Basis for First/early RADAR systems.

Thread Starter

Beetle_X

Joined Nov 2, 2012
67
I would like to study this as a hobby but, I can't find a simple guide on this subject.
First question is , how does a simple/early radar antenna work? I can't understand how a single source of RF waves would cover an area. Wouldn't a rotational antenna have to spin a million times to scan an area and aim up and down?
That would be a good starting point. I apologize for my ignorance in advance.
 

Papabravo

Joined Feb 24, 2006
20,400
I would like to study this as a hobby but, I can't find a simple guide on this subject.
First question is , how does a simple/early radar antenna work? I can't understand how a single source of RF waves would cover an area. Wouldn't a rotational antenna have to spin a million times to scan an area and aim up and down?
That would be a good starting point. I apologize for my ignorance in advance.
A ditectional antenna is used with a point source that spreads out in ALL directions. Think of a 3 dimensional cone. The antenna can't rotate too fast, or it might miss the echos.
 

Thread Starter

Beetle_X

Joined Nov 2, 2012
67
Thank you. That makes sense.
It's like sonar but not effected by the environment as much.
Create a source for a radio wave,point it in the middle of the sky,and see if you get your own RF wave bounced back to you. I realize that the really primate systems had to be manually pointed.

Let's jump to a system with a CRT and the operator is using a maker to mark where the last blip was.
What's the next system?
 

nsaspook

Joined Aug 27, 2009
11,795
The next bit of technology is the PPI (Plan Position Indicator). See the following article:
https://en.wikipedia.org/wiki/Plan_position_indicator
Real cool. I worked on some old tube PPI radar repeaters (AN/SPA-8). The CRT deflection yoke rotated in sync with the radar motor (via Synchro/servo) and was also connected to the ship gyro to compensate for ship movements.
http://jproc.ca/sari/opsroom.html
1680156049702.png1680156668971.png

Items in this photo.

Item 1 - AN/SPA-8 PPI indicator.
Item 2 - AN/UPA-24 control panel for IFF group video decoder.
Item 3 - AN/SPS-12 range scope. It was used to set the final tuning.
Item 4 - AN/SPS-12 control panel
Item 5 - AN/SPA-4 PPI indicator with video input selector switch mounted at left. The master SPA-4 PPI could be switched among several radar systems.
Item 6 - Sperry Mk II navigation radar.
Item 7 - Remote contol C10260/SPS-10 for SPS-10 radar. The SPS-10 radar itself was located on the forward bulkhead of Radar 1, one frame forward and to the right.It had the receiver controls for:

We had a SPS-10 and the SPS-40.
 
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ericgibbs

Joined Jan 29, 2010
18,055
Hi Beetle,
During my 5 years in the R.A.F. 1950-55 I worked a number of ground radar systems.

The oldest was a Chain Home CH, a flood lit system, 4 steel towers 365ft high with a curtain array of antenna strung between the towers.
Approx continuous 200Kilo watts of transmitted power at approx 27MHz, detection range about 150 to 200 miles.

Receiver array, 2 wooden towers about 200ft high with switchable reflector dipoles.

It was called 'Chain Home' as the full radar system had stations like these along the East and South coast of the UK.
Later systems CHL Chain Home Low and Extra Low [which means a lower angle of detection and most times a rotational beam TX/RX antenna, also a vertical tilt systems for height detection.

E
https://duotechservices.com/chain-home-radar-saved-london-in-battle-britain

https://www.radartutorial.eu/19.kartei/11.ancient/karte012.en.html

https://www.radarpages.co.uk/mob/ch/chainhome.htm


Chain-Home-Radar-Saved-London-Battle-Britain.jpg
 
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Ya’akov

Joined Jan 27, 2019
8,176
Some general background on dish antenns:

All directional antennas, dishes included, have a specification called beam width. Expressed in degrees, this is the angle of the (nominally) cone-shaped radiation from the antenna, or the complementary reception.

1680168646266.gif
seen from the side, in cross section, the beam width is the portion marked \( θ_\mathsf{{3\ dB}} \),
which is a general cutoff for gain figures and things like frequency response curves

Beam width is inversely proportional to the gain figure of an antenna. Antennas, being passive devices, can’t have “gain” in the sense a powered device—such as a transistor—can. Instead, they have gain expressed as the increase signal level in dB (deciBels) over an isotropic radiator which is a theoretical ¼ wave antenna in free space¹ in which case it will be denoted dBi, or over an ideal lossless ½ wave antenna in which case it will be dBd.

1680169944422.gif
isotropic radiation (see: footnote 1)​

Hence, all the “gain” is from the concentration of energy due to directionality. Even the highest gain antennas can’t act like the electron gun that creates the raster in a CRT, which seems to be how you are visualizing the situation. The radar antenna‘s beam width will be selected to that that the distance of interest a single horizontal sweep will cover the sky that needs watching.

