boatmike
Well-Known Member
Re: Urban myth...
Hmm... Yes interesting, and having now read more I accept your point, the higher the scanner the worse the clutter. Lets see what it actually means. Having done a few calcs I think taking the nominal 7 degree brewster angle previously mentioned and assuming the vessel remains upright (mines a cat) means the difference between mounting at 4 metres above WL and 12 metres is that at 4 metres the clutter can be expected to cover a 57 metre radius around the yacht where at 12 metres this becomes 171 metres. In practice with a vertical beamwidth of 25 degrees it will extend further than this but this is the nominal centre of the beam. Personally I can live with that especially as my sea clutter filter tunes it out. Assuming the antennae is 60 metres high on a ship though this becomes 858 metres which is significant and on top of matey's cliff (what 200 metres?) it would become 2860 metres. I can see that that would be a problem.
The other side of the problem however is how far can my radar see?
Taking the old formula of Distance in Nm at sea level = 1.144 x height in feet a scanner at say 12 ft would have a horizon of 13 miles. one at 35 ft = 40 miles.
As the maximum range of most yachtie radars is probably about 24 miles the optimum height for the scanner would be 20 ft I guess giving a full range of 24 miles at sea level and a nominal sea clutter radius of 95 yds. Obviously in practice the radar will see objects further than this as the top of a tanker ain't at sea level, but to get a strong echo from its hull it's still advantageous to see more of it especially at extreme range. Apologies for mixing my units but I can't be bothered to convert.
Is that fair? Please feel free to question my logic but I think that means that a radar on an arch is not at the optimum height, but there is no point in going above 20 ft. Mine therefore is higher than it needs to be but basically lives there to miss everything else so I won't move it.
I shall now go and do my late Chrissie shopping and return later to see who thinks I am talking bollox or can't count!
Hmm... Yes interesting, and having now read more I accept your point, the higher the scanner the worse the clutter. Lets see what it actually means. Having done a few calcs I think taking the nominal 7 degree brewster angle previously mentioned and assuming the vessel remains upright (mines a cat) means the difference between mounting at 4 metres above WL and 12 metres is that at 4 metres the clutter can be expected to cover a 57 metre radius around the yacht where at 12 metres this becomes 171 metres. In practice with a vertical beamwidth of 25 degrees it will extend further than this but this is the nominal centre of the beam. Personally I can live with that especially as my sea clutter filter tunes it out. Assuming the antennae is 60 metres high on a ship though this becomes 858 metres which is significant and on top of matey's cliff (what 200 metres?) it would become 2860 metres. I can see that that would be a problem.
The other side of the problem however is how far can my radar see?
Taking the old formula of Distance in Nm at sea level = 1.144 x height in feet a scanner at say 12 ft would have a horizon of 13 miles. one at 35 ft = 40 miles.
As the maximum range of most yachtie radars is probably about 24 miles the optimum height for the scanner would be 20 ft I guess giving a full range of 24 miles at sea level and a nominal sea clutter radius of 95 yds. Obviously in practice the radar will see objects further than this as the top of a tanker ain't at sea level, but to get a strong echo from its hull it's still advantageous to see more of it especially at extreme range. Apologies for mixing my units but I can't be bothered to convert.
Is that fair? Please feel free to question my logic but I think that means that a radar on an arch is not at the optimum height, but there is no point in going above 20 ft. Mine therefore is higher than it needs to be but basically lives there to miss everything else so I won't move it.
I shall now go and do my late Chrissie shopping and return later to see who thinks I am talking bollox or can't count!
