Understanding VSWR and Return Loss

Summary

This presentation delves into the technicalities of voltage, standing wave ratio (VSWR), and return loss in radio frequency systems. It explains how impedance matching between source and load is crucial for efficient power transfer, with standard impedances often being 50 or 75 ohms. The script discusses the impact of impedance mismatch, leading to reflected power, and introduces complex impedances. It further clarifies how antennas' impedance varies with frequency and placement. The presentation also covers how return loss and VSWR quantify reflected power, with methods to minimize it, such as using matching networks and fold back techniques, crucial for protecting RF systems from performance degradation and damage.

Takeaways

• 🔌 Maximum RF power transfer occurs when source and load impedances are matched, typically at 50 or 75 ohms.
• 🔄 Impedance mismatch leads to reflected power, which is generally undesirable as it reduces efficiency.
• 🌊 Reflected power is the power that bounces back from the load towards the source due to impedance differences.
• 🎚️ Complex impedance includes both resistive and reactive parts, which can be matched using a complex conjugate.
• 📡 Antennas have impedance that varies with frequency, affecting the level of reflected power at different frequencies.
• 📊 Return loss is measured in dB and represents the difference between forward and reflected power, with higher values indicating less reflection.
• 🌐 Voltage Standing Wave Ratio (VSWR) quantifies the ratio of maximum to minimum voltage in a standing wave, with a ratio of 1 indicating no reflection.
• 🔄 The reflection coefficient (gamma) is a function of load and source impedances and is used to calculate VSWR.
• 🛠️ Tuning or matching networks can be used to minimize reflected power by adjusting the impedance seen by the source.
• ⚙️ Fold back reduces transmitted power to protect the source from high levels of reflected power, which can cause damage.

Q & A

What is the primary concern in radio frequency systems?

Efficient power transfer is one of the most fundamental concerns in radio frequency systems.

What happens when the source and load impedances are matched?

Maximum RF power transfer occurs when the impedances of the source and load are matched, with all RF power from the source being absorbed by the load.

What is the standard impedance in the RF world?

The standard impedance in the RF world is usually 50 ohms, although systems using 75 ohms as a standard impedance also exist, such as cable television systems.

What is reflected power and why is it usually undesirable?

Reflected power is the power that is reflected from the load back towards the source due to impedance mismatch. It is usually undesirable as it indicates inefficiency in power transfer.

How does a complex impedance differ from a purely resistive impedance?

A complex impedance consists of both a real resistive part and an imaginary reactive part, whereas a purely resistive impedance does not have a reactive part.

What is the significance of the reactance in impedance?

Reactance, which can be capacitive or inductive, changes with frequency and affects how much impedance varies by frequency, which is crucial for devices like antennas.

What is return loss and how is it calculated?

Return loss is the difference in dB between the transmitted (forward) and reflected (reverse) power, calculated as forward power minus reflected power. It is a measure of how much power is reflected back to the source.

What is the Voltage Standing Wave Ratio (VSWR) and how is it measured?

VSWR is the ratio of the highest to the lowest voltages in a standing wave pattern on a transmission line, indicating the level of reflected power relative to the forward power. It can be measured using a network analyzer.

How can the level of reflected power be reduced?

The level of reflected power can be reduced by using matching networks to match the load impedance to the source impedance or by reducing the level of transmitted power, known as fold back.

What are the two special cases of VSWR and what do they indicate?

The two special cases of VSWR are a short-circuit (load impedance equals 0) and an open circuit (load impedance is infinite), both resulting in a VSWR of infinity, indicating 100% of the forward power is reflected back to the source.

Why is it important to minimize reflected power in RF systems?

Minimizing reflected power is important to ensure efficient power transfer, prevent performance degradation, and protect RF components from potential damage caused by high levels of reflected power.

Outlines

00:00

📡 Understanding RF Power Transfer and Impedance

This segment introduces the concepts of voltage, standing wave ratio (VSWR), and return loss in the context of radio frequency (RF) systems. It discusses the importance of impedance matching for efficient power transfer, with a standard impedance often being 50 ohms in RF systems. The video explains how impedance mismatch leads to reflected power, which is generally undesirable. It also touches on the complex nature of impedance, including its resistive and reactive parts, and how complex conjugate impedances can match each other. The summary emphasizes the significance of understanding impedance variation with frequency and its impact on reflected power, using dummy loads and antennas as examples.

