavelength at 32 MHz.

For shorter antenna, when communications will normally be on the lower half of the band, performance can be improved by loading the antenna to the highest frequency normally to be used below10MHz. This gives the antenna added electrical length and reduces the additional power required for tuning to the lower frequencies when the gap is large, making more power available for signal radiation. This is recommended for Moonraker 15 Series and 12 Series antennas.

With vertical antennas, it is not uncommon at some frequencies to have high voltages at the far end from the feed point. In backstay type antennas these voltages come in contact with the supporting insulator and may track across it resulting in arcing and poor transmission, especially when the surface is wet. The far end of a whip antenna, however, is in free space. A corona type effect occurs at the tip, which can cause damage to fibreglass antennas, which cannot support the voltages. It is not unknown for fibreglass rods to catch alight at the tip eventually. Metal rods, like aluminium, however, do not suffer in this way.

Good insulation is essential. Although the initial price of a backstay antenna may seem less expensive, because insulators for backstay antennas need to be able to withstand tons of pressure, they are quite expensive items, especially by the time you get them swaged in. High quality feedthrough deck insulators are essential for both whip and backstay types, as voltages can be very high across the feedthrough at the low frequencies. Bringing the feed straight through the deck will earth out the antenna if the deck is awash, as it often is.

Radiation from this type of antenna has directional tendencies at higher frequencies but is omnidirectional at lower frequencies, favouring higher angles for medium to short range communications, as compared with the vertical types which exhibit omnidirectional radiation with lower angles for longer distances. All vertical antennas operating in the quarter wave grounded mode depend on a good connection to ground (in this case the sea), whether it be the vessel’s metal hull or a ground provided by a radio earth plate. Broadband antennas like the Moonraker FB3-30 need neither earth system nor ATU to perform well throughout the band.

If you prefer to install a backstay or broadband wire antenna rather than a whip, perhaps the most important consideration is what you will do for communications if you lose your mast - and your antenna along with it. Casual recommendations for temporarily stringing up a roll of wire with insulators attached may not seem so simple when sea conditions are so bad that masts are lost! Particularly when an emergency is at hand and unskilled hands may be required to restore communications.

A removable emergency antenna, like the Moonraker S/R series , can have you back in communications in a minute or two, and in these circumstances getting back on the air quickly may make all the difference. If you want to complete in most ocean yacht races these days it is usually a safety prerequisite. The emergency antenna can live below deck and be quickly assembled when required. Only the deckmount feedthrough insulator is permanently installed. This insulator can also be used to feed the backstay wire antenna with a backstay wire connection kit that simply screws into the deckmount and unscrews when the whip is needed.

Emergency whips are usually quite compact, so it is always a good idea to have as much electrical length in the antenna itself by having it tuned to a lower frequency rather than leaving it unloaded. Also always choose the longest physical length available that is recommended for your vessel length, as this will give the best results for the emergency frequency.

Safety at sea is always a prime consideration. Maybe you will never find yourself in difficulties, but if and when it happens, your HF antenna will need to function at its best.

Moonraker Marine Antennas