Why doesn't rigging mess with radar ?

[Dave_S]

In having radar fitted and wondered if the rigging and sails will interfere/degrade the result.

[transmitterdan]

A mast mounted radar will often show some effect where the mast blocks some signal. Other rigging and sails are not going to affect the signal a noticeable amount. Because the rigging is so close to the antenna any reflection is ignored by the radar display. So don't worry too much about rigging or sails. The main thing is to get the antenna high enough to see the desired distance. Radar is a line of site process so the distance is a function of both power and height. No amount of power can see beyond line of sight at the frequencies used by marine radar.

[deblen]

Normal rigging & sails will not appreciably affect radar.

However,avoid mounting metal or other conductive objects on the same level as the radar antenna for the full 360 degrees. Mount these type of objects at least 1/2 meter higher or lower than the radar antenna "window",or you may suffer ghosts,double echos,or degraded performance. Most radar inst. manuals give good instructions regarding this.
Some common offenders are: radar reflectors & stove pipes/charlie nobles.

Carbon/soot/some dirt is conductive,especially when wet. Keep your radome clean.

Cheers from an old tech

[a64pilot]

Originally Posted by transmitterdan

A mast mounted radar will often show some effect where the mast blocks some signal. Other rigging and sails are not going to affect the signal a noticeable amount. Because the rigging is so close to the antenna any reflection is ignored by the radar display. So don't worry too much about rigging or sails. The main thing is to get the antenna high enough to see the desired distance. Radar is a line of site process so the distance is a function of both power and height. No amount of power can see beyond line of sight at the frequencies used by marine radar.

This, he is correct, Longbow Apache radar is blanked by its tailrotor to a great extent, if mast mounted to some extent you will be blanked to the direct rear.

[transmitterdan]

I realize I did not answer the OP's exact question. The reason rigging does not much interfere with radar signals is because the wires are small diameter and not connected together in such a way as to be significant at the wavelength of marine radar. The beam scans quickly and small wires do not spend enough time in front of the beam to "block" the signal. An aluminum or carbon fiber mast is larger and blocks a small angle of the beam width looking aft when the antenna is mounted right in front of the mast. A wood mast also attenuates the signal but not as much.

Even though the mast attenuates the signal to the rear it does not impair the functionality much. Another following vessel would have to stay in a very narrow lane to remain hidden for long. A slight 1-2 degree turn of your boat would reveal such a hidden vessel. For rain it is impossible for it to fall in such a narrow beam so it is easily seen on either side of the mast shadow. The main thing is to remember the small shadow is there and live with it. I would not forego mast mounting just because of the shadow it creates aft. Mast mounting has many advantages such as height and a clear forward view.

Sails are generally made of non-conductive cloth (except for carbon fibers). If carbon fibers make up a significant amount of the sail then that might block some radar energy.

[a64pilot]

Most Radars are pulsed, that is they bang out a signal, then listen. Objects that are very close can't be broken out, Radar will have a minimum range based on it can't transmit and listen simultaneously.
Then there is continuous wave, I can't see how the B&G Radar works, unless it ignore Doppler shift, a moving target will change the return frequency, and that ought to make range estimation incorrect on a continuous wave Radar, perhaps though since vast majority of marine targets are slow it ignores the Doppler shift?
A mast, or any large metal object placed close to the antenna will blank more than you think, how much may depend on antenna size, a small marine Radar, it may blank more than a degree or two.

[JPA Cate]

Agreeing with A64pilot, we have a smallish blind spot directly to the rear. Usually those targets show up because the course isn't totally straight, so it picks them up intermittently.

Ann

[Snowpetrel]

Blind vs shadow sectors. A blind sector completely blocks all radar signal. Eg a mast close to the scanner. Any returns in the blind sector are indirect (reflected) signals from a different bearing. Around the the blind sector is a narrow shadow sector.

A narrower item like a distant mast or Roller furling extrusion causes a shadow sector that reflects or blocks some of the signal, in a shadow sector you will still see real targets, but also tend to get a significant number of false indirect echo's.

I think the wavelength struggles to "see" the narrow wires. Radar reflection is a very tricky thing, and even if it did it would likely just cause very slight random interferance around the screen, much like we often see with the gain wound up.

[donradcliffe]

I hear that the race boats have problems with their new carbon sails blocking satellite reception. I wonder if they might also block the radar.

[Adelie]

Carbon fibers are conductive. With close enough spacing of the fibers the cloth acts as a solid sheet and will reflect radio waves. With wider spacing signal will be attenuated.

[transmitterdan]

Originally Posted by a64pilot

Then there is continuous wave, I can't see how the B&G Radar works, unless it ignore Doppler shift, a moving target will change the return frequency, and that ought to make range estimation incorrect on a continuous wave Radar, perhaps though since vast majority of marine targets are slow it ignores the Doppler shift?

The B&G radar is not continuous wave as that term us usually defined. It is a frequency modulated waveform with a specific ramp waveform chosen so that the received signal can be "compressed" into a much shorter (but bigger) pulse. The pulse height is proportional to the radar cross section of the target. The time of the received pulse can be correlated to the range.

See this old article that does a good job of explaining how FM radar works:

"Chirp" A New Radar Technique, January 1965 Electronics World - RF Cafe

It requires less transmit power because the pulse compression process in the receiver adds the various frequencies together thus increasing gain as much as 50:1. Since noise is uncorrelated with the transmitted frequency the noise is not added nearly as much. Because less transmit power is used it is feasible to use solid-state transmitters which reduces power consumption, size and weight and allows instant-on operation unlike magnetron radars which require a minute or two to warm up.

[transmitterdan]

Originally Posted by donradcliffe

I hear that the race boats have problems with their new carbon sails blocking satellite reception. I wonder if they might also block the radar.

Yes, they might. Carbon fibers are conductive (and fairly lossy at microwave frequencies) so they can be quite good at blocking radar.

[Paul Elliott]

Regarding mast attenuation of the radar signal, one factor that helps the situation is the size of the radar antenna itself. The typical radar scanner antenna is actually a horizontal array of multiple antenna elements, and this array is usually quite a bit larger than the mast diameter. This lets at least some of the elements "see" around the mast. Some elements will be blocked and so the signals will be weaker, but there is usually enough getting through that there is no completely blind spot.

This of course also helps in the case of nearby shrouds, etc, but as transmitterdan points out, these are also usually small relative to the radar signal's wavelength, and so do not do much blocking.

[Dave_S]

Thanks to all, it makes sense.

When I did a survival at sea course, the instructor said that radar was a complicated instrument to read and basically useless without training. The B&G salesman said that was the old ones, these new ones are intuitive and I'll be fine with a quick run down on it.

I assume the truth is in the middle ?

Any views on this.

[JPA Cate]

Hi, Dave,

Google on tutorials for radar return interpretation. Jim found quite a good one, one time.

Ann