The vertical angle of the antenna—be it a dish or some sort of linear array—is generally fixed. But is doesn’t have to be as you will find with airborne radar, for example, where the pilot can choose it based on what she needs to see; or, in the case of weather radar where the angle determines the elevation of the plot and as a result the structures being inspected.

1680171052672.png
a doppler weather radar installation, the antenna is invisible—the sphere is not
the antenna but a protective
radome, a radio-transparent protective structure
for the conventional antenna inside. radomes come in a variety of forms including
fiberglass tubes like the ones protecting vertical antennas used on the VHF/UHF
land mobile bands.

One more point: you might have noticed that radar dishes are not generally circular the way satellite ground station antennas are². This is because the elongated antenna shape is used to influence the radiation pattern, which is the shape of the beam. The rectangular aspect of the radar dish provides more horizontal coverage than the round dish would do which is an advantage in the application.

In practice, no antenna exhibits perfect directionality, and the actual radiation pattern is influence by every metallic object in the near field of the antenna. Another parameter of directional antennas is the. front to back ratio which describes the relationship between the amplitude of the antenna’s transmission or reception in the the “forward” direction, that is the one the antenna is intending to point, and the ”backward“ direction where, ideally, it would be completely silent or deaf.

F04E9DDC-A98C-492F-BAAC-83C6A687AAEC.jpeg
the radiation pattern of a theoretical dish antenna with
corresponding gain regions—note the “lobes” that don’t point
forward even on this “ideal” antenna with their corresponding
reduced and even negative gain figures
(negative gain is
normally called “loss”
)​

Real antennas talk and hear to the rear to some extent, as well as producing side lobes that project perpendicular to the desired direction. All of this energy can be considered loss since it is not useful in the context of the desired signal.


1. An isotropic radiator is an imaginary antenna that radiates equally in all directions, like a sphere. It has 0 dB gain as a result. In practice, there can never be such a thing, and much of an antennas ”gain” is an artifact of the practical implementation and may be undesirable. A theoretically perfect ½λ antenna exhibits a gain of 2.1 dBi, which is the same as 0 dBd—since that is it’s gain figure compared to itself.

2. You might object that many end user satellite dishes look like the radar dishes in that they have a rectangular aspect as well. The ubiquitous Dish Network antennas are an example. The difference is the reason for the shape.

Where the radar dish is trying to influence the shape of the pattern so it is wider, the Dish Network antenna is trying to accommodate multiple receivers that operate on different frequencies. What you are seeing is not a flattened dish but a clever combination of two or three round dishes into a unitary reflector. If you look carefully you will see the individual round depressions in the reflector focused on the corresponding LNA on the arm in front of it.
 

ApacheKid

Joined Jan 12, 2015
1,269
Hi Beetle,
During my 5 years in the R.A.F. 1950-55 I worked a number of ground radar systems.

The oldest was a Chain Home CH, a flood lit system, 4 steel towers 365ft high with a curtain array of antenna strung between the towers.
Approx continuous 200Kilo watts of transmitted power at approx 27MHz, detection range about 150 to 200 miles.

Receiver array, 2 wooden towers about 200ft high with switchable reflector dipoles.

It was called 'Chain Home' as the full radar system had stations like these along the East and South coast of the UK.
Later systems CHL Chain Home Low and Extra Low [which means a lower angle of detection and most times a rotational beam TX/RX antenna, also a vertical tilt systems for height detection.

E
https://duotechservices.com/chain-home-radar-saved-london-in-battle-britain

https://www.radartutorial.eu/19.kartei/11.ancient/karte012.en.html

https://www.radarpages.co.uk/mob/ch/chainhome.htm


View attachment 291056
Let's not forget the magentron either, that British invention that was just given away!

The Tizard Mission.

The technology Britain possessed included the greatly-improved cavity magnetron, which the American historian James Phinney Baxter III later called "the most valuable cargo ever brought to our shores",[5] the design for the proximity VT fuse, details of Frank Whittle's jet engine and the Frisch–Peierls memorandum describing the feasibility of an atomic bomb.
 

ericgibbs

Joined Jan 29, 2010
18,055
Hi,
I also worked on centimetric radars, 208MHz, using the magnetron devices.

PM Churchill did the 'right' thing in providing our US allies with the technology.

E
 

Thread Starter

Beetle_X

Joined Nov 2, 2012
67
If I start with the A scope, it's manually aimed and is like a oscillo scope. Distance is determined of how far the pulse and the echo are from each other.
Magically a ,(Edit; a RHI) can determine altitude. How is this accomplished just by the type of scope?
 
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