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# VSWR and Impedance matching

## VSWR and Impedance matching

(OP)
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?

### RE: VSWR and Impedance matching

Your antenna will now have a 3:1 vswr.

Rich

www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

1.8:1 to 2.2:1

?

### RE: VSWR and Impedance matching

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.

### RE: VSWR and Impedance matching

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

www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

### RE: VSWR and Impedance matching

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

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