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Author Topic: SWR  (Read 15892 times)

Posts: 29

« on: January 10, 2013, 07:07:36 AM »

I'm trying to get an understanding of SWR.   I have several books that circle the subject; none but the discussion in terms I, as a layman, can understand.  Any recommendations?  thx  Jim
Member Limited

Posts: 115

« Reply #1 on: January 10, 2013, 07:48:01 AM »

Here is an online SWR visualization app that's knida cool:

Top trace show incident and reflected waves (red and green).  Lower trace shows the resultant sum of the waves (red) and the envelope of their sum (blue - see that if you enter 2:1 in SWR box, the high points of the blue envelope are twice as high as the low points!).

If you tweak around the paramaters and stare at it for a while, it might help make the connection to what your books are saying Smiley

VTenn Antennas
Bay Area Technical Equip Rental and Test Range

Posts: 9930

« Reply #2 on: January 10, 2013, 09:44:33 AM »

If you send  100 watts up the coax to the antenna and only 95 watts gets radiated due to miss match, the 5 watts coming back down the pipe is SWR. This causes a bit of heating in the finals . a little  won't hurt, but a lot is bad..  that's it in a nut shell.

Posts: 2270

« Reply #3 on: January 10, 2013, 09:47:36 AM »

Not technical, just simple:  SWR is a voltage ratio measured by a SWR meter or bridge.

Assume you are using a rig with a 50 ohm output impedance and 50 ohm coax to feed your antenna.   So far this is considered to be a 50 ohm impedance "system" and is often a typical HAM installation.

SWR (high or low) at the rig is an ONE INDICATION of how close your antenna comes to presenting a 50 ohm impedance to the 50 ohm "system".   In a typical 50 ohm system having an antenna that presents a 50 ohm impedance will result in a 1:1 SWR or a matched system and will generally result in the lowest signal loss in the "system".  In electronics for maximum power transfer the system impedance must be matched.  A 2:1 SWR indicates that your antenna is either 25 ohm impedance or 100 ohms impedance.  A 4:1 SWR indicates that your antenna is either 12.5 or 200 ohms impedance.  With a coax feedline (generally) the greater the SWR the greater the signal loss in the system.  Whether or not the signal loss is acceptable or significant depends on other factors.

This is about as simple as I can explain it.

Dick  AD4U
« Last Edit: January 10, 2013, 09:55:54 AM by AD4U » Logged

Posts: 85

« Reply #4 on: January 10, 2013, 02:58:58 PM »

Imagine the radio signal traveling down the wire to the antenna in the same way as an ocean wave. After they travel down to the antenna, the waves bounce back and meet the waves comming to the antenna.

Now what happens when two waves meet? If two waves hit each other, they combine and raise up, and the values below drop farther down (constructive interferrance, they add to each others highs and lows. Another way for waves to meet is a high wave hitting the low section of another wave... so that wave drops down into the valley of the other wave and cancel each other out, in other words the waves combine and are now equal to normal sea level (destructive interferance, or they cancel each other out. where there should be a low spot between waves is now filled with a hi wave comming the other direction, so they cancel each other out and there is no more wave.

Too high of a SWR means that there that the refletive waves are timing up just right so they start to cancel out often, reducing the amount of good waves that can be used to convey the signal to the antenna. instead of a fluid wave motions passing each other in good timing, instead you get segments where the water height stays the same levels all the time, the waves seem to "stand".

Heres a good picture to reference, the green line is the normal water level. imagine the two blue waves you see travling from left to right. there is a blue wave, then the valley between the waves on the bottom exposing the sea floor, then the another blue wave. The red wave is comming back the other way, and you see to valleys and just 1 wave. The aligntment is so that, its waves are meeting the other waves exactly where its valleys should be. The net result is regular sea level... (just remember that on an ocilliscope where you could see the electric waves youd see that entire shape, not a flat sea level line, that never moved or oscillated, they would just seem to "stand there".)

heres a website that has a gread moving visual thats how it works:
« Last Edit: January 10, 2013, 03:09:09 PM by GRANDKODIAK » Logged


no base station yet

Yaesu FT-7900r VHF/UHF
Cobra 29LX CB

Posts: 1757

« Reply #5 on: January 10, 2013, 10:24:59 PM »

I'm trying to get an understanding of SWR.   I have several books that circle the subject; none but the discussion in terms I, as a layman, can understand.  Any recommendations?  thx  Jim

Read this:

73 de N4CR, Phil

Never believe an atom. They make up everything.

