Radars transmitted power

[Lars_L]

If you count on the mean power then do 4 kW pulse radar transmits with about eight times as high mean power than the broadband radar.

Radar with a bigger antenna has many advantages:
- The transmission beam is narrower and has whit that higher power intensity and that means that the pulse will not be stopped so easily.
- The narrower beam gives better details of the target.
- The bigger antenna is more sensitive when it shall receive the echo.
To compare a 60 cm radar with a 45 cm is wrong, the technical performance is so much better for the bigger that the compare will be unfair. The conclusion is that you should always have as big antenna that’s practical (and economical) possible.

If, despite this, you do a comparison you will always come to a limit. When you with the broadband radar get a echo back that’s that week that the radar cant detect it, there will not be any mark on the screen. If you in the same situation have hade pulse radar, the echo itself would have been eight times as strong (the power in the transmitted signal is that much higher), and the antenna is more sensitive so the radar would easily detect the signal. In this case the factor eight means a lot.

If pulse radars transmitted power would be dangerous for a human, the will a lowering of the power with a factor eight (as the broadband radar do) don’t mean that it will be harmless. The sensitivity for transmitted radar power differ that much between people, that a factor eight don’t make any difference. The conclusion is that independently if it’s a broadband radar or a pulse radar, it should always be mounted that high that there will not be any human in the transmitted field.

[continuouswave]

If a RADAR is transmitting 100-percent of the time, how does it listen for return echoes?

I don't think "continuouswave" RADARS actually transmit continuously. They do transmit longer than pulsed radars, but I don't believe they are key-down 100-percent of the time. If they were, I don't think the receiver could detect any returning echoes in the presence of the transmitted signal.

[transmitterdan]

Their literature says they are FMCW radars:

Broadband 4Gâ„¢ Radar

I don't know the design details Navico use but I think they must have separate transmit and receive antennas. They are strategically placed vertically so the receive antenna does not absorb any significant signal from the transmit. What little signal it does receive is ignored by the signal processing as being too nearby to be a real target.

[CarinaPDX]

IIRC detecting rain was a problem for the earliest Navico broadband radars, and that they had to make some changes in order to detect nearby weather. OTOH heavy rain can obscure targets with pulse radar. I have never experienced a problem with fog (that I know of...) with my pulse radars.

We want radar to detect targets AND weather, and any modern set will do both (to varying degees).

Greg

[continuouswave]

Originally Posted by transmitterdan

Their literature says they are FMCW radars:

Broadband 4Gâ„¢ Radar

I don't know the design details Navico use but I think they must have separate transmit and receive antennas. They are strategically placed vertically so the receive antenna does not absorb any significant signal from the transmit. What little signal it does receive is ignored by the signal processing as being too nearby to be a real target.

If the transmitter in a RADAR were continuously transmitting, and if the receiver "ignored" that signal, then it would be ignoring it all the time. How would the receiver know when to ignore the transmitted signal that it was picking up, since it would be picking it up all the time?

I don't understand how you could perform any sort of distance ranging calculation if the transmitter were continuously transmitting. How would the receiver know the time offset between the transmitted signal and the return signal?

[colemj]

Originally Posted by continuouswave

I don't understand how you could perform any sort of distance ranging calculation if the transmitter were continuously transmitting. How would the receiver know the time offset between the transmitted signal and the return signal?

In the case of FMCW, the frequency is swept and the transmitted frequency at any time in the sweep is mixed with the received frequency to create a beat frequency. The beat frequency is proportional to the distance.

Mark

[ka4wja]

Mark has it about right....

And gosh guys, with names like "continuouswave" and "transmitterdan", 'ya-all shoulda' known this....


Seriously FMCW radar has been around a LONG LONG time....
Aircraft Radar altimeters are FMCW radars, as are/were most early-warning missile defense radars....