05:00

🔍 Measuring Reflected Power: Return Loss and VSWR

This part of the presentation delves into how reflected power is quantified using return loss and voltage standing wave ratio (VSWR). Return loss is defined as the difference in decibels (dB) between transmitted and reflected power, with higher values indicating less reflected power. The segment visually demonstrates standing waves with forward and reflected wave voltages, explaining how VSWR is calculated as the ratio of maximum to minimum voltages. It also covers the historical and modern methods of measuring VSWR, including the use of network analyzers. The mathematical relationship between reflection coefficient and VSWR is outlined, and the impact of increasing VSWR on reflected power is discussed, highlighting the need for minimizing reflections for optimal RF performance.

10:01

🛠 Mitigating Reflected Power with Matching Networks and Fold Back

The final segment summarizes the presentation by emphasizing the importance of matching source and load impedances for maximum RF power transfer. It discusses the use of matching networks, which consist of capacitive and inductive elements, to align the load impedance with the source impedance. Additionally, it introduces the concept of fold back, a method to reduce transmitted power and thereby limit the level of reflected power, particularly useful in high-power sources to prevent performance degradation or damage. The segment concludes with a reminder of the key learnings: the necessity of impedance matching, the quantification of reflected power through return loss and VSWR, and the strategies to reduce reflections for efficient RF system operation.

Keywords

💡Voltage

Voltage, in the context of the video, refers to the electric potential difference between two points in a circuit. It is a fundamental concept in electronics and is critical in radio frequency (RF) systems for understanding power transfer. The video mentions that impedance mismatches can cause reflected power, which affects the voltage levels in a transmission line. Voltage standing wave ratio (VSWR) is directly related to the voltage pattern created by the superposition of forward and reflected waves on the transmission line.

💡Standing Wave Ratio (SWR)

Standing Wave Ratio, often abbreviated as SWR or VSWR when referring to voltage, is a measure of the inefficiency in the transfer of RF power due to impedance mismatches. It is defined as the ratio of the maximum voltage to the minimum voltage in the standing wave pattern on a transmission line. The video explains that a lower SWR indicates a better match between the source and load impedances, resulting in less reflected power and more efficient power transfer.

💡Return Loss

Return loss is a measure of the power that is reflected back towards the source rather than being absorbed by the load. It is expressed in decibels (dB) and represents the difference between the forward power and the reflected power. The video emphasizes that a higher return loss is desirable as it indicates less power being reflected, which is a sign of a better impedance match and more efficient power transfer.

💡Impedance

Impedance is a complex quantity that consists of resistance and reactance, and it is a measure of the opposition that a circuit presents to the flow of alternating current. In RF systems, impedance matching is crucial for maximum power transfer. The video discusses how impedance can vary with frequency and how it affects the level of reflected power, which in turn influences the VSWR and return loss.

💡Reflected Power

Reflected power is the portion of the forward power that is not absorbed by the load and is instead reflected back towards the source due to an impedance mismatch. The video explains that reflected power is generally undesirable as it indicates inefficiency in the power transfer process. The level of reflected power can be quantified using return loss or VSWR.

💡Complex Impedance

Complex impedance is a term used to describe impedance that has both a real part (resistance) and an imaginary part (reactance). The video mentions that every load impedance is complex, and a perfect match can be achieved by using a complex conjugate, which involves reversing the sign of the imaginary part. Understanding complex impedance is essential for designing circuits that minimize reflected power.

💡Reactance

Reactance is the imaginary part of impedance that varies with frequency and is caused by capacitive or inductive elements in a circuit. The video explains that reactance, whether capacitive or inductive, contributes to the total impedance and thus affects how much the impedance changes with frequency. This change is crucial for understanding why certain loads, like antennas, have impedances that vary significantly with frequency.

💡Dummy Load

A dummy load is a type of resistive load designed to have a constant impedance over a wide frequency range. The video uses the dummy load as an example of a load with minimal impedance variation with frequency, which results in a consistent level of reflected power regardless of the operating frequency. Dummy loads are often used for testing and simulating RF systems under controlled conditions.

💡Antenna Impedance

Antenna impedance refers to the impedance seen at the feed point of an antenna, which can vary significantly with frequency and is also affected by the antenna's placement relative to a ground plane or other objects. The video notes that unlike dummy loads, antennas typically have impedances that change with frequency, which is why they have specified frequency ranges for optimal operation.

💡Matching Network

A matching network is a circuit that is used to transform the impedance of a load to match the impedance of a source, thereby minimizing reflected power. The video explains that matching networks often consist of capacitive and inductive elements and are used to achieve the best possible power transfer between the source and load by reducing impedance mismatches.

