Marine radar magnetron

[dr pepper]

Firstly I am aware of the dangers and I am familiar with high voltage in industry, and this question is just academic anyway.

A friends boat has a 40km S band radar scanner, it has a magnetron to generate the Rf, and has a phased array rotating ant.
Transmission time on short range is only 1uS.
How do they get the magnetron to transmit short bursts, I thought magnetrons take a few uS to start up, even with the heater pre warmed.

 

[gophert]

Firstly I am aware of the dangers and I am familiar with high voltage in industry, and this question is just academic anyway.

A friends boat has a 40km S band radar scanner, it has a magnetron to generate the Rf, and has a phased array rotating ant.
Transmission time on short range is only 1uS.
How do they get the magnetron to transmit short bursts, I thought magnetrons take a few uS to start up, even with the heater pre warmed.

The magnetron operates continuously. The signal is only connected to the antenna for short bursts (chirps). Then, the same transmit antenna is immediately connected to the receiver to detect the echo - making it a receiver antenna (on most radars).

 

[debe]

You might find this interesting reading as its similar to your description. I have this Radar & the workshop manual. Its a Koden 32Nm radar. If you want more info just ask. Im of to bed now as its getting late here.

 

[unclejed613]

How do they get the magnetron to transmit short bursts, I thought magnetrons take a few uS to start up, even with the heater pre warmed.

the filament winding on the transformer is isolated, and high voltage negative going pulses are applied to one side of the filament/cathode
, or as is shown in #3, through a pulse transformer.

The magnetron operates continuously. The signal is only connected to the antenna for short bursts (chirps). Then, the same transmit antenna is immediately connected to the receiver to detect the echo - making it a receiver antenna (on most radars).

most search radars use pulsed operation. speed radars use CW operation, and some specialized radars use FMCW. the radar in #3 is pulsed

 

[dr pepper]

Fascinating thanks, and even a schem.
Yes I've heard of sweep & doppler radar, the latter I made a speed trap out of from a Hb100 module.
This unit is a furuno, and its probably older, the motor is a Dc brush motor, positional feedback is from a weird 4 terminal pot (not a synchro) which probably generates sine * cosine waves to generate the circular motion on the Crt.
I wonder if the 820hz repitition rate is also to do with the distance scan from the centre to the edge of the screen & back.
I thought these would burn up a bit more than 1.3A, I spose the outgoing pulse being that short average power isnt that high.
My boat had a radar, it was a raytheon, it was old too, they are still popular, but nowadays are networked to a laptop running integrated bridge software.

 

[ccurtis]

I think that "CIRCURATOR" in the first, block diagram is really a circulator which acts to switch (so to speak) the antenna between the transmitter and the receiver, although no switching actually takes place. The diode limiter in the receiver is used because circulators leak a lot and could otherwise damage the receiver when the Magnetron pulses.

 

[Nigel Goodwin]

I think that "CIRCURATOR" in the first, block diagram is really a circulator which acts to switch (so to speak) the antenna between the transmitter and the receiver, although no switching actually takes place. The diode limiter in the receiver is used because circulators leak a lot and could otherwise damage the receiver when the Magnetron pulses.

I agree, presumably it's a similar device to that used in radio amateur 'relay' systems, where they transmit and receive simultaneously on a single aerial - although there is small frequency offest in that case.

 

[debe]

This is a Furuno radar from the late 80s, picture of the Circulator & antenna parts & the circuit of the transmitter stage. This units display was the older type that used a round display with rotating scan coils. The Koden above used a Raster type TV screen.

 

[dr pepper]

The antenna magnetron and the rx are all coupled with a waveguide, is the gizmo with the green label a directional coupling?
Rotating scan coil simplifies the deflection system as you'd only need one axis, mechanically though it would complicated and the slip rings probably were a source of trouble, I see the magnetron supply also uses a thyristor.
I didnt expect the position signal to be a voltage, I'dve thought they'd sync two stepper drivers clock signals.
I have a ekco weather radar under the bench, but just the display unit, I tried to get the Crt to work as be able to display graphics, I got the tube working, but the inductance of the scan coils makes it too slow, they are in the order of mh's, I'd need 100's of volts to get it to work at video speeds.
Fascinating stuff thanks.

