P-61 Gun-Laying Radar

[Zipper730]

Blind fire tail guns such as "Village Inn" tail radar used on the Lancaster would I think have not been a success.

The RAF used it to some effect it seemed...

Little known is that the Germans deployed a small number of FuG 211 and FuG 215 blind fire radars in combat in 1942.

Now that's interesting!

The reason they were not deployed in greater numbers is likely because of the IFF (Identification Friend or Foe) problem which confounded everyone and that they get caught up in the frequency changes needed in 1943 to overcome windows. You had to fly up close and get a visual. For the Nachtjagdt the rules were if it had 4 engines its not ours so shoot it down.

Wait... they were considered 4-engined bombers not theirs to shoot down? Are you sure you didn't get that backwards?

Above is a picture of the early FuG 202C1 Lichtenstein radar mounted in a Ju 88C. It operated at 490Mhz (60cm)

The small aerial in front of the 16 part (4 by 4) array is the dipole from which the signal radiates, the rear antenna is the 'reflector' which is placed 1/4 wavelength away. it reflects the signal forward and is spaced such a way that the reflection constructively interferes with the dipole to increase directionality. Further forward directionality is promoted by the array interference pattern.

So the antennae worked together to amplify each other and in doing so, increased the ability to accurately track the direction of the target?

By using a rotating switch wave delays were switched in to the dipoles steer the beam alternately left/right then up/down.

This vaguely sounds like a phased-array radar...

This is what the radar operator saw; Below are the radar traces.

The scope on the left is a j scope and shows the range of the target aircraft around its circumference. It is somewhat superfluous in that the other scopes also contain range information. The scope in the middle shows whether the target is to the left or right (in this case the right return is heaviest so the target is to the right) and the scope in the right shows if the target is high or low, in this case high. The big pulse at the end is the ground return.

The azumuth/elevation layout is bizarre: They are exactly the opposite of how I would lay them. The elevation scope looks more like it'd display azimuth and the azimuth looks like an elevation scope.

As for azimuth: The big blip is the target, correct?
As for the elevation: The smaller blip is target, the bigger one is the ground, just to be clear?

The narrow beam of allied microwave radar however made it less susceptible of receiving jamming energy.

So we produced a more focused beam?

If the Germans had of replaced their 60cm 1.5kW transmitter with a 15cm 5kw transmitter (which they had by way of the Telefunken LD7 disk triode) and used a more complicated switch pattern 4 left.right and 4 up down) they would have had a phased array radar that could have fitted in a dome.

So, while the radar wasn't as aesthetically pleasing it was closer to a phased array than what we had?

You need to brush up on your basic electronics, especially wave propagation. It can't be explained simply and unconfusingly without a lot of illustrations, which I can't do on this phone.

You very accurately identified my exact problem: I know almost nothing about electronics and am often in a position where I've made guesses, generalities based on what others have said.

I have no formal education in the subject: I'm curious if you have any ideas where to start. Is there an electronics and wave propagation for dummies somewhere? And if so, how did I miss it all these years

 

[fubar57]

​

LOL......WUT???

 

[XBe02Drvr]

Wait... they were considered 4-engined bombers not theirs to shoot down? Are you sure you didn't get that backwards?

No, you got it backwards. Luftwaffe had no four engine bombers likely to be seen in the night skies over the Reich. (FW-200 was a maritime patrol craft.) Ergo, if it's got four engines, it's a bad guy; shoot him down!
Cheers,
Wes

 

[XBe02Drvr]

I have no formal education in the subject: I'm curious if you have any ideas where to start. Is there an electronics and wave propagation for dummies somewhere? And if so, how did I miss it all these years

No such luck. There's no quick and easy path to electronics. I would suggest getting a beginning text book for an electronics technician course. STAY AWAY FROM ENGINEERING TEXTS! I think there is some form of "Basic Electronics for Dummies" on the market. Try "Volts for Dolts".
You're going to need DC and AC theory, resistance, capacitance, and inductance, impedance, semiconductor theory, amplifiers, oscillators, receiver theory, transmitter theory, antenna and transmission line theory, cathode ray tubes, transducers etc, etc etc.
It took me thirty-four weeks of intensive (morning, afternoon and night) classroom and laboratory instruction, so don't expect rapid miracles. As a leisure activity, plan on a couple years, but I bet you'll find it fascinating once you get going.
Cheers,
Wes

 

[Zipper730]

No such luck.