Highlights

Efficient power transfer in radio frequency systems is crucial and occurs when source and load impedances are matched.

Standard impedance in RF systems is typically 50 ohms, but 75 ohms is also used in some systems like cable television.

Impedance mismatch between source and load leads to reflected power, which is usually undesirable.

Complex impedance includes both real resistive and imaginary reactive parts, and is matched by a complex conjugate.

Impedance varies with frequency, affecting how much reflected power occurs at different frequencies.

Dummy loads usually have a constant impedance over a wide frequency range, unlike antennas which can vary significantly.

Reflected power levels change with frequency, especially noticeable when using antennas as loads.

Return loss is the difference in dB between transmitted and reflected power, indicating the efficiency of power transfer.

Voltage Standing Wave Ratio (VSWR) measures the ratio of the highest to the lowest voltages in a standing wave, indicating reflection levels.

VSWR can be calculated using a network analyzer, which determines the reflection coefficient based on load and source impedances.

A VSWR of 1 indicates no reflected power, while higher VSWR values mean more power is reflected.

Special cases like short-circuits and open circuits result in a VSWR of infinity, reflecting all forward power back to the source.

Tuning or matching networks can be used to minimize reflected power by matching the load impedance to the source impedance.

Fold back reduces the level of transmitted power to protect the source from high levels of reflected power.

Mismatched loads and reflections are common, and matching networks or fold back are strategies to reduce reflected power.

This presentation provides a comprehensive understanding of VSWR and return loss in the context of RF power transfer.

