# Communication question 1

Discussion in 'Homework Help' started by Kayne, May 10, 2010.

1. ### Kayne Thread Starter Active Member

Mar 19, 2009
105
0
Hi there,

have worked though another question. Hopefully got I have it correct this time, if i have done this incorrect can you please tell me where i have made these mistakes.

thanks for your help

A transmitter with an output power of 200W operates at a frequency of 20MHz. It feed an antenna with an impedance of 30 Ω. The cable has a characteristic impedance of 50 Ω and a velocity factor of .90. Cable loss will be ignored and power rating is adequate for the application.

Load is not matched to the transmission line so there will be some reflection of the incident wave energy when it arrives at the load. The incident wave and the reflected wave will both combine to produce an interference phenomenon known as a stationary wave or standing wave.

1. Calculation the Reflection Coefficient at the load

Г = (Rl-Zo)/( Rl+Zo)

Г = (30-50)/(30+50) = -20/80 =-1/4 or -0.25

The relection Coefficient at the load is ____dB

* is there a formula to change
Г to dB??

2. Would there be a Maximum or Minimum in the standing wave

Am I correct in saying that the max and min refers to the VSWR ratio I have worked out in Number 3

VSWR =Vmax/Vmin
=0.75/1.25 = 0.6

So the max = 0.75
and the min = 1.25

I dont think that this is correct becuase the min should be lower than the max? I may have the other question

3. Calculate the voltage standing wave ratio (VSWR) on the line

VSWR = Rl/Zo = 30/50 = 0.6

VSWR = (1+ Г)/(1- Г) = (1+-0.25) / (1--0.25) = 0.6

4. Calculate the actual amount of reflected power and the actual amount of power few to the load.

Г^2 = Pref/Pinc

0.6^2 = Pref / 200
Pref = 0.6^2*200
=72W

The reflected power is 72W

The actual amount of power fed to antenna is
200-72 = 128W.

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
1. Return Loss [dB]=$-20log_{10}(|\Gamma|)$

2. Wouldn't the standing wave have both maxima & minima and everything in between along the line? Depends at which point you observe the standing wave.

3. SWR would be ...
$s=\frac{1+|\Gamma|}{1-|\Gamma|}=\frac{1.25}{0.75}=1.67:1$

4. $\Gamma=0.25$ not 0.6!

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