novice
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

Working Out Frequency

Sat Sep 22, 2012 7:21 pm

The following are the typical specifications for an airfield surface movement radar:
Frequency 10000 MHz
PRF 15000 pps
Pulses length 0.05 usec
Beam width 0.4
Aerial rotation 60 to 75 RPM

What it is i'm having problems understanding what frequency is an understanding it in the above stance, i can understand the PRF of 15000 as it has a pulses every 0.05 useconds that ties in with 15000 pps.
The definition of frequency is that it is the number of complete waves passing a fixed point in one second, denoted by the symbol f and usually expressed as Hertz. Though from the above specifications i would of thought this would of made sense for the pulses has there is a complete pulses every 0.05 microseconds, i just don't understand this 10000 MHz and how it ties in with the rest of the specification, can anyone understnd?
 
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Jetlagged
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RE: Working Out Frequency

Sun Sep 23, 2012 1:09 am

You need to differentiate between the concepts of radar frequency and the pulse repetition frequency. The radar waves have a frequency of 10,000 MHz. This is transmitted as a series of pulses. In this case there are 15,000 pulses per second and each pulse is 0.05 microseconds long. You can calculate from the PRF that the time between the beginning of each pulse is 66.67 microseconds. So the pulses are very short compared to the time between them.

Taking the speed of light as 300 million m/s, each pulse would be 15m in length. The wavelength of the radar is 0.03m, so there are 500 wave oscillations in each pulse.

If the radar was continuous wave, with no pulses, it's frequency would still be 10,000 MHz. So frequency and PRF aren't directly related.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
novice
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Joined: Tue Aug 07, 2012 2:55 pm

RE: Working Out Frequency

Sun Sep 23, 2012 2:57 am

Hi Jetlagged
Sorry but i'm still stumbed by this, i work out at it being 6.6 microseconds based on there being 100000 microseconds in a second thus 100000 divided by 15000 equals 6.6? and i would also like to know how you came up with 15m length based on the speed of light of 300000000 m/s?
Thanks for you help
Novice
 
David L
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Joined: Tue May 18, 1999 2:26 am

RE: Working Out Frequency

Sun Sep 23, 2012 9:26 am

Quoting Novice (Reply 2):
i work out at it being 6.6 microseconds based on there being 100000 microseconds in a second

Ah... but there are 1,000,000 (one million) microseconds in a second. A microsecond is one millionth of a second.  
 
nomadd22
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Joined: Fri Feb 22, 2008 7:42 pm

RE: Working Out Frequency

Sun Sep 23, 2012 12:42 pm

I'm not sure what you're trying to figure out here. You have a 10,000 mhz carrier wave making .05us long pulses 15,000 times a second. That means each pulse is 500 cycles long and you have one every 67us. (1,000,000/15.000)
Jetlagged didn't say a wavelength was 15M. He said it was .03m. Wavelength is just is just 300.000.000 divided by the carrier frequency.
Anon
 
David L
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Joined: Tue May 18, 1999 2:26 am

RE: Working Out Frequency

Sun Sep 23, 2012 5:39 pm

Quoting nomadd22 (Reply 4):
Jetlagged didn't say a wavelength was 15M.

   15m is the pulse length expressed as a distance.

You know the pulse lasts 0.05 microseconds (0.00000005 seconds) and travels at a constant 300,000,000 metres per second.

Distance equals velocity multiplied by time, so 300,000,000 x 0.00000005 = 15 metres. That's the distance between the front of a pulse and the back of the same pulse.
 
nomadd22
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RE: Working Out Frequency

Sun Sep 23, 2012 8:01 pm

Quoting Novice (Thread starter):
i just don't understand this 10000 MHz and how it ties in with the rest of the specification, can anyone understnd?

As far as the original question, nothing really "ties in" with anything. Just about any of those specs can change without affecting the others.
Anon
 
ferpe
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RE: Working Out Frequency

Sun Sep 23, 2012 8:54 pm

I think it might be easier to grasp if we tell the story of the worlds first radar experiment. The Royal Air Force wanted to know if radio waves could be used to detect incoming enemy aircraft to Britton in the mid 30ies. The questions went to Watson-Watts radio astronomy guys IIRC. They said "think so", lets make a test. They let an aircraft fly past a normal radio transmitter in the south of England, I think it had a carrier=station frequency of 30 Mhz. They measured the reflected energy from the aircraft with an anntenna/reciever and display. Indeed the aircraft reflect some radio waves their way, radio ranging and detection was born (the ammis later called it radar).

As the radio station was continuous they could not measure distance, only angle. Later they figured out if you send the radio waves as short pulses you can measure distance as well. The problem with the first radars using 30 Mhz was they needed HUGHE antennas to get any angular precision (chain home had antennas some 100m high I think and forming base receiving pairs hundreds of meters apart), therefore one uses normally 1000 Mhz to 17000 Mhz radars today (the higher the frequency the better precision but the atmosphere attenuation also increases, making a 17000 Mhz radar a short range one). The first radars had separate transmit and receive antennas, this was later rationalized to the same antenna which transmit a pulse and then listens, then send a new pulse and then listens. The interval between sending and listening is the pulse repetition frequency.

The typical ATC and approach radars you see on airports have 3000 to 6000 Mhz carrier frequency.
Non French in France
 
novice
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Joined: Tue Aug 07, 2012 2:55 pm

RE: Working Out Frequency

Sun Sep 23, 2012 11:23 pm

Thanks for helping me out with that wealth of information guys, i have learned a lot on radars, frequencies, wavelengths and pulses that i simply wouldn't of grasped with reading it from a book!
Cheers

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