# impedance of antenna

1. ## impedance of antenna

when we say an antenna has an impedance of 50 ohms, that means the antenna is connected to a load of 50 ohms and the load is not part of the antenna?

2. ## Re: impedance of antenna

Hi,

50 Ohms is the characteristic impedance, you can not measure with an Ohmmeter.
The load impedance does not belong to the antenna. It is for impedance matching, for
maximizing the output power, for optimizing the overall signal-to-noise and to reduce reflections.
(Here the cable impedance and cable length need to be considered, too)

Klaus

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3. ## Re: impedance of antenna

so if we say the antenna has impedance of 50 ohms, that means the material the antenna is made of has impedance of 50 ohms? i know impedance equats resistance, the real part of impedance, plus reactance, the imaginary part of impedance.

4. ## Re: impedance of antenna

Hi,

the material the antenna is made of has impedance of 50 ohms?
No. It has nothing to do with antenna material. At least it should not...

Klaus

5. ## Re: impedance of antenna

hypothetically if i build a radio from scratch, how do i make sure the antenna has an impedance of 50 ohms? if i use a piece of copper wire as the antenna and connect the antenna to a 50 ohm resistor, would that work?

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6. ## Re: impedance of antenna

An antenna with 50 ohms impedance is behaving like a 50 ohms resistor in the operation frequency range of the antenna. An ideal antenna has no internal losses, if it's connected to a transmitter, all input power will be radiated into space.

Respectively, there's no physical resistor representing the real input impedance, you can imagine the antenna as a transformer between input port and free space impedance.

I believe we can help more specifically if you tell what you want to achieve, e.g. use the antenna with a transmitter or receiver?

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7. ## Re: impedance of antenna

use the antenna as a receiver. so how do i make sure the antenna has 50 ohms impedance?

8. ## Re: impedance of antenna

Impedance of 50 ohms is a spec which is associated with voltage and current levels. Suppose the antenna generates a 50 µV signal... Then it implies you can expect 1 µA to flow.

It tells you what sort of band-pass filter would yield an optimum signal, or what pre-amp circuit would work best, or else what impedance matching is needed to make the signal suitable for your existing pre-amp.

Years ago we had twin-lead antenna cable and 300 ohms was the usual spec for it. Twin-lead was used to bring a signal from the antenna to the radio and tv receiver. Commercial equipment frequently gave 1 µV as minimum spec for FM sensitivity.

9. ## Re: impedance of antenna

so measure voltage and current and that will tell me the impedance of the antenna at the particular frequency of operation?

10. ## Re: impedance of antenna

use the antenna as a receiver. so how do i make sure the antenna has 50 ohms impedance?
You rarely achieve exact impedance matching and it's not strictly required for a receiver. Just lose a bit of sensitivity. Different antenna types have specific impedances by design. If it's considerably different from the receiver input impedance (e.g. 50 Ohms for VHF band and above), a matching network or transformer may be used with the antenna.

so measure voltage and current and that will tell me the impedance of the antenna at the particular frequency of operation?
Basically yes. In practice, antenna impedance can be measured with an antenna analyzer https://en.wikipedia.org/wiki/Antenna_analyzer or a vector network analyzer (VNA) https://en.wikipedia.org/wiki/Networ...lectrical)#VNA

11. ## Re: impedance of antenna

Originally Posted by dl09
so measure voltage and current and that will tell me the impedance of the antenna at the particular frequency of operation?
To my limited understanding a tuning circuit is usually present. This is true whether you're transmitting or receiving. If transmitting then you want resonant action, resulting in standing waves. It's a different animal than current electricity. Radio enthusiasts know what kind of power meter to use on a transmitting antenna.

When receiving your antenna is bombarded by photons at every frequency of the spectrum. Some of these broadcasts are stronger than the frequency you're interested in. An antenna can be constructed so it emphasizes a certain desired frequency (or frequency band). In any case a tuning network, or filter (wide-band, narrow-band, etc.) is needed. It's also a convenience spot for impedance matching. Furthermore a complete circuit must be made, since that's the only way current can be measured.

If you have the proper instruments you could carry out a series of experiments. Attach known resistor values. Test response. This might reveal what ohm value brings out optimal performance. And provide a clue as to the impedance in your antenna.

12. ## Re: impedance of antenna

Originally Posted by dl09
when we say an antenna has an impedance of 50 ohms, that means the antenna is connected to a load of 50 ohms and the load is not part of the antenna?
Lets start with the very basics, open dipole antenna this has 75 Ohm resistance at any frequency at transmitting or receiving.
The perfect loss-less line this should include 75 Ohm cable among with 75 Ohm connectors, in other words an entire 75 Ohm system.
Now proper measurement of 75 Ohm system will require SWR/Bridge made for 75 Ohm antenna system.

Ideally one 50 Ohm antenna system , wire , connectors will require it own SWR/Bridge made for 50 Ohm.

Basically both antenna systems they are about parts in use compatibility.
An 50 Ohm antenna system translates that the active dipole this is mechanically modified by including a coil in-line or the low end pole this is at an angle for making some sort of directional transmission.

In other words when A&B ends of a open dipole they have opposite direction this is 75 Ohm, when these A&B ends they come closer by be at an angle up to 90 degrees, the theoretical open air resistance gets lower to 50 Ohms.

Why we do not have 25 Ohms antenna? Because such a model will be problematic, our transmitters they should use more energy and the aluminum dipole antenna style will meltdown at strong currents.
Therefore the industry followed the most Fail-safe impedance model, which works at low risk.

13. ## Re: impedance of antenna

Lets start with the very basics, open dipole antenna this has 75 Ohm resistance at any frequency at transmitting or receiving.
Actually not at any frequency. 75 ohm at the frequency where the dipole length equals λ/2.

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14. ## Re: impedance of antenna

Originally Posted by FvM
Actually not at any frequency. 75 ohm at the frequency where the dipole length equals λ/2.
Come on, no one using using full λ when constructing a dipole, therefore your comment has low value.

15. ## Re: impedance of antenna

Considering that the questions show rather shallow RF knowledge, it should be at least mentioned that the dipole frequency range is small and impedance outside this range completely different.

16. ## Re: impedance of antenna

Originally Posted by FvM
Considering that the questions show rather shallow RF knowledge, it should be at least mentioned that the dipole frequency range is small and impedance outside this range completely different.
At this point lets check your own awareness: Please define sum of radio bands that you are aware of, by not Using Google search ...

17. ## Re: impedance of antenna

The impedance of the antenna (ignoring any parasitic losses) is its radiation resistance to free space, and is determined by the physical/geometric design of the antenna and the frequency.
Some antenna designs have a narrow bandwidth (e.g. dipole, yagi) and others have a wider bandwidth (e.g log periodic).

Note that radiation is power and transferring power means there is a resistive part of the impedance involved.
So the antenna impedance indeed looks like a resistance at the design frequency range, rather like a matching transformer between the free space radiation resistance* and the antenna wire impedance (for some antennas there is also a matching transformer between the wire and the antenna if the two impedances are different).
The tuning of an antenna with an external inductance or capacitance is to cancel any reactive portion of the antenna impedance so that what's left is the resistive radiation impedance (as indicated by a low SWR value).

*Impedance of free space. Z0 = 376.730313668(57) Ω. The impedance of free space (that is the wave impedance of a plane wave in free space) is equal to the product of the vacuum permeability μ0 and the speed of light in vacuum c0.

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