Some time ago Moonraker carried out comparison sea trials to determine the effects of physical antenna length and loading for frequency on HF performance. The trials were conducted over clear water with the vessel moored fore and aft. Ground wave signal strength was measured at 1 nautical mile (1.852 km/1.15 miles), using unloaded Moonraker 1/4 grounded HF whip antennas. Signals were measured at 2.284, 4.1343, 6.2062, 7.697 and 13.3995 MHz.

Antenna Length

Taking the unloaded 3.65m/12ft antenna (12 series) as a benchmark, we were able to measure increased signal strength and determine the gain achieved due to additional length in dB when using a 4.6m/15ft (15 series), 5.5m/18ft (18 series), 6.7m/22ft (22W) or 8.8m/29ft (29W). The results show that any additional investment in length is well worthwhile, particularly on the lower frequencies. When operating on 6.206 MHz a gain of nearly 5 dB can be achieved just by fitting a 6.7m/22ft or 8.8m/29ft antenna.

In order to radiate efficiently at a particular frequency, the antenna needs to be a certain length - or to look like that particular length electrically. Over the HF bandwidth the length required varies considerably. For antennas operating in the 1/4 wave grounded mode the required length varies from 2.4m (7.9ft) at 30 MHz to as much as 36m (119ft) at 2 MHz.

The antenna tuning unit (ATU) or coupler can make the antenna look the correct length electrically but the more correction it has to make, the greater the effect this will have on performance. It is much better to have as much length as possible in the antenna itself, where it can aid in the radiation The more matching that is done in the ATU, the less power is radiated.

An antenna will perform best on the frequency it is the correct length for. This is called its self resonant frequency. Self resonant frequencies for normal marine antenna lengths (1/4 wave) are :

Obviously the more an antenna is physically short for operating on a particular frequency the more performance will be adversely affected. The greatest degradation will occur at the lower frequencies below 10 MHz. This is why we recommend that the longest possible antenna be fitted for the length and use of the vessel.

Effects of Loading

In our comparison sea trials we also measured for the added effect of loading for frequency when operating on 2.284 MHz and 4.134 MHz. These frequencies were chosen for the test as they suffer most from lack of antenna height. Good performance at 2 MHz is of particular concern, 2.182 MHz being an emergency frequency.

As with the length trials, the ground wave signal strength was measured at 1 nautical mile (1.852 km/1.15 miles), using Moonraker 1/4 wave grounded HF whip antennas. Similarly, with the unloaded 3.65m/12ft antenna (series 12) as a benchmark, we were able to measure increased signal strength and determine the relative gain achieved due to loading for either 2.6 MHz or 4.6 MHz in dB.

Test results showed that loading for 2.6 and 4.6 MHz produced further gains over those achieved by length alone. The shorter the antenna is, the greater the benefit, with a possible increase in gain in the region of 39% from loading a 3.65m (12ft) for 4.6 MHz rather than leaving it unloaded.

Choosing a loaded antenna gave significant increases in gain (dB) over an unloaded one of the same length:

As mentioned above, although an ATU or antenna coupler can make an antenna look like the correct length electrically, the more correction it has to make, the greater the affect this will have on performance. It is much better to have as much length as possible in the antenna itself, where it can aid in the radiation. If it is not feasible to have physical length, as with small boats, the antenna can be loaded to look the desired length elecrically.

Fitting an unloaded 3.65m (12ft) antenna and its 1.8m ( 6ft) lead will mean that it is the correct physical length for operating on 13 MHz, but if the user wants to communicate mainly at lower frequencies, the antenna will be far too short.

Obviously, with shorter antennas, the lower the frequency the more performance will be affected. If it is only possible to fit a short antenna, compensation can be made by loading to the highest frequency to be used below 10 MHz. We would recommend loading your antennas to the highest main frequency to be used below 10 MHz (normally 2.6, 4.6 6.3 or 8.3 MHz), especially where shorter antennas are concerned, to maximise performance at this frequency and below. There is no additional charge for loading to one frequency when you buy Moonraker.

Longer antennas can be trapped and loaded so that they are electrically shortened at high and lengthened at low frequencies, enabling the antenna to look the correct length at several frequencies. Above 8.8m (29ft) loading for HF frequencies is not normally required. Multiple resonant frequencies are available for the 6.7m/22ft type 22W and 5.5m/18ft series 18 antennas (up to 2 frequencies) and 8.8m/29ft type 29W (up to 3 frequencies).

Obviously the best solution is to combine length and loading.

Moonraker Marine Whips