Sea-Me and Echomax -

[blackbeard]

Active radar reflectors, transponders, target enhancers, whatever you like to call them - having met a bit of fog in the Channel last year, it occurred to me that they might be a good idea, and a couple of questions occur to me:
Is it worth paying the extra to get one that works in S band as well as X band?
and:
where do you put it? Preferred location apparently is atop the mast and somewhat offset BUT if you put it there it will obstruct the light from the tricolour/masthead all-round-white; perhaps not the best place for something intended to make you more visible. I thought about putting it slightly lower down ie just ahead of the mast but lower than the tricolour (where one normally finds conventional passive reflectors) but the maker's instructions say to avoid this place, also to avoid putting it on the spreaders, due presumably to interference from a large metal tube. (Does this imply that passive reflectors mounted in this way are also ineffective?)
Other possibility is mounted on a tube taking it above the pushpit, but on a small boat it wouldn't be more than about 3 metres above sea level and it would be good to get it higher.

Can anyone shed light (or, if not light, at least a radar echo) on this?

 

[rob2]

I'm sure someone with the regs to hand will be along soon to fill in some detail, but the idea is to mount it offset so as not to exceed the permitted arc of obstruction (only a couple of degrees). It is also a good idea to establish some separation from the VCHF antenna to prevent interference - have you ever picked up ground radar on your car radio at Heathrow? Bearing in mind that your boat will be pitching, rolling and veering, a small arc of obstruction will not align with an observer for any significant time. I have an early X band type mounted on the pushpit, which is not optimum, but I'm told that it gives sufficient range for coastal use. Once offshore, it is more important to get that height and the dual band capability as commercial shipping tend to use the S band once clear of coastal obstructions. I may even put my X band one up there next time I haul out.

Rob

 

[blackbeard]

Thanks for your reply Rob, but I'm still bothered ... the diameter of the Sea-Me is 50 mm and the bracket gives a distance from the mast of about 250 mm. That's going to give a blind arc (ie tri or ARW not visible) of around 10 degrees, give or take, and even with the boat moving about a bit, that in my view is too much.
Also, even if the light is obscured for only part of the time - that might be the part of the time when a watchkeeper is scanning your bit of horizon.
So far, I'm inclined to the view that you are better off with the device on your pushpit even though it would be better, from a purely radar point of view, to have it higher.

Of course not all yachts have nav lights at the masthead ...

 

[rob2]

True, 10 deg would be too much - from memory you're allowed a couple of degs in the regulations. I guess the only real answer would be to raise the tricolour on a pole so they clear the RTE. Note that the dual band RTEs are longer so compound the problem. It's a shame that the lights are not designed with a facility to mount stuff on top of them!

Rob.

 

[pvb]

Other possibility is mounted on a tube taking it above the pushpit, but on a small boat it wouldn't be more than about 3 metres above sea level and it would be good to get it higher.

That would still give warning to ships say 10nm away. Isn't that sufficient?

 

[blackbeard]

10 miles away - yes that should do nicely.
One of the problems with passive reflectors is destructive interference of the incoming, and outgoing, signals when the direct signal from scanner to reflector (or vice versa) meets the signal bounced off the water surface - the latter gives a slightly longer pathway, and if it's half a wavelength longer then the strength of the echo will be much reduced. So it's not just a horizon effect.
At this point, my knowledge of radar reaches its limits so I hope someone who knows can answer. The problem is reduced, apparently, if the reflector is mounted higher; and the problem may be less, anyway, with the stronger signal from an active reflector.
Conventional wisdom suggests, I understand, that the reflector should be at least 5 m above sea level, or 3 m depending on which authority you believe (3 m according to MCA); but in all cases there is a suggestion of "the higher the better".

 

[pyrojames]

Annex 1 of ColRegs, states max 6 degrees obstruction for lights, by masts, topmast or structures etc.

 

[david36]

We have an Echomax XS at the masthead which replaced a single band SeaMe about four months ago after many years valuable service. It was replaced after it was broken off by a v large seabird! We decided on the XS as we could not see the logic of fitting a device which worked on only one band when we could just as easily have one which worked on both. Whether masthead or on a pole at the stern etc tends to depend on your type of sailing and the reason for having it. Masthead mounting produces hits over great distances - 20 plus miles is not unusual. For more coastal type use the lesser ranges from a pole type mount seem perfectly adequate. If you want warning from the alarm that a vessel is coming over the horizon while well offshore then the earlier the warning presumably the better. If the objective is to ensure that some dozy watchkeeper knows that you are nearby then a lesser range probably will do fine. Regarding masking of the tricolour my view is that ouside very crowded waters the chances of a watchkeeper looking outside and seeing your tri are very limited so the small arc of masking is not really a problem to me. In crowded water I would much prefer to use the fore and aft nav lights which I suspect stand much more chance of being seen. But then that is just my opinion.
Fair winds.

 

[BlueSkyNick]

Excuse my ignorance on these products, which I am also planning to install..... How do you know how far away the 'hits' are ?

 

[johnalison]

I presume he is referring to a situation where there are is only one likely source of radar signal.

As I regularly get echoes from ships with my 2Kw radar at around 15 miles I am not over-concerned about the range that my old SeaMe can be seen. Both radar and SeaMe are mounted on a pole at the stern, and as long as a ship can see me far away enough to avoid me, or initiate his ARPA I am content. In practice, anything from 7-10 miles should be enough.

