Relationship between antenna length and wavelength in radar

Question

This is a question about radar design and how it works, because I'm trying to figure out some elements that are unclear to me.

My understanding so far is that to detect an objet with a given size, the signal emitted by the radar needs to have wavelength equal to half of its size. Then, my question is:

• Should the antenna that will emit this signal needs to be of the size of the wavelength?

My understanding tells me that frequency and wavelength, and thus I could use an electronical component to generate the signal with a certain frequency, as an electrical current. Passing this signal in a metallic object will create waves in the air of similar frequency, with an amplitude equal more or less to the amplitude of the signal * the antenna gain.

Given that, if my antenna is 1 meter long, and that I use a signal with 1 GHz frequency, thus giving a 0.3 meter wavelength, how will wavesbe created by the antenna: will there be three (1 meter / 0.3 meter) signal of 0.3 meter wavelength signals parallel between them in the air? Will there be only one 0.3 meter wavelength signal, and 0.7 meter of the antenna be unused? Will there be a 1 meter wavelength signal because the length of the antenna is critical in the equation?

I might miss some knwoledge so please tell me if this is the case.

Also, a related question is: If my 0.3 wavelength signal reaches a 1 meter object, it will be reflected, but what happens if it reaches a 0.1 m object? Will a 0.1 m wavelength signal be reflected? Or will the signal "passes through"?

Answer 1

More detail can be had by posting this to the amateur radio SE. Here are the basics:

In general, to detect an object by a radar reflection requires a wavelength less than the physical size of the object. Half the size is a practical maximum. 1/10th the size furnishes better results. So a 10cm radar is satisfactory to detect a 1 meter object.

The antenna length for a center-fed dipole is 1/2 the wavelength, so a 10cm radar signal would be generated by an antenna with a 5cm driven element. However, to form the signal into a unidirectional beam requires an antenna structure about 10 times the wavelength in size. So a 10cm radar will use a parabolic dish at least 1 meter in diameter to form a beam like that of a searchlight.

You do not feed a 10cm radar signal into an antenna with a driven element one meter in length because the transmitter would be badly mismatched to the impedance of the 1 meter antenna. You would feed the signal into an array of five 5cm halfwave dipoles spaced about 5cm apart instead. You would still need a reflector behind them to make the beam only now it would be 5 to 10 meters in size.