Worth a shot...

I would suggest getting a beginning text book for an electronics technician course. STAY AWAY FROM ENGINEERING TEXTS! I think there is some form of "Basic Electronics for Dummies" on the market. Try "Volts for Dolts".

That's a good one... Volts for Dolts... and it's very fitting with my limited knowledge in this case.

You're going to need DC and AC theory, resistance, capacitance, and inductance, impedance, semiconductor theory, amplifiers, oscillators, receiver theory, transmitter theory, antenna and transmission line theory, cathode ray tubes, transducers etc, etc etc.

Wait... there's such a class at the local community college.

It took me thirty-four weeks of intensive (morning, afternoon and night) classroom and laboratory instruction, so don't expect rapid miracles.

Wow...

 

[wuzak]

Little known is that the Germans deployed a small number of FuG 211 and FuG 215 blind fire radars in combat in 1942. These were known as "Pauke A" and about 10 were produced and trialed in combat. These had an accuracy of 0.5 degrees within +/-10 degrees of forward. The reason they were not deployed in greater numbers is likely because of the IFF (Identification Friend or Foe) problem which confounded everyone and that they get caught up in the frequency changes needed in 1943 to overcome windows. You had to fly up close and get a visual. For the Nachtjagdt the rules were if it had 4 engines its not ours so shoot it down. Proper airborne IFF didn't reach the Luftwaffe but for a few "Neuling" equipped Me 262.

Wait... they were considered 4-engined bombers not theirs to shoot down? Are you sure you didn't get that backwards?

"For the Nachtjagdt the rules were if it had 4 engines its not ours so shoot it down".

I think you misunderstood the statement.

The problem with IFF was that it wasn't 100% reliable. That is, if you didn't get a response you could not be certain that the aircraft was the enemy.

And the British had cracked the German IFF - they could get a response from a German aircraft's IFF so they could track it.

The Germans may have done the same - they certainly were able to track the RAF bombers' tail warning radar Monica, which was withdrawn after the RAF captured a German nightfighter and discovered teh system.

 

[Token]

So the antennae worked together to amplify each other and in doing so, increased the ability to accurately track the direction of the target?

Look at that picture and for a moment forget that there are 16 groups of 2, just consider each 2 element pair. The single set of two elements mounted one in front of the other.


What is being described is a basic 2 element Yagi-Uda antenna. It was documented and identified as a particular function in 1926 in Japan. The back element is parasitic, not active, the front element is driven, and active.


WAY oversimplified here. The energy is radiated from the front element of the pair. Some of the energy goes forward, some to the sides, and some of the energy goes back towards the other element. The energy going back gets reflected off that back element, thus the name for that element being a "reflector". That reflected energy goes forward and is added to the energy of the driven element.


This configuration produces moderate gain and directivity in the forward direction, away from the reflector and towards the driven element.


So that is a single set of two elements seen in that picture, each set of 2 is a Yagi.


But there are 16 sets of 2 elements. Each doubling of elements (2, 4, 8, 16) produces near twice as much gain (not quite double, roughly about 2.7 - 2.8 dB increase on average) and about half the beamwidth.

This vaguely sounds like a phased-array radar...

It is the basis for phased array, but it is not what we normally call phased array.


This is basically an adaptation of the lobe switching technique. By controlling the phase relationship of the energy fed to each set of 8 Yagis you can bend or steer the beam. There are 4 sets of 8, 2 sets of 8 in each plane.


So for the purposes of elevation the 16 sets are divided in two halves, top 8 and bottom 8. For the purposes of azimuth the 16 sets are divided in two halves, right 8 and left 8.


To steer the beam up slightly you slightly delay in phase the feed (energy getting to and from) to the top 8 in reference to the bottom 8. You make the signal arrive to the top 8 a little bit later in time. Often this is done by simply using slightly longer feedlines to the delayed set.


The basics of this theory was described in papers going back to the mid 1920's, and even in ham radio books by the late 1930's. Of course hams did not call it the same thing, they called them "end fire" and "broad side" arrays depending on if the elements were fed in phase or 180 degrees out of phase.

