Radars transmitted power

[colemj]

Originally Posted by continuouswave

The notions of "pulse" and "always transmitting" do not seem compatible.

They are entirely different modes used on different physical units - compatibility doesn't enter into it.

Mark

[transmitterdan]

The simplest analogy is the difference between AM and FM. There is a direct relationship between the frequency domain and time domain. For some reason humans comprehend the time domain better than the frequency domain. Much to the dismay of Mr. Armstrong.

A place to start is to look at the Inverse Fourier transform of a swept sine wave. Another is to look at the Fourier transform of a unit impulse.

I can say for certain that FMCW radars do transmit and receive simultaneously. It's one of their key selling points. Many of them can paint a target 100% of the time with no gaps.

[thomm225]

On a pulse radar, it's so fast anyway it doesn't matter. Continuous would be important for tracking or guiding missles etc but for the simple uses on a boat a pulse radar is fine.

Example: (if I can remember correctly)

One of the Precision Approach Radars (PAR) we had in the military for landing aircraft used by both the Marines and the Navy had a pulse repetition time of 833 microseconds. (followed by a few usec of deadtime but that's not important here) This gave it a range of 60 miles (67 miles in actuality, 833/12.36, but the selector switch read 60) It was normally used in the 10 mile setting for the final appraoches of fighter jets and other aircraft during IFR conditions)

Point is, it's so fast you would never know. Pulse radar is fine for detecting things at sea. A microsecond is a millionth of a second if memory serves.

Also, most all the Airport Surveillance Radars at the major airports in the US are pulse radars.

[continuouswave]

Originally Posted by colemj

OK, so I googled these for you (the answers are pretty easy to find if you want to try it yourself).

Yes, I have used Google to look up answers to give to people, too. I think it is quite common, and I usually don't remark about it.

Originally Posted by colemj

They sweep from 9.3-9.4GHz over 1.3ms...

If the sweep interval is 1.3 x 10^-3 sec and the speed of wave propagation is 3 x 10^8m/sec, this suggests that the maximum range is 3.9 x10^5 meters (divide by two for round trip) meters or 195-km; that is quite a long range for a small boat RADAR.

Originally Posted by colemj

...with a sweep repetition frequency of 200-540Hz (mode dependent and adjustable).

Interestingly, this suggests a 25-70% duty cycle, and not continuous transmission.

Yes, a sweep duration of 1.3 x 10^-3 would suggest the sweep, if continuously repeated, would occur at a rate of 769-Hz. Is the transmitter shut off the rest of the time? Or just not frequency modulated? That is a rhetorical question. No need to Google the answer.

As I said, I better do more reading about continuous wave RADAR. I just assumed these things were still intermittently transmitting.

[continuouswave]

Originally Posted by colemj

They are entirely different modes used on different physical units - compatibility doesn't enter into it.

Mark

Yes, precisely my point. "Pulse compression" seems to imply an exclusion of a signal that was transmitted continuously.

[colemj]

Originally Posted by continuouswave

If the sweep interval is 1.3 x 10^-3 sec and the speed of wave propagation is 3 x 10^8m/sec, this suggests that the maximum range is 3.9 x10^5 meters (divide by two for round trip) meters or 195-km; that is quite a long range for a small boat RADAR.

You are missing the power part of the equation here. 195km assumes either transmit power that isn't available or receiving sensitivity that doesn't exist.

Mark

[colemj]

Originally Posted by continuouswave

Yes, precisely my point. "Pulse compression" seems to imply an exclusion of a signal that was transmitted continuously.

I lost the thread here. Why are you concerned about pulse compression, and how is it related to CW? Did I miss a post about this?

Mark

[transmitterdan]

There are some web sites that try to explain pulse compression in analog circuit terms. It leads to a lot of confusion in my opinion. But the math of chirp radars isn't trivial so it can be difficult to explain without creating confusion. Plus, the people that truly understand it probably don't give that knowledge away freely on the Internet.

[continuouswave]

Originally Posted by transmitterdan

...the people that truly understand [pulse-compression or chirp RADAR] probably don't give that knowledge away freely on the Internet.

You're probably very right on that.

[colemj]

Originally Posted by transmitterdan

There are some web sites that try to explain pulse compression in analog circuit terms. It leads to a lot of confusion in my opinion. But the math of chirp radars isn't trivial so it can be difficult to explain without creating confusion. Plus, the people that truly understand it probably don't give that knowledge away freely on the Internet.

Ah, now I'm back on thread. Pulse compression is referring to the signal processing that is going on. I don't see why this is confusing in terms of CW, though - it is part and parcel of FMCW.

Mark

[amytom]

Originally Posted by thomm225

On a pulse radar, it's so fast anyway it doesn't matter. Continuous would be important for tracking or guiding missles etc

Up through the mid nineties the fire control (missile guidance) radar I worked on used a frequency agile mono pulse Doppler for basic tracking but the terminal phase of missile flight was guided by a CW illumination. The missile honed in on the reflected CW signal. This CW could also be sweep modulated for a backup radar.
After using both modes to track tiny fast targets at 200 mile range I chose a raymarine pulse radar.


Sent from my iPhone using Cruisers Sailing Forum

[Lars_L]

The FMCW radar that’s on the market today for boats transmits with a very low effect and therefore will the echo be VERY low. But by collecting the signal for a while (1 ms) it gets so much information from the echo that image-processing program can do a qualified guess if it’s a target or a disturbance. That the signal goes via an image-processing program makes the image sharper at least for shorter distances. That’s one of the benefits with the broadband radar. But when it starts to be little more distance (3 M), the program makes more guesses on the signal and the image loses in sharpness. Low-end pulse radar (like JRC 1500) does not need or have any image-processing program. The echo is that strong that it can be displayed directly. But then its up to me to decide what is it that I see, is it a target or a disturbance?

[bfloyd4445]

very confusing

First of all to receive and transmit at the same time from the same antenna won't work as one signal will attenuate the other. The radar must switch to receive between pulses.
So, the radiation unless you are close to the antenna is minimal because the pulses are of such short duration and decreasing at the square of the distance away from the array. So if it starts out at 2kw, at 1foot it is 1kw and 2 feet it is .5kw. At least this is my interpretation of how it works


So, no one is in any danger because the guy a couple of slips away runs his radar all day long. You should be more concerned with your cell phone or microwave oven which are often operated in close proxcimity to humans

[Lars_L]

You are right, there need to be one antenna for transmission and one for receiving when you have a continues wave. And the unit have that. The sit on top of each other in the same dome. The dome is a little bit higher but otherwise you cannot see it.

Whit a VHF radio that transmits around the whole horizon, the intensity of the signal decrees very rapidly. On a radar the signal is send in a small beam, and therefore the intensity of the signal decrees much slower.