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# whats is the impedance of the spiral antenna??

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#### akhilpaulv

##### Full Member level 2
when i create spiral antenna in hfss its port defined in 50 ohms. when i search in google they says the impedance of the spiral antenna is 188 ohm!!! now i am confused ??:shock: please anybody help me???

188 ohms approximate the equation Zin = Zo(free-space-impedance) / 2
In real life, because the antenna is not placed in a perfect free-space, the impedance of the spiral is about 100-150 ohms, also depending by factors as: number of turns and turns spacing.
More turns gives higher impedance, and higher spacing gives lower impedance.

188 ohms approximate the equation Zin = Zo(free-space-impedance) / 2
In real life, because the antenna is not placed in a perfect free-space, the impedance of the spiral is about 100-150 ohms, also depending by factors as: number of turns and turns spacing.
More turns gives higher impedance, and higher spacing gives lower impedance.

It s time to start some reading. Try JDKraus, Antennas, to find the details on helicals.

What BW or Q you expect is highly dependant on imperfection of spiral, which of course affects Z and directional gain in real world with Q >> 100 in VHF.

when i create spiral antenna in hfss its port defined in 50 ohms. when i search in google they says the impedance of the spiral antenna is 188 ohm!!! now i am confused ??

Why?

Your "measurement instrument" has 50 Ohm impedance and the antenna has a different impedance. That's ok and you will get the actual antenna impedance from simulations results. There is no need to set HFSS port = antenna impedance.

wave85

### wave85

Points: 2
Impedance of helix is determined by diameter to wavelength ratio and interwinding spacing. It can be radial like rubber ducky isotropic or axial like quad helix high gain. Impedance matching is not as critical and may be different from 50 Ohm as this affects radiated Voltage E field so matching low source impedance for max power transfer can be offset by higher free space impedance using a tapped helix transformer or an intermediate impedance between source and free space.

Not sure which is best for your app.

Equations of Helix antenna (discovered by Kraus) have almost nothing in common to Archimedean spiral antenna, which actually is about in this thread.

Normally impedance of archimedian spiral antenna is near to 110 ohms. if you simulate your design with 110 ohms will give good results which are similar to measured results.

110 ohms should be just fine for a spiral antenna.
I've seen so many people confusing helix (kraus) antenna with spiral (archimedean) antenna. You can compare a spiral antenna with whatever else antenna, with a bented dipole, or a bowtie, but not in a case with a helix.

is it possible to crate a spiral antenna with input impedance 50 ohm??

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