Rotating waveguide antenna (rotating marine radar)

[artis]

I've seen these devices on shores as well as on ships , like a horizontal tube rotating slowly around it;'s axis.

Now from what I know it's a type of radar, and unlike phased array it rotates it's beam physically by means of using a motor to rotate the antenna itself , what I want to know is how it works?
Like what is the antenna type inside the enclosure and what is the beam shape and also is the antenna both a transmitter and receiver ?

 

[artis]

ps. Just to add , it seems there are a variety of these rotating radars , and not all have the same type of antenna , like I also see rotating parabolic antennas but I suppose their beam differs from the flat horizontal ones

 

[sophiecentaur]

I also see rotating parabolic antennas

They are not as common because the 'slice of cheese shape ' gives you a wide vertical beam to cover all elevations and a narrow horizontal beam to give good directivity - that's what you need for ships.
Have you googled this at all?

 

[artis]

@sophiecentaur It's kind of hard to google because I don't even know the correct term for this radar , some call it slot antenna, some just say marine radar.

 

[sophiecentaur]

some call it slot antenna

Do you know what a slot antenna is? I am having a problem deciding what your level of knowledge is. Many of the Radar antennae you see on boats are paraboloid. Slot antennas are a newish 'invention' but both kinds have a wide horizontal aperture for good azimuth resolution and a narrow vertical aperture to give a big range of vertical coverage (they 'know the elevation of all targets, of course but the return signal can be anywhere between very low to just above the horizon and boats tend to rock. The actual shape and aperture of a radar antenna will depend on what sort of beam pattern you need, the frequency of the radar anyhow much you are prepared to spend. But you need a lot more background reading for all this stuff to come together for you.

Why not google "how does a radar antenna work?" Your queries have been too specific and you have missed the answers you need.

 

[artis]

Well here we go again, I do know the basics of how a radar works, it emits EM waves (different frequency for different radar) and then the ones that reflect either get received at a separate antenna or the same antenna.

I asked specifically about these horizontal rotating "slices of cheese" as you rather well described them, I was wondering about their inner workings, the type of antenna inside the plastic enclosure etc,
if you do know how the specific thing works why not tell it to me instead of trying to guess my level etc.

I do apologize @sophiecentaur if this seems disrespectful it's just that I see it this way.

 

[sophiecentaur]

The reason that I have not bothered to give you the necessary information is that involves diagrams plus explanations. I tend to resent it when people ask for information that is available all over the Web.
If I were to try to give you an appropriate level of explanation, then I would need to have an idea about your level of knowledge. So you can't take offence that I asked for it.
If you do as I suggest and do some serious searching then you will find a number of links that are just at your level and with diagrams. Are you really suggesting that I should take the same amount of my own time and search for you?
As soon as you have something from your searches then I will be only too pleased (as will other PF members) to answer specific questions. e.g. "I found this out but I don't quite get parts A B and C about it"

PS I already told you quite a lot but you seem not to have recognised it - that's what I was talking about - your level.

 

[jasonRF]

One common brand, Furuno, advertises that their antennas are slotted waveguide arrays. The name of those antennas accurately describes them. Google should give you some leads.

 

[Paul Colby]

This might help.

Slot_antenna

I've seen these devices on shores as well as on ships , like a horizontal tube rotating slowly around it;'s axis.

 

[sophiecentaur]

One common brand, Furuno, advertises that their antennas are slotted waveguide arrays. The name of those antennas accurately describes them. Google should give you some leads.

Are you sure that the OP is aware of the basic requirement and how the parabaloid does the job. The phased array of slots is a serious step up from that. I can’t read anything in his posts to suggest that a description of a slot array would make much sense. This stuff is a lot harder than the jargon used by sales staff suggests. Too big a jump imo.

 

[jasonRF]

Are you sure that the OP is aware of the basic requirement and how the parabaloid does the job. The phased array of slots is a serious step up from that. I can’t read anything in his posts to suggest that a description of a slot array would make much sense. This stuff is a lot harder than the jargon used by sales staff suggests. Too big a jump imo.

You are probably correct. I was just giving the OP the benefit of the doubt, so gave them the phrase they should google.

edit: please accept my apologies if I derailed the thread.

