S.W.R

S.W.R. (VSWR / ISWR)

 

S.W.R. stands for (voltage V, or current I ) Stranding Wave Ratio.

The SWR of the antenna tells us how much emmited energy from the transceiver is forwarded to the antenna and how much power is reflected. 

Sometimes you see “FWD” and “REF” written on a SWR meter. This stands for the mentioned Forwarded and reflected.

The ratio between those two is our SWR.
1:1 tells us all energy which is sent is gone out of the antenna and nothing has returned.

Im writing energy, often the SWR meter uses the Voltages a long the line too derive the SWR.

There are a lot of good articles on the internet on how to use your SWR meter, or how SWR is derived from it…so i wont go into that too much, just some usefull tips for the beginner :

 

What SWR does NOT tell us is how EFFICIENT the antenna is,

 

Nor anything about features like gain and front to back etc are holding up.
SWR does not tell us how the antenna preforms it actually could be that your antenna with a SWR of 1:1,5 is more efficient. (for example in a 1/4 wave groundplane without matching) It can also be that peak gain / FB / FS are not where SWR is as its best.

 

Therefor when buying a yagi antenna always ask the manufacturer if he can provide antenna plots over a wide frequency response and not just a single “high peak” plot. 

 

 

Up front if you want to measure your true SWR. You will need to do that at the Antenna side

 

And not at the transceiver side. This is cause your feedline may act as an impedance transformer (see chapter coax) Or you may have other causes effecting the feedline ( common mode currents). Those will have influence on the SWR !  

 

The story you heard about cable length,:

That exact length figures are important it probably originates from here: When a coax cable has an electrical length of a half wave or a multiply of this it shows us exactly the same as it does on the other side of this coax cable. So it is possible to read the SWR values below at the TRX, the cable just has to be an electrical half wave long..(calculate half- wavelengths with the velocity factor of the coax cable.)

 

 

SWR BANDWIDTH 

 

 

 

Above we see an example of an SWR curve.

This could be an example of the SWR of your antenna.

You see the boundaries are at 26,6 and 28,5 mhz there the SWR rises above 2,0:1.

The frequency span between the 2:1 points is reffered to as the 2:1 SWR Bandwidth.

In this case it is 1900 Khz. ( 28,5-26,6 =1,9 Mhz)

 

ADVICE: ALWAYS WRITE DOWN YOUR 2:1 SWR BANDWIDTH AS SOON AS YOU INSTALL A NEW ANTENNA. 

 

The reason for this is: over  a periode of time things like corrosion will step in.

Or perhaps there are other reasons why your system will have additional loss.

Imagine this…imagine over the years there will be extra loss for some reason…

perhaps your coax has degraded.

Now you are verifying your 2:1 SWR bandwidth points…

You are using the exact same power into the antenna.

(or preferbly use a VNA they are cheap these days ! ) 

But you wont get the same amount back as you had when you just installed the antenna….

There is loss …. this means your 2:1 SWR bandwidth has become larger.

It is not 1,9Mhz (as the example above) but has become 1,95 or 2000Khz  !

You now know there is something wrong.. . there is additional LOSS!

A “improved” SWR over time always means loss.

 

ANTENNA TOO LONG or TOO SHORT ? 

The SWR can help us with identifying if our antenna is to long or to short.

If your “best” SWR is too high in freqency your antenna is to short.

If your SWR is best at the lower end of your band the antenna is too long.


This is easy to understand :
300 / frequency in Mhz was the wavelength
300 / 27,6 = 10,87 meters, 300 / 27,5 = 10,9 meters so the wavelength is getting longer lower in frequency we already knew that.
If my SWR is better lower in frequency this will mean then my antenna is too long! simple as that.

 

How much loss do I have with SWR?

 

(Bare in mind a 2:1 ration does not mean half the power is reflected.  as it doesnt measure power, it measures voltages.)


What SWR percent power out SWR percent power out SWR percent power out.

SWR
Percent power out
SWR
Percent power out
SWR
Percent power out
1.0:1
100%
1.4:1
97,2%
1.8:1
92%
1.1:1
99,8%
1.5:1
96%
1.9:
90,5%
1.2:1
99%
1.6:1
94,7%
2.0:1
89%
1.3:1
98,3%
1.7:1
93%
3.0:1
75%

A lot of people are obsessed by 1:1 SWR, although that is obviously what we want to see !

It is unlikely anyone can notice the difference in signal strength between 1,0:1 and 1,5:1

Dont be frustrated if that needle pumps up a bit….really…dont…

 

INFLUENCE ON SWR 

 

There could be reasons why your SWR has changed.  A good example of this is rain and snow.

We have our yagi up in the air….and snow and ice has settled on your antenna.

The same antenna where we were not allowed to change element diameter from start.

Obviously the snow and rain wil have that influence.

For that reason you can see a shift in SWR.

Other reasons like nearby object, you have installed a new metal roof, perhaps isolated it ?  or you just installed an additional antenna etc.

You could have common mode currents, thats why it always adviced to use an RF Choke. (or several)

IN short…there are many things that could influence your S.W.R. always make sure the antenna is as free as possible, use RF Chokes and write down your 2:1 S.W.R points And dont worry too much about a slightly higher SWR. And 1:1 doesnt tell us anything about how efficient the antenna is.