BMEA

The average leisure boater relies more heavily on electronics today than ever before. Even ten years ago you would only find a radar on larger boats, but with the advent of low cost LCD systems and compact antennas, a radar system is within the budget of even small boat owners.

However, having a full suite of electronics on your boat doesn’t mean you can sit back and relax, thinking your gadgets will do all your work for you. Even the best, state-of-the-art equipment is still subject to basic forces of nature and it’s important to remember that things such as certain weather conditions can affect the way your equipment performs.

Radio Signals
The weather can have varied effects on the range and quality of radio signals – some beneficial, others not so good. Because the weather is such a complex subject and changes so quickly it is difficult to give definite rules on how your radio equipment will behave in all circumstances – the temperature, wind and amount of water in the atmosphere can vary and combine in many ways. Some of these combinations can cause radio signals to be heard hundreds of miles further than the normal expected range. On the other hand, a different combination of factors may weaken the signal and dramatically reduce the range.

Heavy rain can weaken radio signals because the raindrops absorb power from the radio waves and cause it to scatter, however this has a greater effect at microwave frequencies (see the section on Radar below), rain hardly has any effect on longer wavelengths, so the general rule is that high frequency, short wavelength radio signals will be affected by rain more than low frequency, long wavelength signals.

Loss of signal strength due to rain

One atmospheric condition that can greatly increase radio range is what is called a Temperature Inversion. Normally the warmest air is nearest the surface of the earth and gets cooler as you get further up, but a temperature inversion is where you get layers – known as “ducts” – of cool air sandwiched between the ground and a layer of warm air above, or between two layers of warm air.

Effects of a Temperature Inversion

Normally VHF and UHF signal range is limited to line-of-sight, which in practical terms for sailors is to the horizon. In a temperature inversion, high frequency transmissions may be propagated far beyond this because the warm and cool layers of air are of different densities. The sudden change in density when a radio wave enters the warm air above a duct causes the wave to be bounced back toward Earth. When the wave then hits the ground or a warm layer below the duct it gets bounced back up again. Under the right conditions this effect can be repeated, causing the radio signal to ricochet up and down through the inversion far further than it normally would.

Radar
A radar image can be significantly affected by precipitation (rain, snow, hail, heavy fog etc), causing a reduction in detection ranges and the clarity of the image, especially on small targets. This affects the radar image in two ways -

Firstly, the individual drops of rain absorb and scatter radar signals causing less energy to reach the target and therefore even less returns to radar as an echo. Secondly, the rain itself creates multiple small return signals which fill up the radar image and can obscure “real” targets – this is known as rain clutter.

Rain clutter

The more rain-filled air that is hit by the radar beam, the more clutter that will be reflected back to the radar, so it follows that the narrower the radar beam the smaller the clutter. Therefore when shopping for a radar system you want to go for the system with the narrowest beam both in azimuth (width) and elevation (height) possible.

Whatever the beam width of the system you have, you can compensate for rain clutter by adjusting the radar signal. Most modern radars include automatic rain clutter and gain systems which will do the job very well for you, but you can make manual adjustments to improve the radar image.

You will often find two separate controls on your radar system, one marked “Rain Clutter” and the other “FTC”. Both controls adjust the amount of rain clutter you see, but use different techniques. The Rain Clutter control tunes the return signal around your boat. FTC (Fast Time Constant) reduces rain clutter further away from your boat.

The duration (length) of the radar pulse has an affect on the amount of rain clutter you get returned – the longer the pulse, the more clutter you will get. Reducing the pulse duration will lower the clutter but it will also reduce the strength of the signal hitting the target, so you don’t want to turn the pulse length down so low that it doesn’t even pick up any hard stuff you might end up running into!

It’s not just stuff falling from the sky that can affect your radar signal, the stuff you’re sitting on can interfere as well – if you’re out in very choppy conditions, sea spray or the wave tops themselves can get in the way of the radar signals, causing false images known as Sea Clutter. These false signals can be tuned out in a similar way to rain clutter and most radar systems have dedicated controls for these, usually marked “STC” (Sensitivity Time Control) and “Sea Clutter”.

If you have a radar system, it’s important that you’re familiar with these functions, how to access them and the effects that adjusting them will have. Sailing through a storm in the middle of the night are not ideal conditions in which to be trying to figure out how to get the best of your radar!

Martin Leathead
Cactus Technical Support
http://www.cactusnav.com