The azumuth/elevation layout is bizarre: They are exactly the opposite of how I would lay them. The elevation scope looks more like it'd display azimuth and the azimuth looks like an elevation scope.


As for azimuth: The big blip is the target, correct?

As for the elevation: The smaller blip is target, the bigger one is the ground, just to be clear?

I have attached an annotated image of how I think it works in this case.

The range display is called a J Scope. A circular representation of the entire range coverage in one sweep. In this case it starts at zero range straight up, the 12 o'clock position, and goes around clockwise to maximum range, just short of straight up at about the 11:30 position.


Notice that there is a big blip at zero range of the Range display. This is "main bang", the transmitter power is leaking into the receiver during or slightly after the on time of the transmitter.


Azimuth and Elevation are L Scopes. Azimuth appears to go from zero range at the top of the scope to maximum range at the bottom of the scope. Elevation appears to go from zero range at the left of the scope to maximum range at the right of the scope.

Note that in the attached image the target is at about 1.2 km on the Range scope, slightly to right of boresight, and slightly above boresight. So the target is slightly right and up from the aircraft, and about 1.2 km away.

Most systems using this display technique put zero range at the bottom of the Azimuth scope.

I know the FuG 202 put main bang at the bottom on its Azimuth display, so not sure why this diagram has it at the top. But then I thought the FuG 202 also put zero range on the J Scope at the bottom, or 6 o'clock position and max range at about the 4 o'clock position.

So we produced a more focused beam?

For some radars that is true. The use of microwave frequencies allowed higher gain for a given size of antenna, higher gain means smaller beamwidths, more concentrated power.


However we also had lower frequency systems with wider beamwidths, similar to the one described.


Most allied radars, particularly airborne, were at higher frequencies by mid 1943.

So, while the radar wasn't as aesthetically pleasing it was closer to a phased array than what we had?

No.


First, while the basic concept of the system is similar to Phased Array, using phase relationship to steer a beam, it is not generally called phased array in this application. Not wrong to call it so, just not what is generally meant by the term.


Second, "we" (assuming you mean Allies) used the same techniques, although typically not on aircraft. And so did the Japanese, and I think the Russians. There is no doubt, however, that the Germans used it in greater numbers and to greater affect.


Because we were working at higher frequencies we had tighter beamwidths and other ways to steer the beam, and we had no need to deploy such systems to active units. Most of our airborne systems similar to this were very early or research/experimental, we kind of jumped over this as a front line technology.


Working at the lower frequencies, and longer wavelengths, as the Germans were the antennas were physically larger for similar gain values. While it is mechanically simple to just point a small antenna it is more difficult to point a larger antenna. It was easier for us to just form a more focused, narrow, beam, and mechanically point it as needed. The Germans did not have that option.

T!

 

[Zipper730]

"For the Nachtjagdt the rules were if it had 4 engines its not ours so shoot it down".

Oh, okay... so if it's got four engines, blow it up?

 

[pbehn]

Oh, okay... so if it's got four engines, blow it up? That makes lots of sense...

Yes it does Germany didn't have four engine planes operating at night.

 

[Zipper730]

Look at that picture and for a moment forget that there are 16 groups of 2, just consider each 2 element pair.

Okay, I understand.

What is being described is a basic 2 element Yagi-Uda antenna. It was documented and identified as a particular function in 1926 in Japan.

That's impressive to have developed radar technology such a long time back!

The back element is parasitic, not active, the front element is driven, and active.

Okay, but, at the risk of sounding silly, why was it called parasitic?

WAY oversimplified here. The energy is radiated from the front element of the pair. Some of the energy goes forward, some to the sides, and some of the energy goes back towards the other element. The energy going back gets reflected off that back element, thus the name for that element being a "reflector". That reflected energy goes forward and is added to the energy of the driven element.

So the back element ensures that the energy transmitting rearwards is now going just forward, not in all directions with around twice the energy going forward?

This configuration produces moderate gain and directivity in the forward direction, away from the reflector and towards the driven element.

Antenna gain is basically a measure of the antenna's electrical efficiency, and the ability to produce an energy emission that is not omnidirectional, but instead going in a specific direction?

But there are 16 sets of 2 elements. Each doubling of elements (2, 4, 8, 16) produces near twice as much gain (not quite double, roughly about 2.7 - 2.8 dB increase on average) and about half the beamwidth.

