I need a help about RF!

[calven303]

If the electromagnetic wave is TEM mode , the electric field is orthogonal to the direction of propagation .And there will be no voltage drop along the direction of propagation according to the low frequency theory,but all books say there are voltage drops along the transmission line.How can this be explained using the theory of electromagnetic :?:Is this associated with that the amplitude of the electronic field is always changing:?:
Thanks:!:

 

[dch316]

If the electromagnetic wave is TEM mode , the electric field is orthogonal to the direction of propagation .And there will be no voltage drop along the direction of propagation according to the low frequency theory,but all books say there are voltage drops along the transmission line.How can this be explained using the theory of electromagnetic :?:Is this associated with that the amplitude of the electronic field is always changing:?:
Thanks:!:

hi,
there is no voltage drop as long as the transmission line (TL) is an ideal TL, which never occurs in real world. this means that amplitud would remain constant along the whole TL. What happens in reality is that TL are lossy so there is an attenuation parameter (everywhere called alpha) that makes electric field (and thus voltage) decrease along the TL in the direction of propagation. In Microwave Engineering by David m. Pozar there is a very clear explanation, I used to solve all my doubts from that great RF book.
regards

 

[calven303]

thanks very much!
i can understand that electronic field decrease in the direction of propagation if the transmission line is not ideal,but what i can not understand is the relationship between the electric field and voltage.in my opinion,the electric field is always orthogonal to the direction of the propagation even if the amplitude decrease,so the voltage will not change along the direction of the progation. this only happens in low frequency of course.how to understand it from the view of electromagnetic?
thanks again!

 

[melc]

thanks very much!
i can understand that electronic field decrease in the direction of propagation if the transmission line is not ideal,but what i can not understand is the relationship between the electric field and voltage.in my opinion,the electric field is always orthogonal to the direction of the propagation even if the amplitude decrease,so the voltage will not change along the direction of the progation. this only happens in low frequency of course.how to understand it from the view of electromagnetic?
thanks again!

Imagine that electric field is a red jelly and the magnetic field is a blue jelly.
Both are filling in a specific way called "propagation mode" the waveguide, having both minimum and maximum amplit(d)udes.
Then imagine that you take a saw and cut transwersally the waveguide after it stay in the fridge at -25C. According to your supposition you'll see the same red jelly section (with a little blue inside) on the cutted waveguide no matter if the waveguide was cutted at 1km away or at 20cm away from the microwave generator ? And no matter if you're cutting at multiple of lambda/2 or lambda/4 distances or not ?
There is a very simple relation between the electric field and voltage, they are proportionaly and the voltage varies with the wavelenght value, you'll find a lot of maximum values and a lot of minimum values on the waveguide. But at the opposite end from the generator, the voltage value will drop (both the maximum and the minimum values).

 

[calven303]

thanks dch316 and melc very much!
the electric field is perfectly periodic if the transmission line is ideal,and the eletric field is approximately periodic with the peak value decreasing if the transmission line is not ideal.i understand this.
"There is a very simple relation between the electric field and voltage, they are proportionaly and the voltage varies with the wavelenght value"
why the electric field and voltage are siimply proportional(the electric field is orthogonal to the directiong of propagation here)?in low frequency,the voltage will not change along the path which is always orthogonal to the electric field even if the electric field is large,and how to understand it from the viewpoint of electromagnetic.
thanks!

 

[calven303]

i am eager to konw why i am wrong! i do not want to step forward if i do not konw the reason.thanks!

 

[dch316]

i am eager to konw why i am wrong! i do not want to step forward if i do not konw the reason.thanks!

hi,
as melc said, the electric field value changes from a maximum to a minimum every lambda/2 at every point of the TL.
the voltage takes a form like this V(z,t)=ctte * e^(-jBz)*e^(j2*pi*f*t) in and ideal TL

So it is clear that the amplitud doesn't change along the direction of propagation z. However, the phase changes with time t at every point z and so does the magnitude then. It is clear then that at every point z of the TL the voltage (orthogonal to z direction) changes with time at the frequency f.
There are good pictures of this in the book by pozar.

if the TL is lossy then you should add a factor e^((-alpha)*z) so the voltage drops along the TL BUT it continues being orthogonal to direction z, what changes is the amplitude (before it was ctte and now it is ctte * e^(-alpha*z)).

If i were you I would take a look to Pozar book.
regards