how radius effect on bandwidh and input impedance of a monopole antenna?

[ghasem_008]

Hi
How monopole diameter will affect the antenna bandwidth (Bigger the diameter, larger the bandwidth)?
and
monopole diameter does effect on peak value of input impedance?if answer is yes,what is it's effect?

 

[WimRFP]

The relation bandwidth and diameter (D) has a log relation (proportional to log(D/0.25lambda).

For quarter wave monopoles, the impedance slightly reduces with diameter. In fact this is not because of the diameter (D) itself, but because of to get a thick monopole in resonance, it needs to be somewhat shorter. The length reduction is the actual cause for the slight reduction in impedance at resonance.

For half wave monopoles, there is strong relation between input impedance (at resonance) and diameter of antenna. Thick antennas give lower input impedance.

I have document on the characteristics of half wave monopoles. http://www.tetech.nl/divers/HWmonopoleNL1.pdf

Though it is in Dutch, the images have English text and you will recongnize the formulas related to Q-factor.
You may know that the "VSWR=2 impedance bandwidth" = 0.71*Fc/Q So an antenna with Q = 6 centered at 120 MHz has a bandwidth around 14 MHz.

As monopoles can be simulated with relative small amount of segments, you may experiment with a free/trail EM simulation package.

 

[ghasem_008]

The relation bandwidth and diameter (D) has a log relation (proportional to log(D/0.25lambda).

For quarter wave monopoles, the impedance slightly reduces with diameter. In fact this is not because of the diameter (D) itself, but because of to get a thick monopole in resonance, it needs to be somewhat shorter. The length reduction is the actual cause for the slight reduction in impedance at resonance.

For half wave monopoles, there is strong relation between input impedance (at resonance) and diameter of antenna. Thick antennas give lower input impedance.

I have two question:
1 - how can I find this relation between bandwidth and diameter?
2 - why a thick antenna need to be shorter for resonance?I can not understand your reply.

 

[WimRFP]

An indication of the relative bandwidth for half wave monopoles can be found on page 8 under "relatieve bandbreedte" in the referenced document. Note that the actual bandwidth depends also on the size of the groundplane.

(Thin) wire antennas can be treated as transmission lines where there is loss due to radiation. At the end of the antenna the E and H field are distorted (fringing). This is the same as for a plate capacitor. Between the plates the E-field lines run straight from plate tot plate, but at and near the edges there is a fringe field. This introduces additional capacitance.

Using Thick conductors results in more fringe field hence more virtual capacitance at the end of the monopole. This capacitance shifts the resonant frequency downwards. To compensate for this effect, you need to reduce the length.

 

[ghasem_008]

thank you very much for your attention...
your reply is perfect!

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Also I have another question.
I simulated a monopole antenna by FDTD and change monopole radius.I saw that when radius increased,input impedance magnitude (namely |Rin| and |Xin|) decrease.why this situation occurs?