Resonance Problems

Discussion in 'General Electronics Chat' started by nelsonys, Jun 7, 2013.

1. nelsonys Thread Starter New Member

Apr 6, 2011
21
0
I am now studying about resonance on transmission line.

Is there any recommendation on good study materials (books or any online materials), or even drop some explanation here about the mechanism of resonance and the relationship between reflection (standing wave) as well as lambda/2? Further topics about resonance such as radiation(antenna theory), wave mode, etc...

What I knew about resonance is just about the LC elements that cause resonance. Would like to further my understanding by relating VSWR, wavelength, multiples of half wavelength into resonance as well as other advanced topics.

Besides, I'm now trying to learn about antenna. For an open-ended patch antenna, waves propagate to the edge of the antenna reflect back and create standing wave. According to the allaboutcircuits material, open-end creates full reflection (Vmax) at the edge, but what I don't understand is why is it have to be of the length of lambda/2?

Besides fundamental resonant frequency, there are higher order resonant frequencies related to n multiples of lambda/2. Does this mean that in a certain transmission line, there will be infinite numbers of resonant frequency (ignore losses)?

We know that it's the resonance that creates strong radiation in the antenna, but radiation in the antenna is narrow bandwidth despite there are n multiples of lambda/2 exist in the transmission line. Why is it so?

I'm sorry for my vague description, but I really hope anyone of you could provide me insights on this matter.

2. circuitfella11 Member

May 10, 2013
56
5
lambda/2 is used because it is where the maximum voltage output is found, while the current is zero.. this part of study is mostly found on the behavior of voltage versus current in antennas.. most books and materials often discuss this, but sometimes vague.. it is easier to grasp the concept from school, instructors, experienced individuals, etc..

i don't know if you are referring to some logarithmic properties of antennas..

hope this helps

3. vk6zgo Active Member

Jul 21, 2012
677
85
"Besides fundamental resonant frequency, there are higher order resonant frequencies related to n multiples of lambda/2. Does this mean that in a certain transmission line, there will be infinite numbers of resonant frequency (ignore losses)?"

Yes!-BUT!

A transmission line is not an inherently narrow band device,& if the source & load impedances are the same as the Characteristic Impedance of the line,(an ideal) transmission line of any length will transmit all frequencies from DC to ∞ equally well.

In the real world,transmission lines have frequency dependent losses,but are still very wideband in their own right.

"We know that it's the resonance that creates strong radiation in the antenna, but radiation in the antenna is narrow bandwidth despite there are n multiples of lambda/2 exist in the transmission line. Why is it so?"

I'm not sure where you are going with this one,but if you are asking why these resonances don't cause harmonics of the original frequency,no,they can't!
Non linearity is required to do this,& the transmission line & antenna are linear devices.
They can however, cause the radiation of existing harmonics from the RF source.

If as is more likely,you are asking,why can't we use an antenna at frequencies where it is 0.5λ, λ,1.5λ,2λ,etc,yes you can,but not all of these lengths yield a load impedance matching,or even close to that of the transmission line,with the result that feeder length becomes critical.

My suggestion is to borrow a copy of the ARRL Antenna Book from your local library,& read their discussion on this matter.