- #1
david845
- 9
- 0
I really hope i can get some answers to the questions that have been plaguing me for some time. I'm more conceptual, and math does not satisfy my curiosity so I was hoping the experts here could fill in some blanks:
In making a jump from learning DC/low frequency AC to RF, i studied transmission lines and the finite speed of light. From my understanding, even connecting a single wire to the hot terminal of a battery would produce a transient current until the battery "sees" the open circuit and returns to open circuit conditions where no current flows. As this happens at almost the speed of light we cannot notice the transient conditions. The incident voltage wave from connecting an alternating voltage source to the TL only travels slightly slower than speed of light (c) through the medium. To me, this means that if the physical length of the TL is much shorter than the wavelength of the signal of a corresponding frequency (like 60HZ AC power lines), then simple circuit theory applies because 1/2 wave cycle could never complete itself and reverse direction before the wavefront reaches the load. (Am i correct so far?)
Consequently, a TL that is physically long compared to the wavelength breaks down circuit theory. I do understand that in a TL terminated in an open circuit that the wave will reflect at the open circuit and will superpose with subsequent oncoming waves and produce a standing wave. Is the standing wave important in order for the antenna to radiate? It seems with the ends open circuited there has to be reflection, but every book i read says if the load is terminated with a matched impedence then there is no reflection and all the energy is lost to radiation...how is this possible? If a dipole antenna is merely a continuation of the TL (2 conductors separated and left open circuited) how can there not be any reflection and all the power is transduced into radiation? Why do we need to view the TL as a series of lumped elements (R,L,C - though i understand self Inductance and mutual capacitance how does it affect the voltage/current in the TL?). Is a resonant antenna 1 where the 1/2 wave cycle reverses as it JUST reaches the open circuit?
I appreciate ANY help in answering as many questions as you can. I've seen a lot of people with the exact same question - but I also have never seen an answer that satisfied any of them.
In making a jump from learning DC/low frequency AC to RF, i studied transmission lines and the finite speed of light. From my understanding, even connecting a single wire to the hot terminal of a battery would produce a transient current until the battery "sees" the open circuit and returns to open circuit conditions where no current flows. As this happens at almost the speed of light we cannot notice the transient conditions. The incident voltage wave from connecting an alternating voltage source to the TL only travels slightly slower than speed of light (c) through the medium. To me, this means that if the physical length of the TL is much shorter than the wavelength of the signal of a corresponding frequency (like 60HZ AC power lines), then simple circuit theory applies because 1/2 wave cycle could never complete itself and reverse direction before the wavefront reaches the load. (Am i correct so far?)
Consequently, a TL that is physically long compared to the wavelength breaks down circuit theory. I do understand that in a TL terminated in an open circuit that the wave will reflect at the open circuit and will superpose with subsequent oncoming waves and produce a standing wave. Is the standing wave important in order for the antenna to radiate? It seems with the ends open circuited there has to be reflection, but every book i read says if the load is terminated with a matched impedence then there is no reflection and all the energy is lost to radiation...how is this possible? If a dipole antenna is merely a continuation of the TL (2 conductors separated and left open circuited) how can there not be any reflection and all the power is transduced into radiation? Why do we need to view the TL as a series of lumped elements (R,L,C - though i understand self Inductance and mutual capacitance how does it affect the voltage/current in the TL?). Is a resonant antenna 1 where the 1/2 wave cycle reverses as it JUST reaches the open circuit?
I appreciate ANY help in answering as many questions as you can. I've seen a lot of people with the exact same question - but I also have never seen an answer that satisfied any of them.