Quadrahedral radar reflectors.

[doris]

Having read the comic this month and found out that the 'tube' type of dismountable reflector that I have is useless, I spoke to the makers of the best rated item. No good for popping up and down as and when, for that best you get a couple of the quadrahedrals. No probs say I and it will be cheaper too. However now having them, there are no instructions.....yes I know that everyone says that they must be mounted in the 'catch rain' position but what is this. There are holes at the 'top' and 'bottom' fairly obviously for tying some string to but not much water is going to get caught in that position. Any help out there??

 

[l'escargot]

There is a diagram here - the one shown has round plates but the position is the same.

 

[Thistle]

Re: Quadrahedral radar reflectors.

So how, given the one pair of holes at opposite corners, do you hang it in this position?

 

[l'escargot]

Re: Quadrahedral radar reflectors.

To be quite honest, when I've actually bothered, I've always just hoisted the thing. It may be in the right position when you get it up and you are tied to the pontoon in flat calm. As soon as you are heeling it isn't.

I work on the principle that whatever way you put it up, sometimes it will be orientated correctly and sometimes it won't.

 

[Twister_Ken]

No law to say you can't drill extra holes to get into the catch-seagull-droppings positiion

 

[fireball]

Well - we believe our tube worked fine ... passing bramble bank in the dark and fog - listening to 2 large ships rumble past us (we were outside the channel!) then - we were buzzed by the patrol boat as he made his way back up into Southampton ... probably thought we were dozy idiots - out in the dark and fog ...

 

[capt_courageous]

If the metal plates are square - just put it on the floor and it will naturally sit in the catch rain position. The wrong way is with one of the plates horizontal and the other one vertical although this seems the easiest way to hang it up.

 

[Evadne]

Mine has a pair of three legged bridles. You attach one leg of a bridle to three of the corners to hang it from, and the other bridle goes on the underside to steady it. As said above, teh correct attitude is the one it sits in on the ground. I usually hoist mine on the spinnaker halliard between the backstays.

 

[squidge]

Hi , this is something that i have been giving some thought to and have now rigged a dedicated halyard for a quadrahedral.The problem i have is how to hoist it so that it stays in position. I have thought of making some brackets to attach it to the back stay , with a line from the underside of the reflector to the pushpit to prevent it swinging into the main. Still not ideal when healing so what about adding a weight to the bottom of the quad and making a gimbal?

 

[Robin]

I'm not sure I read your post correctly, but TWO reflectors is surprisingly perhaps NOT a good idea. From tests in PBO and I believe scientific theory suggests that one radar return can be out of phase with the other and either double the signal strength or cancel it entirely. It seems like bigger is better but multiples are not , va va voom.

 

[Thistle]

Re: Quadrahedral radar reflectors.

True. But why do the manufacturers force us to modify the product before it can be used? Having a single set of holes which don't help you to hang the thing in the correct orientation isn't very helpful. Surely it wouldn't be too difficult (a) to have holes drilled in the right places and (b) print decent instructions on the bag.

 

[jimi]

Re: Quadrahedral radar reflectors.

Erm .. I beleive this is quasi scientific balderdash. Radar refectors act by reflecting and the more reflective surfaces the bigger the echo. If that were not the case the the best stealth plane or boat woulf be one covered in radar reflectors all cancelling each other out?

 

[Robin]

Don\'t shoot the messenger....

I just quoted it not wroted it! /forums/images/graemlins/tongue.gif

 

[jamesjermain]

Scientific balderdash

There is apparently good scientific evidence for this phenomenon. Car silencers work on the same principles using sound waves.

According to the MoD testing station, QuinetiQ, the echo signature of the boat and that of the radar reflector can also cancel each other out making the reflector worse than nothing. This is a rare phenomenon but not unknown (see the YM article 'Can you see me' in the June issue p72)

 

[jimi]

Re: Scientific balderdash

But presumably the cancel out will be rare and momentary whereas the usual return will be larger for most of the time?

 

[StugeronSteve]

Re: Quadrahedral radar reflectors.

I presume that the two reflectors would have to be out of phase in such a way as to set up some sort of destructive wave form. Surely the constantly changing angles, from which the signal is reflected, would make any effect of this sort momentary.

Is there not also a similar probability that the two reflectors would be in perfect phase and intensify the signal?

 

[snowleopard]

Re: Quadrahedral radar reflectors.

if the two reflectors differ in distance from the ship emitting the radar pulse by 1/4 of a wavelength, 3/4, 1 1/4 etc. the two echoes will be exactly out of phase and if they are of the same strength they will cancel each other out. As the wavelengths are in the order of centimetres and the boat is moving around, these coincidences will be momentary. A big-ship ARPA radar needs to get returns on only 50% of scans to keep track of you so it is extremely unlikely that the second reflector would stop you being seen.

 

[capt_courageous]

The size of the signal your corner reflector reflects back to the radar increases with what is called its radar cross section. Instinctively this gets bigger as the plates get bigger. Its better than that since the RCS goes up to the 4th power of the plate size. That is if you get one twice the size you get 2*2*2*2 = 16 times the return. Size matters. One just a bit bigger beats two any day. Forget phases and cancellation etc. unless you want a degree in very difficult sums. The double catch rain position is an attempt to help with the problem of boat heeling etc. Sometimes it makes things better - sometimes worse. Must go now - my brain hurts.

 

[doris]

So reading all these super erudite replies, if I put one reflector up either side they are highly unlikely to cancel each other out and even if they do it will be momentary and we might even get seen!! Or if one was pulled up the backstay by the topping lift angles sorted, seagull crap caught!!

 

[bedouin]

I would say that using two octohedral reflectors is really only a benefit if you can control the orientation.


Octohedral reflectors give very good reflections at certain angles, but have big nulls where they give virtually no reflection at all. At the good angles the reflection from a single reflector is plenty strong enough to be seen, adding a second will not help detection. At the bad angles even two reflectors will not be any good (twice not-a-lot is still not-a-lot).

The best solution is to fix the orientation of the reflectors so that the peaks of one coincides with the nulls of the other (this is what the Firedell or Echomax Do).

As an alternative you can always hoist an Echomax just as you would an Octohedral type. If you were to add a small mass to the base and allow it to hang vertically even when the yacht was healed you would end up with a reflector that works much better than it would rigidly fixed to the mast.