Transcripts

hello and welcome to this presentation

on understanding visible and return loss

in the short presentation we'll discuss

the technical concepts behind voltage

standing wave ratio and return loss as

well as how these quantities are

measured this burn return loss are both

related to the transfer of radio

frequency power and efficient power

transfer is one of the most fundamental

concerns in radio frequency systems

maximum RF power transfer occurs from

the source of RF power and the load or

sync of that power have impedances that

are matched in this case all of the RF

power from the source is absorbed by the

load and in most cases this is exactly

what we want the standard impedance in

the RF world is usually 50 ohms but

you'll also come across systems that use

75 ohms as a standard impedance for

example cable television systems so what

happens if there's a mismatch or

difference between the source and load

impedances in this case the impedance

mismatch causes some of the power from

the source or the forward power to be

reflected from the load back towards the

source this power is called the

reflected or reverse power

we'll use the terms interchangeably in

this presentation reflected power is

almost always undesirable there are very

few cases where we would want any power

reflected from the load back towards the

source so far we've shown our impedances

as purely resistive values but in

reality every load impedance is a

complex impedance consisting of both a

real resistive part and an imaginary

reactive part a complex impedance is

matched by a so-called complex conjugate

in which the sign of the imaginary part

is reversed at this point it might be a

good idea to pause for a brief refresher

on impedance remember that an impedance

Z is a complex value that consists of

two parts a resistance R which does not

change with frequency and a reactance X

which does change with frequency

reactance can be further divided into

capacitive and inductive reactance which

are not surprisingly usually created by

capacitors and inductors our complex in

peanuts has both a magnitude and a

direction it's

very important to remember that because

of reactance total impedance varies by

frequency but how much does impedance

vary by frequency that depends on the

load a dummy load for example is usually

a very resistive load that's designed to

have a constant impedance over a wide

frequency range most antennas on the

other hand have an impedance that

changes substantially by frequency for

this reason most antennas have a

specified frequency range over which

they can or should be used note also

that the impedance of an antenna in the

real world is also dependent on the

placement of the antenna relative to a

ground plane or other nearby objects so

if we were to use our mostly resistive

dummy load as a load the level of

reflected power would remain low and

roughly the same even as we change the

frequency from a hundred megahertz to

two hundred megahertz five hundred

megahertz or even a gigahertz if however

we use our antennas the load the level

of reflected power will be a function of

frequency in this example at 100

megahertz the reflected power is only

four watts at 200 megahertz the level of

reflected power goes down to less than

one watt but at 500 megahertz the level

of reflected power is now twenty five

watts and increases to fifty watts at a

gigahertz most real-world devices fall

somewhere in between these two somewhat

extreme cases of little impedance

variation by frequency and large or

irregular impedance variation by

frequency so clearly it's important that

we have some way of quantifying the

level of reverse or reflected power and

in most cases we want to do this

relative to the level of forward power

they're actually two different ways that

this relationship is quantified

these are return loss and voltage

standing wave ratio commonly called

either VSWR or visit our let's start by

looking at return loss return loss is

nothing more than the difference in DB

between the transmitted and reflected

power in other words forward power minus

reflected power equals return loss for

example if our forward powers 50 DBM and

our reflected power is 10 DBM we have a

return loss

40gb larger values for return loss mean

that less power is reflected so we

usually want return loss to be as large

as possible and of course return loss

must always be a positive number since

the level of reflected power is always

less than the level of forward power

even in the case of a load that reflects

100% of the forward power some power

will be lost along the path from the

source to the load and back the other

quantity used to measure or quantify the

level of reflected power relative to the

level of forward power is something

called vis waar or voltage standing wave

ratio here the blue trace is the forward

wave voltage the red trace is the

reflected wave voltage and the purple

trace is the combined voltage on the

line

note that the amplitudes of the forward

and reverse voltage remain constant but

the amplitude of the combined voltage

trace the purple trace rises and falls

over time creating let's refer to as a

standing wave voltage standing wave

ratio is simply the ratio of the highest

to the lowest voltages in our standing

wave in this example the peak value is 3

and the minimum value is 1 so we have a

visitor of 3 many years ago bazar was

calculated by physically measuring

voltages at different points along the

transmission line but today this Vark

can be automatically measured and

calculated using a network analyzer

mathematically we calculate vis wire by

determining the reflection coefficient

gamma which is a function of the load

impedance z sub L and a source impedance

at Z Sub Zero don't forget that these

impedances are complex frequency

dependent values once we have gamma vis

war is calculated by plugging gamma into

another simple equation we can also

easily convert between vis waar and

return loss now that we know how to

calculate vis waar let's look at what

happens as visible R increases if the

source and load impedances are matched

then vis waar is 1 and we have no

reflected power all powers absorbed by

the load and none of it is reflected

back towards the source it of his war of

1.5 only 4% of the total power is

reflected by the time we get to a

visible RF 3 a quarter of their forward

power is reflected back to the

source this is still acceptable for many

applications but the percentage of

reflected power increases dramatically

as vizla increases further at a vis war

of six only about half of the forward

power is absorbed by the load and the

remainder of the forward power is

reflected back to the source head of his

VAR equals ten two-thirds of the forward

power is being reflected back there are

two special cases we should discuss in

terms of vis wire the first of these is

a short-circuit in this case the load

impedance equals 0 and gamma equals

minus 1 in the case of an open circuit

load impedance is infinite and gamma

equals 1 if you plug either 1 or minus 1

into the visible our equation you get

the same result a vis waar of infinity

which means a hundred percent of the

forward power is being transmitted back

towards the source needless to say

having a hundred percent of the forward

or transmitted power reflected back to

the source is usually neither expected

nor desired setting aside these two

extreme cases what do we do about

reflected power in general one way to

minimize the level of reflected power is

to place a tuning or matching network

between a source and the load the

matching network consists of impedances

usually in the form of capacitance and

inductance design such that adding this

additional impedance matches the load

impedance to the source impedance in

this example we want to transform our

complex load impedance to match the

purely resistive 50 ohm source impedance

by selecting appropriate values in the

matching network we can change the

overall load impedance to match the

source impedance another way to reduce

the level of reflected power is to

reduce the level of transmitted power

this is called fold back and is

primarily used in higher power sources

such as broadband amplifiers the main

application of fold back is protecting

the source from high levels of reflected

power which can cause performance

degradation and even permanent damage in

some cases for example let's assume that

our source as a maximum safe reflected

power of 40 watts if the level of

mismatch is low save is y equals 1.5

then with 100 watts of forward power

only four watts will be reflected back

to our source if this war were to

increase to six the level of reflected

power of fifty watts would exceed the

safe limit by lowering transmit power

down to 80 watts then the level of

reflected power now falls again within

the safe limit so let's summarize what

we've learned first maximum RF power

transfer occurs when the impedance of

the source and the load are matched

impedances are complex frequency

dependent values and therefore a given

impedance is matched by its complex

conjugate which we get by reversing the

sign of the reactance or imaginary part

of the impedance a mismatch between

source and load causes some of the

transmitted or forward power to be

reflected by the load and returned to

the source the greater the degree of

mismatch the greater the level of

reflections we can quantify the amount

of reflected power as return loss or as

the voltage standing wave ratio or visit

are the conversion between these two

quantities is very straightforward

mismatched loads and reflections are not

uncommon and the two main ways of

reducing reflected power are the use of

matching networks and fold back this

concludes our presentation on

understanding vis warren return loss

thanks for watching