So more energy is put on target, and more accuracy can be produced in the process?

It is the basis for phased array, but it is not what we normally call phased array.

Okay. So, it's more a progenitor?

This is basically an adaptation of the lobe switching technique. By controlling the phase relationship of the energy fed to each set of 8 Yagis you can bend or steer the beam. There are 4 sets of 8, 2 sets of 8 in each plane.

So for the purposes of elevation the 16 sets are divided in two halves, top 8 and bottom 8. For the purposes of azimuth the 16 sets are divided in two halves, right 8 and left 8.

To steer the beam up slightly you slightly delay in phase the feed (energy getting to and from) to the top 8 in reference to the bottom 8. You make the signal arrive to the top 8 a little bit later in time.

The term of phase relationship is something that I'm starting to learn, it seems to mean one of the following

• The position of a point in time on a wave-form cycle, with a complete cycle being the time it takes from the wave to start and complete and return to it's original cycle (wave starts in the middle, goes up to the peak, down to the valley, and back to the middle?)
• The expression of relative displacement (the minimum distance from the start to the end of the wave?) between two corresponding peaks, valleys, of two waves on the same frequency.
• With sinusoidal/sine-wave functions: It has to do with the wave being varying degrees in or out of phase with each other (constructive/destructive interference), or the length of the wave that has passed from the start to it's current point (I think)

Often this is done by simply using slightly longer feedlines to the delayed set.

The antenna physically was extended out/retracted in?

The basics of this theory was described in papers going back to the mid 1920's, and even in ham radio books by the late 1930's.

I never used a ham radio... I really should have at least bothered to learn morse code (I know S = ... O = ---)

I have attached an annotated image of how I think it works in this case.

View attachment 467294

Okay, so the small blip is the target, and the main bang is basically an artifact from the radar itself?

For some radars that is true. The use of microwave frequencies allowed higher gain for a given size of antenna, higher gain means smaller beamwidths, more concentrated power.

That makes sense...

However we also had lower frequency systems with wider beamwidths, similar to the one described.

Were there any advantages to them?

No.

First, while the basic concept of the system is similar to Phased Array, using phase relationship to steer a beam, it is not generally called phased array in this application. Not wrong to call it so, just not what is generally meant by the term.

Okay

Second, "we" (assuming you mean Allies) used the same techniques, although typically not on aircraft. And so did the Japanese, and I think the Russians.

Yes, I meant the allies

 

[Token]

That's impressive to have developed radar technology such a long time back!

The Yagi-Uda antenna was not designed as a radar technology, it was just another development in the road of radio frequency systems and devices. A relatively simple way to produce a somewhat directional beam using electromagnetic energy.


Radar is simply a specific application of radio frequency energy. To RF an antenna is an antenna, whether it is a radar system, a voice radio, or a satellite data transmission. Technologies that apply to your cell phone antenna also apply to radar, or in the case of the Yagi antenna a technology that applied to communications radio applied quite well to radar.

Okay, but, at the risk of sounding silly, why was it called parasitic?

It is not active, i.e. no energy is directly applied to that element. It draws power off of, away from, the active, or driven, elements in the antenna design. Correctly done it then can redirect that energy in a desired direction. You can also have parasitic directors instead of reflectors. They would serve to bend the energy of the system towards themselves instead of away from themselves.

So the back element ensures that the energy transmitting rearwards is now going just forward, not in all directions with around twice the energy going forward?

Lets not say twice as much energy going forward, lets just say it increases the amount of energy in a specific desired direction.


Think of it this way, you have a lightbulb in the middle of a room. The light goes everyplace more or less evenly. Now put a mirror on one side of the light bulb. All the light that goes in the direction of the mirror is reflected, and now goes in the opposite direction, adding to the light on the opposite side of the bulb from the mirror.

Antenna gain is basically a measure of the antenna's electrical efficiency, and the ability to produce an energy emission that is not omnidirectional, but instead going in a specific direction?

I would drop the efficiency part of that statement, and simply say gain is the ability to produce energy inn a desired direction, the more directional the beam the more gain the antenna will have, all other things being equal.

So more energy is put on target, and more accuracy can be produced in the process?

