# Tag Info

8

For practical purposes, all frequencies. If you cut a transmission line into infinitesimal segments, each segment can be modeled as: simulate this circuit – Schematic created using CircuitLab ($G$ is conductance, the inverse of resistance) The characteristic impedance is: $$Z_0 = \sqrt{ R + j\omega L \over G+j\omega C }$$ ($\omega$ is the angular ...

5

Antenna efficiency over 130% would mean a perpetual motion machine, so probably not.

5

Both the real and the imaginary terms of the complex impedance across the feedpoint terminals will change as the feedpoint moves away from the electrical center of the antenna. As an example, a NEC4.2 calculation for a nominally 1/2 wavelength radiator in free space when the feedpoint is located 0.05 wavelengths from one end is about 1340 -j1100 Ω. A ...

4

Using a NEC-2 model, here's how R and X of a free-space 20-meter dipole made from #14 copper wire vary with feedpoint position (50 = center fed; 0 = end fed):

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The characteristic impedance of a line does change with frequency. At high frequencies the loss components are effectively zero and $Z_0$ depends only on $L$ and $C$ (per unit length). But at low frequencies, $R$ (and $G$) cannot be ignored and contribute significantly to the line impedance. There is a "corner frequency" where one regime takes over ...

3

Well, according to the spec tables in the manual the thing expects a 50 Ω antenna impedance. Mismatch means a loss of signal energy. How much exactly depends on the length of the 75 Ω transmission line – this makes a system much harder to design predictably. I wrote all the following (up to the next horizontal line), then stopped to wonder "what might ...

3

I think if you're worried about the load on the other dipole, you need to go back and look at how Z parameters work. $Z_{21}$ doesn't assume anything about the impedance on the two dipoles. It's just one of the impedance parameter term - $Z_{21}$ is the voltage developed on dipole 2 due to a current in dipole 1. It's not the final voltage or current present ...

3

I think the problem you will run into is that people will want this to be plug and play. They may not have a directional coupler, or know or feel like to calibrate it and all that other stuff. Even if they do it, they may wonder if they did it right and if the reading will be accurate or not. I think you will need to handle this part in your design and ...

2

According to Microwaves101, the "load pull" technique may fulfill your need: Load pull involves varying the load impedance presented to a device under test and monitoring a single or set of performance parameters. When used in conjunction with a signal source and signal analyzer (spectrum analyzer, power meter, vector receiver…), load pull can be ...

2

Smith chart shows that 50 Ohm antenna (assumed perfect 50 Ohm real!) connected to 75 Ohm coax cable can result in impedances between 50 and 112.5 Ohm. SWR 1: 1.5. So this so far not mentioned possible solution in this thread may be usefull for your problem: for a single frequency, or for a small frequency band, the use of cable with a multiple of half ...

2

I don't think the "matching to free space" should be taken so literally. I've never seen an actual equivalent circuit in a textbook or used it to derive any property of an antenna. Sure an antenna is the interface between the transmission line (which has an impedance) and free space (which also has an impedance, same units but different in nature). ...

2

The output impedance is not that important: the (lowest) allowed load impedance is important. And the assumption is right: from the output impedance it is possible to calculate the effect of load on the output voltage. But be aware of frequency deviation as a function of load variation, and also eventual stop of oscillation with serious load. This is not a ...

1

S11 is one of the S-parameters. It uses 1 port on the VNA to look into something and look at the reflection coming back to determine reflection coefficient / VSWR. If the device was calibrated properly then it can also tell you the complex impedance of what it is "seeing". To calibrate you have to use open, short, and load (99% this is 50 ohm), but ...

1

The impedance given in a coaxial cable data sheet is its "characteristic" impedance, not its actual impedance. What that means is that "50 Ω" coax will have the least loss when the output impedance of the transmitter is 50 Ω with no reactance, and the impedance of the load (the antenna) is also 50 Ω with no reactance, irregardless of ...

1

Ideally, the impedance is not dependent on frequency. Practical limitations should be stated in the specs. Also note, that the effective impedance seen at the end of the coax is affected by power travelling "upstream", for example reflected/standing waves. These have to be taken into consideration when there is an impedance mismatch down the ...

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