# Why antennas have usually 50 ohm resistance

Joined Dec 13, 2023
6
Why antennas have usually 50 ohm resistance and ? what the source of this 50 ohm resistance

#### Externet

Joined Nov 29, 2005
2,216
Welcome.
It is not resistance; it is impedance and it is not 50 ohm, it is near 50 ohm as a compromise standard. There is another standard of 75 ohm as that figure nears the transition from emission into air/space in less lossy performance. The 50 Ω impedance happens at one frequency and not a band of frequencies.
I will look for a reference link that explains it better than me and add later.

#### Externet

Joined Nov 29, 2005
2,216

#### ZCochran98

Joined Jul 24, 2018
303
The short answer is that it's derived from a compromise between the impedance that causes the minimum loss in a coax cable (77 Ohms) and the impedance that allows for maximum power handling (30 Ohms) in the same coax cable.
Here's a solid article on the topic.

#### Papabravo

Joined Feb 24, 2006
21,225
Edit to Add. You cannot use a multimeter, which measures DC resistance in Ohms, on a coaxial cable. The 50Ω on a cable is the magnitude of a complex number, which you may remember is computed as:

$$Z\;=\;\sqrt{Re^2+Im^2}$$

AND this value is frequency dependent.

#### nsaspook

Joined Aug 27, 2009
13,253
Edit to Add. You cannot use a multimeter, which measures DC resistance in Ohms, on a coaxial cable. The 50Ω on a cable is the magnitude of a complex number, which you may remember is computed as:

$$Z\;=\;\sqrt{Re^2+Im^2}$$

AND this value is frequency dependent.
What is frequency dependent?

It's a ratio of EM electric field and magnetic field energy components (voltage and current) as they propagate thru the coax dielectric.
Wave impedance: https://www.electricity-magnetism.org/wave-impedance-formula/

Last edited:

#### WBahn

Joined Mar 31, 2012
30,034
Edit to Add. You cannot use a multimeter, which measures DC resistance in Ohms, on a coaxial cable. The 50Ω on a cable is the magnitude of a complex number, which you may remember is computed as:

$$Z\;=\;\sqrt{Re^2+Im^2}$$

AND this value is frequency dependent.
No, it is the characteristic impedance of a transmission line, which is well-modeled by the Telegraphers Equation.

$$Z_0 \; = \; \sqrt{\frac{R \; + \; j \omega L}{G \; + \; j \omega C}}$$

While, in general, it is a complex value, in well-made transmission lines R and G are very small compared to L and C and so this is closely approximated by

$$Z_0 \; \approx \; \sqrt{\frac{L}{C}}$$

which is a real number with very little frequency dependence over a very wide range of frequencies.

#### Externet

Joined Nov 29, 2005
2,216
Do not deviate from the original question; is was about antennas and see comments oriented toward transmission lines. An antenna exhibits 50Ω at only one frequency.

#### nsaspook

Joined Aug 27, 2009
13,253
Do not deviate from the original question; is was about antennas and see comments oriented toward transmission lines. An antenna exhibits 50Ω at only one frequency.