Higher gain means a tighter beam width. Tighter beam widths mean more directional accuracy is possible. But higher gain also means more radiated power for the same amount of transmitter peak power, and it means more receiver sensitivity in a specific direction. All meaning you can track targets further away.

Okay. So, it's more a progenitor?

Sure, you could say that. It is a core concept that would eventually allow what we call phased array systems today.

The antenna physically was extended out/retracted in?

No, the antenna was not extended, the feedline carrying the energy to the antenna was lengthened or shortened.


You have a transmitter and two antennas. Each antenna is physically 4 meters away from the transmitter, but each has 5 meters of cable connecting the transmitter to the antenna. These two antennas are fed in phase, the transmitted energy takes the same time to go from the transmitter to each antenna. Now lengthen the cable going to only one antenna to 6 meters, and leave the other 5 meters long.


The radio energy will take longer to get from the transmitter to the second antenna, the one fed with a 6 meter cable, than it will take to get form the transmitter to the first antenna, the one fed with a 5 meter cable. The energy will arrive at the second antenna delayed.


The exact phase shift of said delay will depend on the frequency of operation, but delayed it will be.

Okay, so the small blip is the target, and the main bang is basically an artifact from the radar itself?

Yes.

Were there any advantages to them?

Realistically there were no advantages to the lower frequencies at that time. It was simply a technology limitation driven by the upper frequency you could make acceptable power at. Once the Cavity Magnetron became a usable fact and microwave frequencies, and no longer just a laboratory piece, the technology allowed higher freqs with good power in compact packages. The Allies had a usable microwave cavity magnetron, the Germans did not.


However, today lower frequency radars are making a comeback. Current Stealth Technology is not as affective against lower frequencies.


T!

 

[Zipper730]

The Yagi-Uda antenna was not designed as a radar technology, it was just another development in the road of radio frequency systems and devices.

Oh, okay

Technologies that apply to your cell phone antenna also apply to radar, or in the case of the Yagi antenna a technology that applied to communications radio applied quite well to radar.

That's quite fascinating.

It is not active, i.e. no energy is directly applied to that element. It draws power off of, away from, the active, or driven, elements in the antenna design.

Okay

Lets not say twice as much energy going forward, lets just say it increases the amount of energy in a specific desired direction.

Okay

Think of it this way, you have a lightbulb in the middle of a room. The light goes everyplace more or less evenly. Now put a mirror on one side of the light bulb. All the light that goes in the direction of the mirror is reflected, and now goes in the opposite direction, adding to the light on the opposite side of the bulb from the mirror.

That makes enough sense

I would drop the efficiency part of that statement, and simply say gain is the ability to produce energy inn a desired direction, the more directional the beam the more gain the antenna will have, all other things being equal.

Understood

Higher gain means a tighter beam width. Tighter beam widths mean more directional accuracy is possible. But higher gain also means more radiated power for the same amount of transmitter peak power, and it means more receiver sensitivity in a specific direction. All meaning you can track targets further away.

Useful thing to have!

Sure, you could say that. It is a core concept that would eventually allow what we call phased array systems today.

Okay...

No, the antenna was not extended, the feedline carrying the energy to the antenna was lengthened or shortened.

You have a transmitter and two antennas. Each antenna is physically 4 meters away from the transmitter, but each has 5 meters of cable connecting the transmitter to the antenna. These two antennas are fed in phase, the transmitted energy takes the same time to go from the transmitter to each antenna. Now lengthen the cable going to only one antenna to 6 meters, and leave the other 5 meters long.

The radio energy will take longer to get from the transmitter to the second antenna, the one fed with a 6 meter cable, than it will take to get form the transmitter to the first antenna, the one fed with a 5 meter cable. The energy will arrive at the second antenna delayed.

So the electrical wiring is stretched or pulled in?

Yes.

Okay

However, today lower frequency radars are making a comeback. Current Stealth Technology is not as affective against lower frequencies.

I've heard that somewhere before...

 

[Token]

So the electrical wiring is stretched or pulled in?

The total feedline length is lengthened and shortened in various ways.


An example might be coaxial cable and switches. The coaxial cable is the feedline.


