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Among other reasons, those old tube sets were nowhere as efficient as today's solid state miniaturized circuitry. The transmission lines and antennas were more lossy and less efficient at converting generated power to effective radiated power. Internal timing was less precise, as was the test equipment to calibrate and monitor it. Antennas were less effective in focussing the radiated power, and had less inherent gain to catch the return, and thanks to compact and efficient phase lock loop circuits, modern radar receivers are able to achieve usable S/N with much weaker returns. And then there's digital signal processing.peak power of a Hughes E-5 (F-94c) or of an AN/APS-21 (F3D-2) is described with 200 kW while the latest Irbis E (Su-35) has a peak of 20 kW, an AN/APG-81 (F-35) of about 17 kW, or the Raven of the newest Gripen "more than 10 kW".
In the half century plus since the radars you cited were designed the largely un-radared world has become covered with a network of ATC radars, the raw data of which is available to the relevant military commands that defend regional airspace. This network is by no means all encompassing, but the big picture of the airspace is much less the mystery it once was.BTW, I do not agree that for modern fighter aircraft detection range is less required because of GCI than in earlier decades. The big picture of the airspace is important as ever and there is not always an AWACS at hand
Returning to the original topic of this thread: William Wolf describes on page 417 of his excellent book "U.S. Aerial Armarment in World War 2" following P-61 installation:
"There were seven P-61B-25s built which were test beds for the Western Electric APG-1 Gun Laying Radar that was linked to the GE remote controlled dorsal turrets. The SCR-720 radar fed data into the analogue computer which directed the turret guns onto the target. One P-61B-15 and the first six P-61B-20s were also modified to this configuration. These aircraft were tested by the Air Proving Command at Eglin Field, FL and at the night fighter training unit at Hammer Field, CA."
Thank you for the reply. Yes, average output among the radars you mentioned may be similar but it still makes no sense to me why for example the
peak power of a Hughes E-5 (F-94c) or of an AN/APS-21 (F3D-2) is described with 200 kW while the latest Irbis E (Su-35) has a peak of 20 kW, an AN/APG-81 (F-35) of about 17 kW, or the Raven of the newest Gripen "more than 10 kW". Is it really just a matter of pulse repitition?
Token, thank you for this great and detailed explanaition. So does this mean that the microwave source of modern (airborne) radars (TWTs) is generally not capable to deliver the same high peak pulse performance (I am not talking about average output) like old magnetron based radars? I have read the Irbis E of the Su-35 is built up with two TWTs just to get peak power up to 20 kW. On the other hand the AN/ASG-18 of the YF-12 is said to have had a peak output of 600 kW or more; similar numbers are available for the original interceptor version of the MIG-25). And this radar of course had a TWT and not a magnetron as microwave source.
Ad P-61: I also think that William Wolf has mixed things up a little bit. I am aware that the SCR-720 with its helical scanning is not able to feed data into a computer (in contrary to the APG-1/2). BTW it was already well explained in earlier posts
The power of 600 kW for the Mig-25 radar is mentioned in "Modern Air Combat" by Bill Gunston, a book of the 1980s. Not sure if this is correct. I do not remember the source where I read about the peak power of the AN/ASG-18. However it used two TWTs in tandem to obtain the highest possible detection range with its 40 inch dish.
Of course stealth considerations are an important driver to limit power output but maximum detection range still seems to be a primary requirement for at least a certain class of "strategic" fighter aircraft like Mig-31, Su-27 follow-on series, F-22, etc. It would generally be interesting to know if modern high end fighter aircraft radar range performance is really the maximum possible or if it is more a balance between stealth and detection range.
The power of 600 kW for the Mig-25 radar is mentioned in "Modern Air Combat" by Bill Gunston, a book of the 1980s. Not sure if this is correct. I do not remember the source where I read about the peak power of the AN/ASG-18. However it used two TWTs in tandem to obtain the highest possible detection range with its 40 inch dish.
A crusty old Chief at Avionics A School told us about a rig like that which he had seen when he was a young Airman. He said the old radar sets had a fairly narrow scan pattern (45° left and right), and they wanted wider coverage, so they put one on each wing. They were both aimed at 45° off straight ahead and linked to one scope and coordinated so the first scanned from 9 o'clock to 12, where the other took over and continued on to 3, then started back. On the scope it looked like a single sweep with 180° coverage.a single F6F-5N was built up with two radar scanners (one on each wing). Do you have any idea what the reason could have been?
Well this was all word of mouth and long ago (1970), but as I remember it, he described a display that looked like half a PPI with the beam sweeping back and forth rather than rotating and with a small cross hair that slid up and down the centerline to display antenna elevation angle relative to horizon. (Gyro stabilized) A -o- icon displayed the interceptor's flight attitude. Apparently the goal was for the two sets to be so coordinated that the shared display would hand off seamlessly from one set to the other at the 12 o'clock position and all look like one. Theoretically, anyway. The fact that it was sitting off in the weeds neglected suggests it maybe wasn't one of the better ideas after all. I think the F3D was the hot new thing at the time.Wes,
never thought that anybody had an answer to this because of the many decades back. Great, thank you.
So one crt showed the combined picture of both radar sets?
Spindt type FEAs have been a thing since the late 1960's. But lots of more traditional guns are in use.Some time ago I have read that current TWT development looks at microtip emitters (field emission array cathodes) to replace the electron gun although it has become a little bit quiet about it.
BTW is it correct that AESA radars (in contrary to PESA) do not have TWTs or klystrons as microwave source but instead generate the microwaves via semiconductors?