# Transmission Lines calculations

Discussion in 'Homework Help' started by Kayne, May 7, 2010.

1. ### Kayne Thread Starter Active Member

Mar 19, 2009
105
0
Hi,

Have worked though another transmission line question is someone able to tell me where I have made the mistakes.

Thanks

A section of air spaced-two wire transmission line is to be made form a roll of 3mm diameter copper wise. The velocity factor is unity. The line is to be 1.5 wavelengths and operates at 15MHz.

1.What length of wire is required to make the line.

λ = 3x10^8/15x10^6 = 20metres

2.What is the centre to centre distance must the wires be placed to make a line with a characteristic impedance of 400Ω

Zo=276Log2D/d
400/276 =Log 2D/3
10^1.449=2D/3
28.119/3=2D
9.373 *2= D
D= 18.746mm

When the transmission line is used to feed a load at 7.5MHz it is found that a standing wave exists. The load impedance is found to be 100 Ω

3.Calculate the Reflection Coefficient and then VSWR

Г =(Zl-Zo)/(Zl+Zo)

=400-100/400+100
=3/5

VSWR = 1+ Г/1- Г
={1+(3/5)}/ {1- (3/5)}
=4

4.Distance between adjacent voltage minima in the standing wave pattern

λ = 3x108/7.5x106 = 40metres

I dont think that this is correct??

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Haven't checked all answers but part 2 is incorrect. Just a math error.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
VSWR=4 is correct

The SWR minima / maxima are spaced at 180 electrical degrees or at half line wavelength intervals.

4. ### Kayne Thread Starter Active Member

Mar 19, 2009
105
0
TNK I can see the error that I made, wrote it down wrote in my note book.

2.What is the centre to centre distance must the wires be placed to make a line with a characteristic impedance of 400Ω

Zo=276Log2D/d
400/276 =Log 2D/3
10^1.449=2D/3
28.119*3=2D
84.410 /2= D
D= 42.20mm

Is there a formula to work Number 4 or am I on the right track...

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
You are on the right track - the maxima & minima are separated by λ/2 not λ.