In the previous example, 2 antennas 4 meters from the transmitter each fed with 5 meters of feedline. I have a 5 meter section of coax going to each antenna. And at one end of each feedline I have a coaxial switch that allows me to select an additional meter of coax in the path or to bypass that meter of coax. Throw a switch and the total length of cable feeding an antenna is now 6 meters, flip the switch the other way it is now 5 meters. I can now switch in and out, at will, a delay equal to the propagation time through that additional meter of coax, a delay, or phase shift, that can now be added or subtracted to each, either, or both, antennas.

I've heard that somewhere before...

While lower freqs are sometimes good for acquisition radars they are, for a variety of reasons, generally poor for shooters in most applications. So even assuming the use of lower freqs for search radars 100% negates stealth (and it does not) the shooter still cannot track and shoot the target unless they also are at lower freqs.


This is not a new concept or issue. It has been known from the very earliest days of intentional radar cross section control. The general issue and physics involved are well documented in related papers going back to the 1940's. But occasionally some news report, magazine article, etc, pops up making it seem like this is a new finding, and that stealth technology is now "dead" or "useless". This revelation is simply not true, but can't really be explained in a 60 second sound bite, so most people don't understand.


What it has done, however, is shift some of the radar development that has happened. In general since WW II the trend has been towards higher and higher frequencies for radar, abandoning the older VHF and UHF freqs that were at one time commonly used. Since Stealth became an acknowledged thing there has been a resurgence of lower frequency radar, almost certainly mostly driven by concerns for defeating stealth.


T!

 

[Zipper730]

AN/APG-4: Was a radar configured as a 500lb bomb used as an air intercept radar of Corsairs and Hellcats.

That's a nutating-scan right?

British efforts for blind fire radar died with the death of Arthur Ernest Downing who was shot down in a friendly fire incident while developing chaff resistance for the planed for tracking capable AI Mk.IX in 1941.

Do you have anymore details on this event?

By using a rotating switch wave delays were switched in to the dipoles steer the beam alternately left/right then up/down.

So basically the rotation either would activate different circuits or coil up the line increasing or decreasing feed line length?

These latter radars injected the firing directions into either the EZ42 or EZ45 gyro 'reflector sight' which had been modified to take inputs from Toss bombing computers or the blind fire computer (Elfe) to deflect the cross hairs appropriately. It was hope to put the signals straight to the auto pilot.

I didn't know toss-bombing even existed in WWII, let alone a proposal to feed automated signals from the radar straight to the autopilot (which is something we'd see years later)

The total feedline length is lengthened and shortened in various ways.

An example might be coaxial cable and switches. The coaxial cable is the feedline.

Another member (I posted a reply to him too for the same reason, but it will also be a reference as well) mentioned rotating switch relays...

In the previous example, 2 antennas 4 meters from the transmitter each fed with 5 meters of feedline. I have a 5 meter section of coax going to each antenna. And at one end of each feedline I have a coaxial switch that allows me to select an additional meter of coax in the path or to bypass that meter of coax.

So you basically divert power off at 5 meters, or at 6?

While lower freqs are sometimes good for acquisition radars they are

Why if I may ask?

This is not a new concept or issue. It has been known from the very earliest days of intentional radar cross section control. The general issue and physics involved are well documented in related papers going back to the 1940's.

They attempted stealth back in the 1940's? Do you mean the XB-35?

But occasionally some news report, magazine article, etc, pops up making it seem like this is a new finding, and that stealth technology is now "dead" or "useless". This revelation is simply not true, but can't really be explained in a 60 second sound bite, so most people don't understand.

Honestly, it'd be nice if news was more comprehensive...

Since Stealth became an acknowledged thing there has been a resurgence of lower frequency radar, almost certainly mostly driven by concerns for defeating stealth.

Makes sense...

 

[XBe02Drvr]

They attempted stealth back in the 1940's? Do you mean the XB-35?

The fact that research papers addressed theories of radar cross section control doesn't mean that actual hardware attempts to achieve it were made back then. Complete invisibility as we think of stealth today was at that time a "holy grail", unachievable with the technology at their disposal. There was still tactical advantage to be had by making an aircraft less noticable on radar, allowing it to get much closer to its target before being detected. "New radar contact, 12 o'clock high, five miles, closing, 900 knots!"
Cheers,
Wes

 

[Token]

So you basically divert power off at 5 meters, or at 6?

