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tensor20 Antenna Theory Regular
Joined: 12 Jul 2011 Posts: 18

Posted: Tue Jul 12, 2011 1:52 am Post subject: Frequency Independent Antennas 


Hello Forum,
I recently found out about frequency independent antennas. They seem to be wideband antennas. The plane spiral antenna is an example.
The work based on Rumsey's principle which states that if the antenna structure in defined only in terms of angles then the antenna is wideband...
Does anyone know what that means? How is it different from a fractal?
How does the current distributes itself for different frequencies?
How do these types of antenna work over such wide bandwidth?
Does the antenna contain smaller and larger parts of itself that resonate at different frequencies?
thanks
tensor20 

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tensor20 Antenna Theory Regular
Joined: 12 Jul 2011 Posts: 18

Posted: Tue Jul 12, 2011 1:57 am Post subject: 


I just found an old reply by Schubert where he says that
"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."
High frequency means small wavelength...why would the inner core cancel out and the edges be responsible for the radiation?
What if the frequency was low? I guess the opposite would occur: the inner part would radiate and not the outer one....
tensor20 

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bigSteve Antenna Wizard
Joined: 14 Mar 2009 Posts: 265

Posted: Wed Jul 13, 2011 2:06 am Post subject: 


Angles are the same  no matter what the frequency. Hence, if all dimensions were relative to angles instead of sizes (in wavelengths), the antenna would be frequency independent.
Obviously, no real antenna is size independent. However, long horn antennas are defined by their angles. They act somewhat as waveguidefree space wave converters. As long as their size is large than a halfwavelength, the fields travel down the horn and radiate. In this sense, above a certain threshold the response of the horn is defined by the angles that the horn flares at. In this sense, it is "frequencyindepdendent", but only over a certain range.
Spiral antennas act the same way. Whenever the circumference is longer than about one wavelength, they tend to have similar properties. The "inner core" is filled with the current, and for higher frequencies consecutive wraps of the spiral cancel out. The last wrap is not paired and therefore is the only part that contributes to radiation. 

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tensor20 Antenna Theory Regular
Joined: 12 Jul 2011 Posts: 18

Posted: Wed Jul 13, 2011 2:28 am Post subject: 


Hi BigSteve,
thanks for your reply. But let me understand things even better:
as the frequencies goes up consecutive wraps of the spiral cancel out....
why? So if we start from the center and go around 360 degrees twice the currents cancel out?
When you say that the dimensions of the antenna are given in terms of angles and the angles are the same for all frequencies......
If we take a regular antenna (not a frequency dependent one) we could describe its geometry using polar coordinates r and theta the same way we do for a freq. indep. antenna....So what is the difference exactly?
thanks and sorry for the extra clarification
tensor20 

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