antenna resonance

[robismyname]

Being new to antennas I was told that resonance occcurs when the inductive reactance cancels with the capacitive reactance at some frequency. My question is how does an antenna resonate. I dont recall seeing any indoctors or capacitors inside my 2.4GHz stub antenna.

 

[zz0468]

Being new to antennas I was told that resonance occcurs when the inductive reactance cancels with the capacitive reactance at some frequency. My question is how does an antenna resonate. I dont recall seeing any indoctors or capacitors inside my 2.4GHz stub antenna.

The antenna itself exhibits both inductance and capacitance.

 

[robismyname]

ok so what you are saying is that the antenna is symbolic of a cap and induc.

It behaves like a cap and induc?

 

[Just_Me]

ok so what you are saying is that the antenna is symbolic of a cap and induc.

It behaves like a cap and induc?

In RF, everything behaves like a capacitor, a coil and a resistor to some extent.

P.S. AC is just slow RF, same rules apply.

 

[rescuecomm]

From a transmit point of view, the antenna is resonant when the SWR is 1 to 1. All the RF power is being radiated and none is reflected back into the transmitter. An antenna that is too short is capacitive, one that is too long will be inductive.

Bob

 

[Just_Me]

From a transmit point of view, the antenna is resonant when the SWR is 1 to 1. All the RF power is being radiated and none is reflected back into the transmitter. An antenna that is too short is capacitive, one that is too long will be inductive.

Bob

Only true if resonance and match are at the same point.

SWR has to do with the match between the transmission line and the antenna, that may happen at or off accutane resonance.

 

[wannabescannist]

Being new to antennas I was told that resonance occcurs when the inductive reactance cancels with the capacitive reactance at some frequency. My question is how does an antenna resonate. I dont recall seeing any indoctors or capacitors inside my 2.4GHz stub antenna.

Just because you don't see a capacitor or inductor doesn't mean that it does not exhibit capacitance and inductance. Same with transmission lines, which are sometimes modeled as two wires with capacitors in parallel and inductors in series on one of the wires (I'll leave out resistance & conductance for now).

 

[robismyname]

why was SWR brought up? I thought resonance occurs when inductive reactance = capacitive reactance. If this is the case how do you get the antenna to resonant if it plugged to a transmitter, assuming that the antenna is the right wavelength for the transmitter frequency.

 

[zz0468]

SWR has nothing to do with resonance. You are correct - resonance occurs when inductive and capacitive reactance are equal.

I'm not quite sure I understand the second half of your question enough to give you an answer.

 

[robismyname]

What Im asking is the following:

You cant use an antenna unless it is connected to an antenna. Once the antenna is connected to the transmitter what makes the antenna resonate? In other words how do you get the inductive reactance and capacitance to be equal in the antenna once it is connected to the transmitter? Doesn't the antenna resonance frequency have to be the same frequency as the transmitter?

 

[rfradioconsult]

A half wave dipole antenna at resonance (the recognized standard) has a nominal feedpoint impedence of around 72 ohms. A quarter wave vertical with ground plane has a nominal feed point impedence af around 30 ohms at resonance. Most casual users assume their antenna is performing at maximum when they see a 1:1 VSWR; however they usually are actually detuning their antenna to obtain the "perfect" match and will go to the extreme to get a match, thus actually moving the resonant point away from their desired frequency by altering the physical length of the antenna.

 

[zz0468]

You cant use an antenna unless it is connected to an antenna. Once the antenna is connected to the transmitter what makes the antenna resonate?

It is the physical properties of the antenna itself that determines what it's resonant frequency is.

In other words how do you get the inductive reactance and capacitance to be equal in the antenna once it is connected to the transmitter?

Just to clarify, it is the inductive REACTANCE and the capacitive REACTANCE that is equal at resonance. The reactance changes with frequency, and it is only at the resonant frequency that they are equal. The actual inductance and capacitance values of the antenna element don't change. It's reactance in ohms is a variable, determined by the frequency.

When the inductive reactance value, in ohms, is equal to the capacitive reactance value, in ohms, the antenna is said to be resonant. At that point, the complex impedance is purely resistive. If that resistive value happens to be equal to the characteristic impedance of the feedline, the swr is 1. If not, than the swr is something other than 1. The antenna is still resonant, however.

The resonant frequency does not change when it's hooked to a transmitter or receiver.

Doesn't the antenna resonance frequency have to be the same frequency as the transmitter?

No. It frequently is, but it doesn't HAVE to be. HF transmitters are VERY FREQUENTLY operated quite successfully on non-resonant antennas.