AC V,I, and P Distribution on a Transmission Line

[hashmos]

Hi Guys,

I looked so hard on the net, trying to understand something that I am struggling with for the whole day, but I ended up frustrated.

I am working on my design project and was assigned a part conecnred with antennas, I am using the Antenna Theory Analysis and Design by Balanis to have basic understanding of antennas,

What is confusing me is when he talks about the AC being applied across two conductors connected to an antenna. He choses different locations of the conductors and assign to them different charges (+ve) and (-ve) given at a specific time.

How can this happen, I always thought that AC means a constant current and volatge distribution, at a spcific time, along the transmission line. That is, for time t1, measuring the voltage at 2 meters from the source will yield the same result as when we measure it at 5 meters(at least this is what I thought)

AC, as I remeber, is a changing current and voltage with respect to time, NOT displacment..

Please enlighten me :(

Thanks in Advance

 

[waht]

AC, as I remember, is a changing current and voltage with respect to time, NOT displacment..

If you are used to 60 Hz from the electric outlet then displacement is not an issue because the next closest dip occurs miles away.As you go higher in frequency, wavelength gets shorter.

$$\lambda = \frac{c}{f}$$

At 100 MHz FM radio band, wavelength is 3 meters. At 2.4 GHz (wifi) , wavelength is 13 cm. At 10 GHz wavelength is 3 cm, and so on.In a transmission line or in air, the voltage will peak or dip every quarter wavelength displacement. There is a cool device called a slotted line

http://www.microwaves101.com/encyclopedia/slottedline.cfm

in which you slide a probe in a waveguide transmission line operating at 10 GHz and observe peaks and dips.

 

[hashmos]

Thanks waht for ur explnation,

so what you are saying is that electromagentic waves travel through the wire, which casues the AC current as the E field changes... ?

 

[waht]

At a more fundamental level that's what happens. But we generally convert the time-varying EM field into voltage and current.

 

[vk6kro]

Here is a bizarre example of what happens at high frequencies...

If you take two pieces of wire, 1 metre long and 1 cm apart, and feed an AC (RF) signal into one end of the parallel line, if the frequency is about 75 MHz, the AC generator will see almost a short circuit.
The lines are not touching each other, yet they seem to be a short circuit.

The effect is real enough to destroy transistors which try to feed AC signals into such loads.

This is called a quarter wave line and the effect is due to the AC signal traveling along the line, getting reflected and coming back to the start point just as the next wave is being fed in.

If the line is exactly the right length, it takes just the right time for the wave to travel along the wires and back again to be able to interfere with the incoming next cycle of the AC signal.

 

[hashmos]

thanks vk6kro, the example is bizarre but gave an idea and I think it is asscoaited with the concept of standing waves,

ur input was quite benifical,

thanks again

 

[vk6kro]

Yes, exactly right. The effect is caused by standing waves.

Wires arranged like that are called transmission lines. An alternative transmission line is coaxial cable which is more common now but works in a similar fashion to take power from a transmitter and deliver it to an antenna.

The example transmission line given would have an impedance of about 300 ohms, meaning that if you put a 300 ohm resistive load on the far end of it from the transmitter, no power would be reflected.