# Standing Wave Ratio

Discussion in 'Homework Help' started by mo2015mo, Dec 1, 2013.

1. ### mo2015mo Thread Starter Member

May 9, 2013
157
1
Hi guys..

What does it mean when we say the value of Standing Wave Ratio is 5 practically ??

where
SWR = 1+|T|/1-|T|

and what is transmitted coefficient T = η2-η1/η2+η1 practically??

2. ### WBahn Moderator

Mar 31, 2012
20,247
5,758
Dividing anything by 1 just gives you what you started with, so your first equation becomes:

SWR = 1 + |T|/1 - |T| = 1 + |T| - |T| = 1

If this isn't what you meant, then perhaps you should start exercising more care in making sure that what you write reflects what you mean.

Similarly for

T = η2-η1/η2+η1 = η2 - (η1/η2) + η1

This has been a common theme with a lot of your posts. You really need to work on adopting the kind of precise expression that engineering relies on. You'll never be perfect at it -- I don't think anyone ever is and I KNOW I'm not -- but you will save yourself a lot of grief if you try hard at it.

So, try again and we'll go from there.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
In the very old days of radio using long two-wire balanced open transmission lines of several wavelengths, operators developed a simple method of finding the VSWR on the line. They could connect a voltmeter directly across the two wire lines and measure the voltage at any point along the line they wished. Two readings would be of interest - the maximum voltage or voltages at certain points along the line and the minimum voltage or voltages at other different points along the line. The ratio of the maximum to minimum observed values was called the VSWR [Voltage Standing Wave Ratio]. It is measure of the degree of mismatch between the line characteristic impedance and the load impedance - usually that of an antenna.

There's quite a bit more to the story but you can readily discover that for yourself with some research & reading.

A line VSWR of 5 doesn't necessarily indicate an undesirable condition as some might think.

4. ### Papabravo Expert

Feb 24, 2006
11,160
2,184
Consider an RF source, a transmission line, and an antenna as a system. Now insert a measuring instrument between the source and the transmission line. A low SWR tells you that most of the forward power (from the source to the antenna) is being radiated and very little is being reflected. Conversely a high SWR tells you that most of your forward power is being reflected by the antenna back to the source. This condition can and will damage some transmitters. Most operators try to avoid this condition.

In the OP's post the vertical bars refer to "magnitude" and NOT absolute value. The value inside the vertical bars is a complex number, and the magnitude of a complex number is a real number. The quantity:
Code ( (Unknown Language)):
1.
2. (1 + |T|) / (1 - |T|) will always be greater than 1, and
3. |T| < = 1
4.
The case where T = 0 + j0 is the case of a perfect match with no reflection. It is virtually unachievable in practice.

Last edited: Dec 2, 2013