How does an antenna tuner work
|What must or can a tuner do ...|
In order to perform its transformation function, an antenna tuner should handle any load that is within its specified limits. But everything has its limits, because no antenna tuner can adapt everything!
1. The greatest possible power transmission from the transmitter via the feed line to the antenna is only given if the antenna represents a real effective resistance for the transmitter without a capacitive and / or inductive reactive component.
2. There is an impedance matching between the antenna and the transmitter.
An SWR greater than 1: 1 occurs when the load on the antenna system differs from the specified output load impedance of the transmitter, which is usually 50 Ω.
The table below shows some representative resistance values with none or. one capacitive or inductive reactive components that lead to different SWR values. It should be noted that general categories could be established for all load impedances, all of which have different solutions.
A special case of "real adaptation".
The load is a purely ohmic impedance of 50 Ω. In this case the SWR is 1: 1 and you don't need an antenna tuner. You can use it with all loads that are well adapted to the transmitter without a change. There may be a little extra loss here or there, but in general it will work fine.
The load has a resistance impedance other than 50 Ω which does not allow proper operation. These can be loads with a SWR of 2: 1, such as 25 Ω or 100 Ω, a SWR of 3: 1 SWR such as 16.7 Ω or 150 Ω or a SWR of 10: 1 such as 5 Ω or 500 Ω. All of these loads have in common that they could be adapted using an ideal transformer.
Keyword broadband transmitter. Just like you can adapt an 8 Ω loudspeaker to a 600 Ω line with an NF transformer. With HF applications, such transformers must of course be constructed differently, because the presence of reactive components must be avoided. A classic example is the matching arrangement used in a broadband antenna. It offers a nearly resistive load of around 600 Ω over a frequency range of 4: 1 and is often fed by a broadband transformer with 12: 1 impedance through a 50 Ω system. While most would call it a transformer rather than an antenna tuner, there is no question that there is a scenario that performs the function we are talking about.
The load consists of a combination of an ohmic 50 Ω component and a series reactive component. A load with a parallel reactance can be converted to one with a series reactance, but we'll focus on the series version as it's easier to visualize. This makes an antenna tuner that is easy to imagine.
When the series reactance of the load is inductive [+ XC.], the entire antenna tuner must contain a component with a capacitive reactance of the same order of magnitude [-XL.].
The serial combination of the two is now 0 Ω reactive, so the resistive component is 50 Ω.
Note that unlike the broadband transformer, this will likely only work on a single frequency. If the antenna has inductive reactance it will tend to fluctuate with frequency [XL. = 2 x π x F x L], at least over a limited frequency range.
To keep the total impedance ohmic we need a variable capacitor. Thus, the capacitive reactance can be adapted to the changing inductive reactance of the load. Note that this "antenna tuner" now has a button on the front and looks "more like an antenna tuner".
The load is a combination of active components and blind components that make a SWR larger than we can handle.
This is different from the other three scenarios. You will find this situation very often in practice, which all general-purpose antenna tuners have to deal with, at least over a certain SWR range.
There are a variety of circuit configurations that can be used that perform a combination of canceling the reactive component and transforming the resistive impedance. Only a small selection is shown here.
In many cases, if the circuits are made up of the same quality components, they will provide equivalent performance. As with all general purpose tuners, there are certain advantages and disadvantages, and some are more suitable for some applications than others.
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