One concern I have is the likelihood of my own radar interfering with the SeaMe by swamping it as it is nearby, though not within its beam angle. The indicator light stays on when I am transmitting, so I have no means of knowing if a ship can "see" me. I tend to play safe by only transmitting when I am looking at the screen.

 

[dt4134]

One concern I have is the likelihood of my own radar interfering with the SeaMe by swamping it as it is nearby, though not within its beam angle. The indicator light stays on when I am transmitting, so I have no means of knowing if a ship can "see" me. I tend to play safe by only transmitting when I am looking at the screen.

So you think that there is a risk that your radar will transmit a stronger signal on the same frequency at the same time which will stop a ship receiving the 'enhanced' pulse from the SeeMe/EchoMax.

Does the radar on merchant ships encode a signal on teh radar signal to ensure it is only their own echo they detect? I thought that was only military radar. It would require the SeeMe or EchoMax to echo the encoding too.

I'd be interested to know the answer but I suspect it is not that complex and it is purely the timings of the signals that matter, in which case you're probably OK (might be a minor problem with the echo of the SeeMe from a target confusing your radar if it is trigerred into transmitting by your own radar pulse, but I presume the transmitted SeeMe pulse would be weaker so any echo from the target would be weaker still).

 

[salamicollie]

Ships radar is un encoded - and transmits on the same bands hence a Radar set has Interference rejection to try and minimse this - seen as a ring of noise when a ship passes close by to you.

I think what the above poster is meaning is that when his radar is transmiting the RTE is picking up his TX and will bounce it back, because it is so close his own radar it will tend to ignore it (too quick a return) and while it is being painted with your beam it may not react to the ships... all depends on seperation, own radar power beam width side lobes....

 

[johnalison]

Thank you for your replies to my question. I think you are saying "it might or it might not".

My best bet would seem to be to try it out with a friend's radar at a range of about 3 miles, when the effect of a SeaMe working or not should be apparent.

 

[david36]

Excuse my ignorance on these products, which I am also planning to install..... How do you know how far away the 'hits' are ?

Apologies for the delay in responding, I have been busy laying-up for the winter months. As suggested by others, if your radar shows only one vessel within 24 miles then it is a fair bet that the alarm is being caused by that vessel. AIS would no doubt be more definitive but we have so far avoided that bit of kit. I hope that helps.
Fair winds.

 

[LittleSister]

Mast / passive radar reflector interference

I thought about putting it slightly lower down ie just ahead of the mast but lower than the tricolour (where one normally finds conventional passive reflectors) but the maker's instructions say to avoid this place, also to avoid putting it on the spreaders, due presumably to interference from a large metal tube. (Does this imply that passive reflectors mounted in this way are also ineffective?)

Yes, to an extent. The current conventional position of passive reflectors mounted on the front of the mast means they are, at least in theory, not radar visible to a vessel approaching from dead astern, as the reflector is shielded by the mast (which tends, by virtue of its shape to disperse, rather than reflect back, the radar waves).

The Echomax site has a table which shows the angle of the arc over which the reflector is fully shielded, which depends on the width of the mast, the width of the reflector, and the distnce between the two. (This data is all theoretical.) IIRC a blind arc of 15 to 25 degrees is not unusual. I would imagine that the arc over which the reflection is seriously reduced is much greater than this.

I have always thought it strange that this fairly obvious potential shortcoming doesn't receive more discussion and publicity, and searched in vain for empirical data about it. I would be pleased to learn more if anyone has any info.

 

[blackbeard]

Thanks, LittleSister, for above.

Effect of mast seems pretty well what might have been expected (though I can't find the tables you mention of the Echomax site, maybe just looking in the wrong place;
although I have found a lot of information there, some of it rather worrying).

Like you, I'm puzzled that the effect is not widely known - it's fairly obvious that a large piece of metal near the radar reflector might have some effect. And I wonder how many owners with passive radar reflectors realise how ineffective they might be, though there's a reasonable chance that a good reflector might be much better than nothing.

 

[prv]

The current conventional position of passive reflectors mounted on the front of the mast means they are, at least in theory, not radar visible to a vessel approaching from dead astern, as the reflector is shielded by the mast

I've always assumed this didn't apply to my mast-mounted reflector because the mast is made of wood. But actually, I've no idea if my assumption is correct.

Anyone know?

Pete

 

[Jamesuk]

We have Sea me right at the top above everything.

We also have just installed a passive radar reflector called the Tri-lens (Looks like three boobs well that was the comment)

The Sea me is great I usually look at the Red light LED When it flashes it means something near you has Radar on. So we have a look at AIS then Radar. Its brilliant actually. Although does nothing for non Radar users.

Enjoy. I would put it at the very top. Make up a new bracket.

James

 

[LittleSister]

Thanks, LittleSister, for above.

Effect of mast seems pretty well what might have been expected (though I can't find the tables you mention of the Echomax site, maybe just looking in the wrong place;
although I have found a lot of information there, some of it rather worrying).

Like you, I'm puzzled that the effect is not widely known - it's fairly obvious that a large piece of metal near the radar reflector might have some effect. And I wonder how many owners with passive radar reflectors realise how ineffective they might be, though there's a reasonable chance that a good reflector might be much better than nothing.

I can't find anything on the Echomax site now, either, but this (see link below) shows the same sort of thing, but I see they're saying the mast obscures the reflector across an arc of about 30 to 40 degrees aft of the boat! http://www.northseanavigatorinc.com /mast%20shadow.htm