There is a cable (feedline) that carries the transmitter power from the transmitter to the antenna. In my example the cable length will be either 5 meter or 6 meters (5 meters plus a 1 meter extension added) in total length.


Why is this important? Because RF takes time to travel the length of whatever cable it is being conducted by. The specific time will vary with the cable type, but it is the cable length divided by speed of light multiplied by the propagation velocity of the cable. So the RF will take longer to travel the length of the cable in the 6 meter example than it does in the 5 meter example. How much longer? Using a pretty common coax velocity factor it will take about 5 nanoseconds longer. For a 90 MHz radar (like the German FuG 220 Lichtenstein) using a cable of that velocity factor this 5 nsec is roughly 160 degrees of phase shift.


The lengths I threw out in my examples were random, but the real FuG 220 used a 90 degree phase shift to steer the beam. So the feedline lengths would be varied (via switching) a bit more than 0.5 meter in total length.

Why if I may ask?

Shooters are limited by whatever weapon system they are tied to. If you have an AA gun that can shoot 6 km why have a radar driving it that can see 200 km? Sure, you want the radar to be able to work a bit beyond the weapon range, but not that much.


But acquisition and search radars want to detect a target as far away as possible. Lower frequencies are easier to make high power with. All other things being equal more transmitter power means greater detection range. Lower frequencies have lower space loss, this means the signal gets attenuated less over distance. All other things being equal lower space loss means increased power arriving at a given target at a given range.

They attempted stealth back in the 1940's? Do you mean the XB-35?

I did not say they attempted stealth back then. What I said was (discussing current acknowledged stealth technologies and their inefficiencies at various frequencies) "This is not a new concept or issue. It has been known from the very earliest days of intentional radar cross section control. The general issue and physics involved are well documented in related papers going back to the 1940's." Even if they were not attempting stealth in the 1940's the basic factors that would lead to it, and limit it, were understood to some extent by then.


The currently acknowledged stealth technologies (in regards to radar aplicaitons) are surface reflectivity and shape. The basic physics of these issues are well understood back to at least the 1940's, if not the 1930's. The term "radar cross section", how reflective a given target is under specific angular considerations, has been in use since the mid 1940's.


With regards to reflectivity, some surfaces reflect RF energy (and thus radar) better than others. How well a surface reflects RF energy tends to be tied to the frequency of the RF energy. This was known long before stealth was a thing. Stealth applied this in an intentional and controlled way.


With regards to shape, a surface that is not parallel to the wavefront will tend to reflect energy away from the wave source, not returning it to the source location. This was known long before stealth was a thing. Stealth applied this in an intentional and controlled way.


T!

 

[Token]

Second, "we" (assuming you mean Allies) used the same techniques, although typically not on aircraft. And so did the Japanese, and I think the Russians. There is no doubt, however, that the Germans used it in greater numbers and to greater affect.

It bothered me when I wrote this that while I knew the Allies had used such techniques I could not think of the specific system or application. Turns out that several radars used this technique in some form or another.


A prime example of this would be the SCR-545A radar. In fact it used an almost identical arrangement to the Lichtenstein radars, 16 half wave dipoles in 4 groups to allow lobe switching by phase shift in the feed lines. This radar, however, used a large reflective surface behind the fed elements, instead of parasitic elements. The SCR-545 was a ground based radar, so fewer weight constraints allowed for the large reflector.


An interesting fact I stumbled on was that the Army used modified SCR-545s to simulate Wurzburg and Giant Wurzburg radars for ESM training / testing purposes. I have not yet found data on specifically how the -545 was modified to fill this application.


T!

 

[Koopernic]

It bothered me when I wrote this that while I knew the Allies had used such techniques I could not think of the specific system or application. Turns out that several radars used this technique in some form or another.


A prime example of this would be the SCR-545A radar. In fact it used an almost identical arrangement to the Lichtenstein radars, 16 half wave dipoles in 4 groups to allow lobe switching by phase shift in the feed lines. This radar, however, used a large reflective surface behind the fed elements, instead of parasitic elements. The SCR-545 was a ground based radar, so fewer weight constraints allowed for the large reflector.


An interesting fact I stumbled on was that the Army used modified SCR-545s to simulate Wurzburg and Giant Wurzburg radars for ESM training / testing purposes. I have not yet found data on specifically how the -545 was modified to fill this application.