 

[debe]

Yes the Green labeld object is the Circulator/ Direction coupling. I scraped the Furuno as the CRT was just about had it. The Kodan is still an operational unit. The red circle is the stepper motor that rotates the scan coils on the Furuno unit.

 

[dr pepper]

So it look like there are 2 stepper motor oscillators synced over the bearing pulse line.
Doesnt look very complex, 2 stepper osc's, magnetron and a rf amp.
I spose you need to add the current used by the display to the 1.5a of the scanner, which would probably be around 5a I was expecting.
My Ekco weather radar display has 1/2 a circle display, probably ahead only, the If is in the display unit, I didnt work out how the display worked, it has x/y deflection coils, I think the scanner generated quadrature signals which could just be amplified & applied to the coils, with the equiv of a Tv horizontal mixed in for the distance axis.
Funny how you regard some things as complicated, but when you get into it, they are not so bad, however if you were to start from scratch maybe it would be a marathon.

 

[debe]

Your weather radar probably had a scanner that swept back & forth sideways in the nose of the aircraft. These are pictures of the Koden display & info on the antenna drive, which generated an Azmth signal in the drive motor to sync the antenna to the display.

 

[dr pepper]

Yes the weather radar was a low frequency unit, in the Vhf range, and there was a huge about 1.5m dish in the nose of the plane, it must have rotated 180 degrees, I know this as one was up for sale on ebay.

8085 processor, not seen or heard of one of those for a while, didnt it come as a microcontroller version too.
The video board looks like it straight out of a monitor, maybe it was.

 

[debe]

This is the block diagram of the display & the circuit of the Processor board (best I could get of the circuit). generaly these older radars were pretty reliable, apart fom Magnetrons wearing out & antenna drive motors. The drive motor was faulty on my Koden unit & I had to think out of the square to repair it as they are not available. Yes the display is realy just a basic monitor.

 

[dr pepper]

At some point I've used all those logic devices, including 2764 rom's, but that was a long time ago.
I guess you could replace the magnetron, motor and even the monitor with an Lcd.
I gathered you'd had trouble with the motor drive from the sketch + dimensions hand drawn on the manual.
I'm going to route in the garage this weekend, I might have a scanner assy somewhere, dunno what to do with it, fiddle about with it for an hour & put it back probably.

 

[debe]

Heres some pictures of the rebuild of the motor. The original motor inside the housing I dismantled & removed the armature from the shaft. Then I shortened the housing to just use it as a bearing assembly, & shift the AZI pulse generator further up the shaft. The brass coupling to join the new motor was salvaged from a very old VCR. The replacement motor was a new adjustable speed cassete motor cost $12. This repair has been quite effective & quite cheap.

 

[dr pepper]

Thats just the kinda thing I'd do, make it have it.
Question is would you trust it at sea, I'd be Ok if I ran it for a couple of days non stop first.
The thing with steppers is that with no brushes they last longer, Bldc are the same.
Found mine in the shed, not much use its just an empty radome, nothing much inside an empty plastic biscuit tin.
I've been refreshing my memory on magetrons, I knew they were resonant, I'd forgotten how the Rf was taken to the outside world, but of course it just an antenna, I spose when the magnets weaken you'll get more electrons breaking free from the arc orbit & not making it to the tuned cavities.
If you put a magnet near an arc, even a spark plug you get a slow circular motion, I always wondered if thats the same effect, does plasma have similar behavior to electrons in a vacuum.

 

[dr pepper]

Ok I did some research.
When the magnet weakens in a magnetron the electrons are not pulled into a loop the same, and more than intended crash into the outer copper ring without causing the cavities to resonate.
This increases current consumption and reduces the Rf output, the tube basically turns into a thermionic diode.