What is the load impedance?

[lost in electronics]

I have a RG58/U cable, my SWR is 2 at 26.3Mhz.  how do I find the load impedance?

[AnalogKid2DigitalMan]

Not a good match.

This forum is primarily discussion of National Instruments hardware and software, not an RF site.

Good luck good buddy, catch you on the flip side, watch out for the gators and Bears.

[gchristi1]

LabVIEW has a Smith Chart, play around with it, try to understand it, you'll learn a lot and you'll eventually know the answer.

[EBL]

The load impedance is a function of the length of your coax cable. Assuming your RG-58/U cable is exactly 50 ohms (and lossless), the SWR of 2 indicates that your load impedance lies somewhere on a circle on the Smith Chart which is centered at 50 ohms and intersects 25 ohms and 100 ohms. Hence the magnitude of your load impedance will be no lower than 25 ohms and no higher than 100 ohms. But the impedance may be complex, such as 40 + j30 ohms. The impedance will rotate around the circle as you change the length of your cable or as you change frequency.

Since you know the frequency, you can calculate the load impedance if you know the (electrical) length of your cable.

For a good discussion, see:

http://en.wikipedia.org/wiki/Smith_chart


Hope this helps,
Ed

[EBL]

> Since you know the frequency, you can calculate the load impedance if you know the (electrical) length of your cable.

Actually, I wrote this sentence too hastily. Knowing only the SWR (the magnitude of the reflection), you can't calculate the exact impedance, even knowing the length of the coax. More information would be needed, particularly about the phase.

Ed

[gchristi1]

Nice explaination EBL. That's why I fell in love with the RF-microwave field. If a cable is smashed, it may look close to a short or it may look like an open. It just depends. At first, it seemed so counter intuitive.

[johnsold]

What Ed is talking about is the impedance seen at the SWR meter, which is a function of the actual load impedance and the intervening cable characteristics. The actual load impedance is fixed at a value which has a magnitude of twice or half that of the impedance for which the SWR meter is calibrated. Without the phase information (which the SWR meter does not report) you cannot determine any more than that about the load impedance.

If the cable is lossy, even this description is not accurate. It you measure the SWR at the input to 10 km of RG-58/U cable at 1 GHz, you will have no clue as to what is connected to the other end.

Lynn

[tbob]

Even as low as 450MHz, I've seen a situation where the SWR meter showed about 1:1.1, which is good.  The antenna was completely chopped off.  I think it was because of the length of the coax cable being a multiple of the wavelength.