If you wish the details, have a look at a couple of these sites...
Wolfram Demonstrations Project
Radar Basics
Continuous-wave radar - Wikipedia, the free encyclopedia


Now as for the Navico FMCW marine radar (that they call broadband, 3G, 4G, etc.), when compared to normal-pulsed-radar, its primary advantages are:
a) the "instant on"....not needing to keep a magnetron warmed-up in standby (saving some power)
b) the slight overall power savings (not as drastic as their marketing makes it seem, as they are only comparing the power used by the scanners, and not reminding you that the typical display uses twice the power of the scanners....)
c) high resolution on very close-in targets

And, its disadvantages, compared to normal-pulsed-radar....
a) poor detection of rain...
b) less useable range (although they are getting better)
c) slightly less target discrimination at middle to longer ranges...due to wider antenna beam of current units (although they are getting better)
d) limited to just the one manufacturer...



I hope this helps...

John
s/v Annie Laurie

[transmitterdan]

I do know this. I do it for a living. The Wikipedia page on "chirp" has some good info too. Basically it's possible to convert a swept frequency signal into something that approaches an impulse. The height and time delay of that received "pulse" is what drives the video of the radar. It does not need to turn off the transmitter to receive because of the high isolation between tx and rx antennas.

[colemj]

Originally Posted by transmitterdan

It does not need to turn off the transmitter to receive because of the high isolation between tx and rx antennas.

I didn't mean to imply this, and will further state that the isolation is not only in the physical domain, but also in the frequency and time domains of the signals each are processing at any given time.

Mark

[transmitterdan]

Leakage from the tx to rx just causes a small impulse at t=0. This is the same as "ground clutter" in pulse radar. It's simple to ignore the pulse at t=0 which is what I inelegantly tried to explain to CW.

[continuouswave]

Originally Posted by transmitterdan

Leakage from the tx to rx just causes a small impulse at t=0. This is the same as "ground clutter" in pulse radar. It's simple to ignore the pulse at t=0 which is what I inelegantly tried to explain to CW.

I don't understand the concept of t=0. If the transmitter is always on, how can there be a time reference to when it started transmitting? A time t=0 might be ten weeks ago when you turned on the RADAR.

Also, the frequency modulation cannot continue indefinitely. At some point it has to stop.

[colemj]

Originally Posted by continuouswave

I don't understand the concept of t=0. If the transmitter is always on, how can there be a time reference to when it started transmitting? A time t=0 might be ten weeks ago when you turned on the RADAR.

Also, the frequency modulation cannot continue indefinitely. At some point it has to stop.

The frequency is swept at a linear rate from a set lower frequency to a set higher frequency. Any given time along this sweep is precisely known. T=0 is the start of the sweep at the set lower frequency. The time of the sweep is precisely controlled, so all points along the sweep line are precisely known.

This modulation stops when you turn off the power.

Mark

[continuouswave]

Originally Posted by colemj

The frequency is swept at a linear rate from a set lower frequency to a set higher frequency. Any given time along this sweep is precisely known. T=0 is the start of the sweep at the set lower frequency. The time of the sweep is precisely controlled, so all points along the sweep line are precisely known.

What is the interval at which the FM modulation is repeated?

[continuouswave]

Originally Posted by ka4wja

,,,with names like "continuous wave" ...'ya-all shoulda' known this....

Having never worked in aviation or the military, I have very little RADAR background.

As for the "continuous wave" user-id, my background is in A1, on-off keying and Morse code--that sort of CW or continuous wave.

The concept of FMCW auto-dyne RADAR is new to me. I will have to do some studying. I thought these new RADARs were pulse-compression devices. The notions of "pulse" and "always transmitting" do not seem compatible.

[colemj]

Originally Posted by continuouswave

What is the interval at which the FM modulation is repeated?

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

They sweep from 9.3-9.4GHz over 1.3ms (+/-10%), with a sweep repetition frequency of 200-540Hz (mode dependent and adjustable).

Interestingly, this suggests a 25-70% duty cycle, and not continuous transmission. However, that most likely has something to do with power and signal processing, because it certainly isn't necessary for reception of signal.

Mark