PWR4360-59B
Senior Airman
- 379
- May 27, 2008
I just did a search on this topic, and pretty much got nothing. Where is a good site about all the latest in radar tech, well and the old stuff as well?
High power radar
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Wavelength
Airman 1st Class
- 108
- Jan 31, 2010
http://www.cdvandt.org/Fug200-paper-Hans-Jucker.pdf
The Hohentwiel radar design referred to in the article obtained relatively high pulse power by delivering a high voltage pulse to the anode of the transmitting tube, via a high voltage pulser such as a thyratron. This was the way to high pulse powers during WWII.
Normally, a lower power pulse was delivered to the control grid of a triode, but this limited radiated power. For example, the German Freya air warning radar delivered a precisely timed and shaped low powered pulse to the control grid of the transmitting triode. This created a radiated power of about 35kw using grid modulation of the late war TS41 transmitting triode, but by using anode modulation the radiated power output was increased to 200kw. Increasing the pulse voltage from a spark gap pulser resulted in power outputs for Freya exceeding 1000kw. Anode modulation does create certain problems, such long warm up times, and poor pulse shape. Random timing of the pulses is also a problem, making coherent radar techniques problematic.
Another way to effective high power is not through high pulse power per se, but through longer pulse widths. The illumination energy delivered to the target is the radiated pulse power multiplied by the pulse duration. For example, the 50kw ship board Hohentwiel alluded to in the Junker article would have illumination energy, of 100kw because it uses a 2 microsecond pulse width. The longer the pulse width; the lesser the bandwidth requirement of the receiver. This results in a more sensitive receiver with a far superior signal to noise ratio.
High illumination energy combined with a more sensitive receiver can result in excellent performance. This is the typical approach used by modern solid state radar designs. The trade off, with out the use of coherent radar techniques, is very poor resolution for distance. Modern coherent radar techniques such as pulse compression and other types of pulse modulated radar can get excellent resolution performance while still using long pulse widths, but they must also use very high bandwidth at the receiver while employing greatly compressed pulse widths.
The Hohentwiel radar design referred to in the article obtained relatively high pulse power by delivering a high voltage pulse to the anode of the transmitting tube, via a high voltage pulser such as a thyratron. This was the way to high pulse powers during WWII.
Normally, a lower power pulse was delivered to the control grid of a triode, but this limited radiated power. For example, the German Freya air warning radar delivered a precisely timed and shaped low powered pulse to the control grid of the transmitting triode. This created a radiated power of about 35kw using grid modulation of the late war TS41 transmitting triode, but by using anode modulation the radiated power output was increased to 200kw. Increasing the pulse voltage from a spark gap pulser resulted in power outputs for Freya exceeding 1000kw. Anode modulation does create certain problems, such long warm up times, and poor pulse shape. Random timing of the pulses is also a problem, making coherent radar techniques problematic.
Another way to effective high power is not through high pulse power per se, but through longer pulse widths. The illumination energy delivered to the target is the radiated pulse power multiplied by the pulse duration. For example, the 50kw ship board Hohentwiel alluded to in the Junker article would have illumination energy, of 100kw because it uses a 2 microsecond pulse width. The longer the pulse width; the lesser the bandwidth requirement of the receiver. This results in a more sensitive receiver with a far superior signal to noise ratio.
High illumination energy combined with a more sensitive receiver can result in excellent performance. This is the typical approach used by modern solid state radar designs. The trade off, with out the use of coherent radar techniques, is very poor resolution for distance. Modern coherent radar techniques such as pulse compression and other types of pulse modulated radar can get excellent resolution performance while still using long pulse widths, but they must also use very high bandwidth at the receiver while employing greatly compressed pulse widths.
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PWR4360-59B
Senior Airman
- 379
- May 27, 2008
Its been awhile posting here. Yes and antenna gain has a bunch to do with effective radiated power, sure would like to know if the Nexrad weather radars can operate non pulsed continuous wave at high power?
