equivalent circuit of monopole antenna

[ghasem_008]

Hi.
equivalent circuit of a cylindrical monopole antenna is Parallel RLC or series?
I think that it's a parallel RLC,but in several paper I saw that a series RLC was used.
is there anyone who explain it,perfectly?
thanks

 

[WimRFP]

What length (electrically spoken)?

 

[ghasem_008]

is there any difference between equivalent circuit of electrical or physical length?

 

[rahdirs]

^ For the same physical length,a dipole can be electrically small or electrically large depending on the frequency.Similarly,for a monopole

 

[ghasem_008]

yes,I know.
my monopole antenna is a quarter wavelength.
now,what is equivakent circuit?

 

[FvM]

There's no simple equivalent circuit representing the wideband behaviour (resonances at n*λ/4). Around the λ/4 resonance, a series RLC circuit with R representing the radiation resistance is the best fit.

P.S.: To elaborate a bit, you get parallel RLC behaviour around λ/2, series around 3λ/4 and so on. Resonance Q depends on antenna l/d ratio.

 

[ghasem_008]

why in 2n(λ/2) we have a parallel circuit,while in (2n+1)λ/2 we have a series?
Also,why in a λ/4 monopole antenna,when inner radius of cylinder increase,BW of antenna increase?
while when radius increase,I think that ohmic resistance decrease.
NOW by a series RLC,Quality factor is:

QAUALITY FACTOR = 2pi*f*L/R = 1/(2*pi*f*R*C)

so,when R decrease,quality factor increase and therefore Bandwidth decrease!

while,when we use from a parallel circuit,quality factor(=2*pi*f*R*C) decrease and bandwidth increase.

I'm confused!

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it is related on derivative sign of reactance?namely in 2n(λ/2) we have d|X|/dw<0 and in (2n+1)(λ/2)
we have d|X|/dx>0 ?
I want to know it's physcial meaning from EM wave point of view.