T!

Both sides used lobe switching and conical scan. The Germans were first to deploy conical scan in Würzburg C around Feb 1941 when it was became opperational with FLAK troops. They used lobe switching on an expermintal Seetakt radar to get 0.1 degree accuracy in 1934 but held of deployment till about 1941 because it impacted range and they eventually settled on a system that lobbed the receiving side only to avoid disclosing the switching pattern and because it didn't broaden the beam which caused peripheral targets to be taken in. There was also a long range development of the Freya radar called Wassermann that consisted of stacked Freya radars that used genuine phased array techniques for height finding. An even longer ranged radar called Mammut (mammoth, code name billboard by the British) that used phased array techniques to scan up to 58 degrees to either side and behind. They were aware of the technique. They didn't use switched delays but infinitely variable taped delays to adjust the phase. Mid war Freya radars had lobe switching not only to more precisely locate the enemy but to precisely guide the night fighter within a degree or so of the target.

Hohtenweil radars used on Fw 200 sent out a beam along the underside of the wing to scan left and right as the aircraft flew along but they also had a forward facing array which because the radar was derived from. FLAK radar had lobe switching. Fw 200 were thus able to carry out blind night attacks. There weren't many Fw 200 around but the Luftwaffe wanted to fit them to Me 410 torpedo bombers.

The US Army had lobe switching on its SCR~268 and the USN on it's early fire control radars.

 

[Zipper730]

It bothered me when I wrote this that while I knew the Allies had used such techniques I could not think of the specific system or application. Turns out that several radars used this technique in some form or another.

A prime example of this would be the SCR-545A radar.

That's fascinating

In fact it used an almost identical arrangement to the Lichtenstein radars, 16 half wave dipoles in 4 groups to allow lobe switching by phase shift in the feed lines. This radar, however, used a large reflective surface behind the fed elements, instead of parasitic elements. The SCR-545 was a ground based radar, so fewer weight constraints allowed for the large reflector.

Or drag...

An interesting fact I stumbled on was that the Army used modified SCR-545s to simulate Wurzburg and Giant Wurzburg radars for ESM training / testing purposes.

Makes enough sense...

They used lobe switching on an expermintal Seetakt radar to get 0.1 degree accuracy in 1934

That would be around 6.57 yards at 400 yards?

 

[Token]

Both sides used lobe switching and conical scan. The Germans were first to deploy conical scan in Würzburg C around Feb 1941 when it was became opperational with FLAK troops. They used lobe switching on an expermintal Seetakt radar to get 0.1 degree accuracy in 1934 but held of deployment till about 1941 because it impacted range and they eventually settled on a system that lobbed the receiving side only to avoid disclosing the switching pattern and because it didn't broaden the beam which caused peripheral targets to be taken in. There was also a long range development of the Freya radar called Wassermann that consisted of stacked Freya radars that used genuine phased array techniques for height finding. An even longer ranged radar called Mammut (mammoth, code name billboard by the British) that used phased array techniques to scan up to 58 degrees to either side and behind. They were aware of the technique. They didn't use switched delays but infinitely variable taped delays to adjust the phase. Mid war Freya radars had lobe switching not only to more precisely locate the enemy but to precisely guide the night fighter within a degree or so of the target.

Yes, pretty much everyone was aware of the phase control techniques before active hostilities started in WW II.


But not all lobe switching was or is achieved by phase control, some of it is simple antenna switching.


The Germans fielded some very advanced, for the time, techniques early in the conflict. The British and the US were using the same techniques in the lab and for development, but typically not for fielded systems.


The British settled for less advanced techniques with things like Chain Home. That system really was not cutting edge radar, in fact it was rather basic, but the integrated way it was used multiplied its effectiveness substantially.


The US knew that they would have to go to the war, the war was not coming to them, so the US development centered more on mobile systems. The really excellent SCR-584 remained in service in various ways for over 40 years. Even the rather mundane SCR-270 (the same type of radar that detected the incoming Japanese raid on Pearl Harbor) stayed in service for over 30 years (although not in the US), because it was "good enough" and mobile / transportable.

The US Army had lobe switching on its SCR~268 and the USN on it's early fire control radars.

The SCR-268 also lobed on receive (LORO, Lobe On Receive Only).


T!