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Why a dipole element is cut to 1/2 wavelength of frequency?

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S4C

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Dipoles

Good Morning,

A basic question concerning Dipole elements. I apologize for my ignorance, and any help would be greatly appreciated.

As I understand it: A dipole element is cut to 1/2 wavelength of a desired frequency.

Question: Why 1/2?

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Background: Im studying Chaff and why it works as a Radar Jammer. I know that chaff is made of many strips of dipole elements cut to 1/2 of the wavelength of the target "victims" radar frequency. This allows the reflected RF to present itself on the "victims" radar scope.

Why 1/2? Why not the full wavelength? What would would happen if it was cut to the full wavelength instead of 1/2? I think the reason is that the dipole element would not reflect if cut to the full wavelength...but I dont know why.

Any help...?

I am a novice at best. Please can anyone explain this in Laymans terms?

-S4C
 

Re: Dipoles

The equivalent reflection cross section area per weight of material is maximum with the half wave material. One problem is the frequency selectivity. Making the material longer will allow it to work over a wider frequency range.

The half wave is resonant and so has the maximum currents induced into it. These currents cause radiation.

The Allied code name was window. The Axis code name was dupple. Rope was the Allied code name for long strips (not frequency selective) with cardboard squares on one end that would float to earth extended.

Such material was not confined to the sky. The Germans made sets of three dipoles on orthogonal axes to make the Allied ground mapping radar less effective. These dipole triplets were affixed to strings which were strung above the ground to degrade the radar imaging of the river through Berlin. This river was used as a landmark for bombing purposes.
 

Re: Dipoles

Here you can find the answer to your question, with pictures to understand what happened with the currents in a dipole, if the length is not ½ wavelength.
Is mentioned, if the length of a ½ wave dipole is only 10% off, the efficiency can drop down to 7%.
**broken link removed**
 

Re: Dipoles

Thanks for the replies guys. Did I mention I was a novice at best? LOL

These explanations are flying a little too far over my head. I went to the website mentioned...read the article...couldn't get much out of it, though im sure it is written in a very basic language.

After further research I believe this to be the case:

When dealing with Chaff, a dipole cut to the full wavelength of a victims RADAR freq will have the same effect as a dipole cut to 1/2 the wavelength.

1/2 wavelengths are used to save space and/or weight.


If this is wrong please correct...ummm...in laymans terms please. You may interpret "laymans terms" to mean idiot if you like! :)

Thanks,
-S4C
 

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