Posts: 3916


« Reply #6 on: January 11, 2013, 06:24:02 AM »

I'm trying to get an understanding of SWR.   Any recommendations?

On a transmission line, there exists the possibility of two RF waves traveling in opposite directions. The forward wave travels from the source (transmitter) toward the load (antenna). The reflected wave travels from the load toward the source. Each of these waves is associated with a voltage, current, and power. If the reflected wave's amplitude equals zero, the SWR is 1:1, i.e. zero reflected power and the transmission line is matched (flat).

When two (coherent) RF waves are traveling in opposite directions in a transmission line, the result is a standing wave with hills and valleys. Assume that the voltage hill is called Vmax and the voltage valley is called Vmin. The SWR is the ratio of Vmax to Vmin. (SWR is also the ratio of Imax/Imin.) The following equations hold for forward power, Pfor, and reflected power, Pref.

Pfor = Vfor*Ifor

Pref = Vref*Iref

Let's call the forward voltage amplitude Vfor and the reflected voltage amplitude Vref. The following equations indicate the relationship between those four voltages all of which can be measured.

Vmax = Vfor + Vref

Vmin = Vfor - Vref

So SWR = Vmax/Vmin = (Vfor+Vref)/(Vfor-Vref)

Note that if Vref=0 then SWR=1:1

I would suggest that you study the voltage reflection coefficient, rho, at the same time you study SWR. |rho| is the amplitude of the voltage reflection coefficient; rho also has a phase.

|rho| = Vref/Vfor and is the fraction of Vfor that is reflected from the load.

SWR = (1+|rho|)/(1-|rho|)

|rho| = (SWR-1)/(SWR+1)

Here's an example: The forward voltage is 100 volts and the reflected voltage is 70.71 volts. What is the SWR? What is the value of |rho|.

SWR = (100+70.71)/(100-70.71) = 5.83:1

|rho| = 70.71/100 = 0.7071

Quiz: If the characteristic impedance of the feedline equals 50 ohms, what are the amplitudes of Ifor and Iref? What are the amplitudes of Pfor and Pref? How much power is being delivered to the load? Show that the power reflection coefficient is the square of the voltage reflection coefficient.

Pref/Pfor = |rho|2

Here's some standing wave animations:


Posts: 5

« Reply #7 on: January 12, 2013, 10:07:05 AM »

Here is video about wave behaviour that includes standing waves.


Posts: 593

« Reply #8 on: January 12, 2013, 12:38:21 PM »

All the replies are good source material.
First thing is to get by the term SWR by realizing they are letters for   Standing......Wave......Ratio.
Take the words separately and what they mean, then put them togather.
To be more correct it's VSWR. The V is voltage which is actually what most meters are reading at that physical point in the transmission line.
Good luck..

Posts: 815

« Reply #9 on: January 12, 2013, 03:25:02 PM »

IMHO, this is the best video on standing waves:

a must view for anyone who wants to understand (or better understand) this subject.


Posts: 1595

« Reply #10 on: January 14, 2013, 03:30:05 AM »

Interestingly, of all the SWR meters in my shack - home made and commercial - not one measures the SWR directly! They all measure the voltage and current at a (near) single point along the transmission line and then infer the SWR by making an assumption about the characteristic impedance of the line.

I'll bet not many readers have a meter that actually measures Vmax and Vmin, then computes Vmax/Vmin Smiley

Steve G3TXQ

Posts: 6221

« Reply #11 on: January 14, 2013, 08:05:24 AM »

In any event, don't put too much effort in getting the absolute lowest SWR that you can.  As long as it is fairly low (1.5 to 1 isn't bad at all) your rig will be fine.  Today's rigs will even tolerate a higher mismatch.  The best way, however, of checking the resonance of your antenna is an antenna analyzer, not a SWR bridge.  That is what you should use.

Posts: 187

« Reply #12 on: January 14, 2013, 08:21:40 AM »

I'll bet not many readers have a meter that actually measures Vmax and Vmin, then computes Vmax/Vmin Smiley

Steve G3TXQ

For better or worse, I'm old enough to have used a slotted line probe and a RF voltmeter.  You move the probe back and forth until you find the Min and Max and do simple division.  As I recall..  Cool

Posts: 1595

« Reply #13 on: January 14, 2013, 08:25:21 AM »

OK - that's one! Any more Wink

Steve G3TXQ

Posts: 187

« Reply #14 on: January 14, 2013, 09:04:04 AM »

And, thankfully, my testing was vhf.  Can you imagine a slotted line to work 160 meters?
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