 

[davenn]

I asked specifically about these horizontal rotating "slices of cheese" as you rather well described them, I was wondering about their inner workings, the type of antenna inside the plastic enclosure etc,

here's a whole page of rotating waveguide joints. Many as used in rotating radar antennas

https://www.bing.com/images/search?...&form=HDRSC3&first=1&scenario=ImageBasicHover

 

[davenn]

there's nothing as such "in there" as they are a hollow waveguide
there are just some variations depending on the designer/manufacturer

 

[Ibix]

So it's basically a radio frequency periscope? Neat...

 

[davenn]

Have had these 2 pic's stashed away on the PC for many years

But sometimes things go wrong with rotary WG joints
The rotary joint ceased and the flexible section of WG kept turning till it tore in half

 

[sophiecentaur]

But sometimes things go wrong with rotary WG joints

I imagine that maintenance visits up the mast or onto the roof are no fun. There's a lot of salt and weather up there to do damage.

 

[artis]

@jasonRF thanks for stepping by and no you did not derail anything, I appreciate your input.

@sophiecentaur well it's one thing for you to tell me that I'm not up to task (which you might be right about) but it's another to answer other people's input in this thread by telling them what should I be told and what not, I don't find that constructive or helpful.

In fact my main inquiry was about the type of antenna used in this narrow horizontal rotating type of radar and judging by all the input so far it seems it is indeed a slotted waveguide. Which would explain the geometry of the rotating part of the radar. So a slotted metallic waveguide coated with external plastic to keep dust and moisture out, much like the parabolic link antennas used on radio towers as communication channels between towers as far as I know.@davenn since you posted the pictures let me ask, the rotating flange part of the radar , where the stationary part goes over to the moving/rotating part , I suppose there is a small separation there between the stationary and the moving part, but in order to keep the integrity of the EM wave within the waveguide the separation has to be smaller than 1/4th of the wavelength?

Well in the case of a parabolic antenna , they have a feedhorn in front of the antenna which directs the EM waves against the antenna dish which then forms their shape, the return EM signal is then transmitted/reflected in reverse and picked up by the "dish head" as satellite technicians like to call it, and can then be received,

can someone explain how the rotating slot antenna functions as a receiver? Obviously a radar would be useless if it just radiated and did not receive.
My own guess would be that the slots work both ways, they do radiate as the E field changes over the gap due to changing current, but an outside E field would cause the gap to develop charge across it's long sides and a changing current, much like with light the diffraction pattern is the same irrespective if one changes the side from which the wave comes.

 

[Paul Colby]

can someone explain how the rotating slot antenna functions as a receiver? Obviously a radar would be useless if it just radiated and did not receive.

Provided there are no chiral or handed materials involved, antenna systems obey the reciprocity relation. Let $J_1$ be a current generating a RADAR signal at frequency $\omega$ and let $J_2$ be a current of a remote source, then,

$\int E_1\cdot J_2 dv = \int E_2 \cdot J_1 dv$

where $E_1$ is the electric field generated by $J_1$ and the same for 2. Receive gain is directly related to broadcast gain.

P.S. I gave a link to a wiki that covers exactly the type of antenna you asked about in post #9.

 

[artis]

yup I read that link @Paul Colby , thank you.
So basically the slot antenna can receive it's own radiated reflection much like any other antenna and the received signal strength to radiated signal strength ratio is then a matter of how far away from the antenna the radiated signal got reflected ?

 

[Paul Colby]

So basically the slot antenna can receive it's own radiated reflection much like any other antenna and the received signal strength to radiated signal strength ratio is then a matter of how far away from the antenna the radiated signal got reflected ?

Yes, lookup the RADAR range equation. Return signal power drops like $1/R^4$ for obvious reasons. Oh, and there is also the time delay of the return power that is measured.

 

[sophiecentaur]

can someone explain how the rotating slot antenna functions as a receiver?

It's fundamental. Any transmitting antenna can function as a receiving antenna - just swap the transmitter for a receiver. In many cases, the expense of a massive structure that can handle high transmitter power would be bad value if used as a receiving antenna. In fact many transmitting antennae are much more directional (so as to beam the transmitter power over a city, for efficiency) than the antennae used for receivers.
You are conflating two functions in this comment / question. A rotating joint is needed with whatever rotating directional antenna is used. The slot antenna operates by using an array of a number of slots in the side of a cavity / guide which produce waves that are in phase with each other - producing a 'beam' in one particular direction.

