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[fisico32]

Hello Forum,

what is the conceptual explanation of why a spiral antenna is a broadband antenna?

A spiral antenna is self-similar: an enlarged version of the antenna is the antenna itself but simply rotated...

Does that mean that an incident field of a certain fixed frequency will "see" the structure as resonant? A high frequency will use the central area of the antenna. A lower frequency will use a larger portion of the antenna.....

But if that is so, even a straight wire antenna should be broadband: a high frequency would use just a segment of the antenna while a low freq a larger section of the same antenna....

thanks
fisico32

[Schubert]

For a wire antenna (dipole), they are not broadband because the impedance changes very rapidly. As a result, the power is reflected from the antenna and cannot be converted into radiation.

Spiral antennas have great impedance versus frequency characteristics, which makes them broadband. The current flows on t he whole length of the wire. For the higher frequencies, the inner core cancels out and the edges are responsible for the radiation.

[fisico32]

Thanks for the reply!

antenna impedance, for what I know, is the ratio of the voltage to current at a specific point in the antenna.....So the impedance can vary along the antenna structure....

I read that for a half-wave dipole antenna the impedance is nominally 75 ohms....Does that means that the voltage/current is the same everywhere on the antenna?

Beside the antenna impedance there is also field impedance: E/H.
How does this relate to the antenna impedance?


Back to the spiral antenna: so you say that for high frequencies the inner part of the antenna has significant current while for low frequencies the outer part of the antenna has significant currents...Is that correct?


"....The current flows on t he whole length of the wire..." isn't that true for any antenna?

[Schubert]

Antenna impedance is the impedance at the feed of the antenna, i.e. wherever the coaxial cable or transmission line connects to the antenna itself. It is the ratio of the voltage to the current at the feed point, and is a complex number. The impedance varies along the antenna structure; moving the feed will change the antenna's impedance.

For an infinitely thin half-wave dipole, the impedance is 73 + i*42 ohms. If the dipole is trimmed slightly, to about 0.485 lambda, the impedance will be entirely real and about 70 ohms.

For plane waves (or the radiated fields in the far field of an antenna), the ratio E/H is the characteristic impedance of the medium. For instance, in air, E/H = 377 Ohms. in general, this characteristic impedance is a function of the permittivity and permeability (material properties of the medium).

A lot of people ask, why isn't the antenna impedance matched to the characteristic impedance? I.e. is an antenna better in air if the impedance is 377 Ohms? Basically, no. The antenna impedance doesn't care about the characteristic impedance of the medium.

Back to the spiral antenna: The magnitude of the current is relatively constant along the spiral. The phase varies with distance, as the spiral will typically be at least a wavelength long. The current flowing on the inner part of the spiral will tend to cancel at higher frequencies (due to the current flowing hte opposite way on the other arm).