ThomasP
Senior Master Sergeant
Hey PWR4360-59B,
No. While such an option could be built into the NEXRAD (S-band) the specific requirements for the NEXRAD did not include such and would have entailed a pointless expense. The NEXRAD could roughly be compared to the radar component of the AWG-9 FCS (X-band, on the F-14A/B TOMCAT) when using pulse doppler track-while-scan mode in the sense that different areas of airspace can be differentiated due to relative motion and looked at as though they are individual entities. The only continuous wave mode built into the NEXRAD is of relatively low power and used for calibration and testing of parts of the system.
No. While such an option could be built into the NEXRAD (S-band) the specific requirements for the NEXRAD did not include such and would have entailed a pointless expense. The NEXRAD could roughly be compared to the radar component of the AWG-9 FCS (X-band, on the F-14A/B TOMCAT) when using pulse doppler track-while-scan mode in the sense that different areas of airspace can be differentiated due to relative motion and looked at as though they are individual entities. The only continuous wave mode built into the NEXRAD is of relatively low power and used for calibration and testing of parts of the system.
Token
Senior Airman
Why would you want a CW radar in such an application? CW is great for increasing the average return from a target, but at the cost of other factors. Narrow bandwidth necessarily reduces information you can derive from the target. For example, in a CW radar you have high (compared to a simple pulsed system) average power returned from the target, combined with a sensitive, narrow banded, receiver, increasing probability of detection. But, it cost you things like range resolution. Of course, modern compressed pulse systems, like FMCW radars, can bring the best of both worlds, but at the cost of complexity.
For radar, antenna gain is a key factor in ERP. When you have a 200 kw transmitter and you put that into a 30 or 40 dBi antenna you are multiplying the effective power by 1000 or 10000 times. I.e., your 200,000 Watt transmitter is now as effective as a 200,000,000 Watt, or higher, transmitter into an antenna with a gain of 1.
T!
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- #6
PWR4360-59B
Senior Airman
- 379
- May 27, 2008
A quick study of nexrad units, upgrades that started in 2010 to the 159 units of WSR-88D completed in 2013. 700KW at the klystron output and a 53 db gain center fed parabolic antenna. Just curious of what the heating capability would be say at 5 miles from the antenna using CW? Then if say 10 units both focused at the same point?
Hunting rabbits?
Token
Senior Airman
I know those numbers come from Wiki, but I am suspect of the gain quoted. The WSR-88D uses a parabolic dish that has an aperture of 8.5 meters and a center freq of about 2800 MHz. Given an efficiency of say 70% (very good) that makes the gain something more like 46 dB. Even using the raw dish diameter of 9.1 meters, vs the aperture, the gain would be something under 47 dBi. Given normal efficiencies the real number is probably more along the lines of 45.5 dBi.
And the peak power of 700 kW is of less importance to this question (heating energy on a target) than the average power. The max PRF of 1282 and the min PW of 1.57 usec yields a duty cycle of about 0.2%. So the average power for this transmitter is going to be about 1400 Watts.
But you did say CW, so lets do the math on 700 kW CW using the most probable gain of about 45.5 dBi (about 57 % efficiency for the aperture). And this yields a beamwidth of about 0.88 deg. Lets ignore feedline losses and just use those numbers.
OK, quick back of the napkin calculations follow, not something I can guarantee but should be in the ball park.
At 5 miles this yields a field strength of about 30.52 W/m^2, or 3.052 mW/cm^2. 10 systems looking at a single point would be additive, so 30'ish mw/cm^2. That is certainly well above the 10 mw/cm^2 RADHAZ MPE controlled access level, but far from what would do any real heating. I would not want to hang out there for lunch, but I guarantee most people in the modern world have been exposed to that kind of power level at one time or another.
By the way, the real WSR-88D average power would yield something like 0.061 W/m^2. Even with 10 of them pointed at a given point at 5 miles the power level would be way, way, below the 1 mW/cm^2 maximum uncontrolled exposure level.
T!