But seriously, why do you not google this information for yourself? There have been so many key words offered to you in the various posts in the thread which will help with your search. Go for 'Images' and you will find dozens of pictures which, in some ways, are worth many words.

 

[sophiecentaur]

and the received signal strength to radiated signal strength ratio is then a matter of how far away from the antenna the radiated signal got reflected

Did you ever look up RADAR even? The distance is measured by the Time Delay between transmitted and received reflection.

 

[artis]

I do get the distance , if the time delay is constant the target is stationary but if the time delay increases or decreases then the target is moving. Seems like they would also involve the change in frequency for the reflected signal aka doppler shifting for a target that moves
I did google the slot antenna , seems like the marine radar ones have a horizontal waveguide with horizontal slots in it,m due to interference the highest E field is achieved in the middle of the antenna , the beam seems to be narrow in the horizontal direction but very high/wide in the vertical one which makes sense since the antenna physically rotates in the horizontal direction but the beam then scans everything from high up in the sky to down to near earth/water level.

So a military ship can then track both sea vessels and air borne devices in it's vicinity.something that I can't find in a quick google search is how for example in the case of the slot antenna the received signal is transmitted back to the analyzer electronics? I mean let's take the simplest case of a large object that is stationary, so the waveguide transmits a certain frequency, given the object doesn't move the reflected signal will be just time delayed but not doppler shifted.Here is my own attempt at explanation. Assuming a stationary object some distance away the reflected waves will interfere with the transmitted ones causing slight changes in the impedance of the antenna, at some distance away the reflected waves will be in phase with the transmitted ones , I do wonder how does the radar know the distance of the object in this case? If the object is stationary and at the right distance away so that the reflected waves are in phase , if the radar works in CW then all it "sees" is a constant increased output?
One more thing , as for the rotating slot antenna it still skips me how it can know not only the relative speed of object to radar which is more understandable, but also it's height.

 

[Paul Colby]

You could buy a book on RADAR.

 

[sophiecentaur]

@artis
Imo, you are too preoccupied with the slot antenna idea. It is only one form of antenna that's used in Radar - depending on the wavelength and application used. If you are expecting google searches which involve "slot antenna" to yield the basics about Radar then you will probably be disappointed. Most of the basic tutorial information about Radar is very old and goes back to WW2. I suggest you open your acceptance angle a bit wider and avoid getting into stuff that is just not appropriate for you YET.

Here is my own attempt at explanation. Assuming a stationary object some distance away the reflected waves will interfere with the transmitted ones causing slight changes in the impedance of the antenna, at some distance away the reflected waves will be in phase with the transmitted ones , I do wonder how does the radar know the distance of the object in this case? If the object is stationary and at the right distance away so that the reflected waves are in phase , if the radar works in CW then all it "sees" is a constant increased output?

This is an example of Word Salad which doesn't actually give a coherent picture of the processes that Radar uses. It's because you are trying to run before you can walk.
I already told you that distance is calculated by the time delay for the echo and the speed of the waves. If you don't use that knowledge than you will never get a grasp of the system. I suggest you stop assuming you 'know' things and start learning about the system from square one. If you don't then you will have wasted a lot of your own time.
PS Look up Pulse Position Indicator on Radar. That gives you the basics of all rotating Radar systems. You must have seen one on a war film at some time - they have been used for decades.

PPS I think your idea about Interfering waves may be from what you have read about the very earliest forms of radar but that's not a good way into the system.

 

[artis]

I do know that radars aren't just rotating types.

I do get the time delay , sure EM travels at some 0.7c? or thereabouts through mediums such as air, so for a transmitted wave to reflect off of a target will take some time , bit less time if the target is approaching, in this case also the wavelength will be upshited.

