VSWR and Impedance matching

[med534]

Hi,

If my antenna has a VSWR of 1:1.1 at an impedence of 100 ohms,
but is fed with a 50 ohm impedence, what effect will this have?

 

[biff44]

Your antenna will now have a 3:1 vswr.

Rich

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[VEBill]

1.8:1 to 2.2:1

?

 

[Higgler]

Right VE1Bll,

the voltage reflection coefficient if it was perfect VSWR into 100 ohms is;
Tau (reflection coef.) =(100-50)/(100+50)= 50/150 = 0.33
VSWR is (1+Tau)/(1-Tau) = 1.33/0.66, so about 2:1, which is about -10 dB return loss or 90% efficiency, or 0.5 dB loss in efficiency.

That's not too bad. Most wide frequency systems use this VSWR level as a standard specification without burning things up or losing too much.

If you buy some 75 ohm cable, make it 1/4 wavelenth (electrical length at your operating frequency), and insert it between the 50 ohm and 100 ohm device, you'll get a better result in VSWR. Probably 1.3:1 (depends on frequency and connector quality).

kch

PS: don't forget the "a" in impedance. Remember you're slowing down ants.

 

[biff44]

Sorry, my bad. Just goes to show, you should always look up the equation instead of just doing it in your head from memory.

Above is true.

Simpler equation:

S = Zl/Zo for Zl>Zo
and
S = Zo/Zl for Zl<Zo

So S = 100/50 =2

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[VEBill]

Don't forget that the starting point was not 1.0:1, but was "1.1:1" (or something like that...) at 100 ohms.