What I was suggesting or rather asking is what happens when a stationary target is at a specific distance from the transmitter such that the echo adds constructively in phase , and the radar is in CW operation, because for pulsed operation I can see how one could receive the echo but how about CW , I mean the echo arrives but it arrives constantly adding to the transmitted wave , would the distance to target in this situation be analyzed from the strength of the in phase interference ? Because as the target would get closer to the next "in phase echo" position the signal magnitude would increase ?https://en.wikipedia.org/wiki/Plan_position_indicator
is this what you are talking about @sophiecentaur ,
Well sure I have seen these screens , they had to have the long glowing phosphor crt's in order not to miss the last position constantly.

 

[sophiecentaur]

such that the echo adds constructively in phase

You've done it again. What could you possibly mean by this? How could you possibly compare the Megawatt output pulse with a Microwatt reflected pulse and make any sense of that? Why don't you just read and learn rather than trying to invent your own version and then desperately try to hang onto it.
Where did you hear that EM waves travel at 0.7c through air?

Yes PPI (plan - not pulse) was the original display.

 

[Tom.G]

Hey @artis, re RADAR distance measurement.

• You are in a very large room.
• You clap your hands, once.
• A short time later you hear an echo of you handclap.
• You measure the time between you handclap and hearing the echo.
• If you measure 1/2 second you know it took 1/4 second for the sound to hit something reflective and 1/4 second for the sound to return to you.
• Knowing that sound takes roughly 5 seconds to travel a mile, you calculate that the reflective surface is 1/20 of a mile away, or 264 feet.

That's how RADAR works, a short pulse is transmitted, and the time is measured until an echo is received. The rest is details, like what is the antenna beamhsape, or what is the smallest detectable object at what distance.

Now please,
GET A BOOK ON RADAR AND LEARN THE RUDIMENTS,
or
https://www.google.com/search?&q=radar+fundamentals+pdf.
or
at least use wikipedia.

Cheers,
Tom

 

[artis]

@sophiecentaur , well EM wave speed in lossy mediums is less than c , maybe the 0.7 figure came from a insulated wire rather than Earth's atmosphere, anyways , I think I sorted out my problem here.

I forgot about the near and far field effects of an antenna and was thinking in the near field terms where if a target would be say few meters from the antenna then indeed it would affect the antenna noticeably.
So the echo from a distant target is from the far field of the antenna which has decoupled from it and now propagates through the medium until it hits an obstacle made from a different medium like a plane with aluminum body , reflects and so the echo travels backwards. So the echo from the transmitter/receiver antenna's perspective is just a signal picked up much like any other signal, so then it's up to the analyzer electronics to calculate the time of the transmitted pulse and the time of the echo received , rather time difference.
@Tom.G I'm rather sloppy that's true.
as for the range measurement , I guess the only radar that cannot measure range to target is an unmodulated CW type as it would only receive doppler shift info to tell target speed, I wonder are police radar guns like this
But still if I can rephrase, let me ask like this,
Are there any instances where a radar's transmitted waves (yes I know they are high power , at least when they leave the antenna, near the antenna) can interfere with the reflected echo in such a way that is detrimental to the echo signal and lessens the chance of it being picked up?

 

[Tom.G]

doppler shift info to tell target speed, I wonder are police radar guns like this

Yes they are. I understand the police also have a Laser version of the Radar gun that measures the time-of-flight of the light beam, I don't know the details though.

Are there any instances where a radar's transmitted waves (yes I know they are high power , at least when they leave the antenna, near the antenna) can interfere with the reflected echo in such a way that is detrimental to the echo signal and lessens the chance of it being picked up?

From a practical standpoint, no. The transmit power is so high that the receiver antenna input has to be shorted to ground or otherwise blocked to avoid frying the receiver.

That effect can be seen in other scenarios though. For instance if you are listening to a distant, weak, radio station and move the receiver you may find spots that the signal completely disappears, and other spots where you get a strong signal. That is due to something called 'multipath reception.' You are receiving a direct signal from the transmitter and also a signal that has been reflected off of something, often a large building. (Or maybe multiple reflections from multiple objects in multiple paths.)

Another instance of this occurs when watching TV using an antenna rather than a cable connection. Sometimes the signal will fade out and return several times. That is generally due to an airplane flying thru/near the signal path between you and the transmitter; you get alternating signal cancellation and enhancement from the signal reflected from the plane!

Cheers,
Tom