Understanding VNAs - Antenna Measurements transcript

hello and welcome to this presentation understanding vna's antenna measurements in this presentation we'll provide a short technical introduction to antenna impedance measurements using a vector Network analyzer this presentation assumes a basic understanding of network analysis including return loss s SWR s parameters and the Smith chart if you're unfamiliar with these topics or if you'd like A Brief Review please see the separate presentations on each of these topics as you probably already know a changing current in a conductor produces an electromagnetic field that will radiate outwards from that conductor conversely if this radiated electromagnetic field encounters or cuts across a conductor this will induce a current within that conductor when one or more of these conductors are designed and implemented specifically for radiating or for receiving signals these are called antennas there are many types of antennas with different sizes geometries properties Etc but one thing that's common to all antennas is that they are frequency dependent that is they are efficient radiators or receivers only overrate limited frequency range the parameters and performance of antennas can often be predicted by means of simulation or modeling but physical measurements of antennas are necessary to verify performance under real world conditions there are actually two types of antenna measurements one type of measurement is radiation measurements which quantify how well the antenna radiates a signal this includes the antenna's gain and directivity beamwidth deficiency Etc however in this presentation we'll be looking at the other type which is antenna impedance measurements the impedance of an antenna determines how much of the input or transmit power is absorbed or radiated by the antenna and how much is returned to the trans mitter this is determined by injecting a signal into an antenna and then measuring the magnitude and phase of the signal reflected or returned from the antenna as mentioned a few moments ago this will change often substantially as a function of frequency there are several different methods or instruments that can be used for measuring antenna impedance but the preferred method is using a vector Network analyzer to perform a reflection or so-called S11 meth measurement Vector Network analyzers can make scalar and Vector measurements of both forward and reflected power and they're available in both benchtop and handheld form factors modern vnas have a wide frequency range usually up to single or double- digigit gigahertz and they also have a high dynamic range in addition VNA support various calibration procedures which enables very high measurement accuracy and repeatability although this present ation will focus on antenna measurements vnas are general purpose instruments that can measure display and record so-called s or scattering parameters for a wide variety of devices and applications this means that in addition to measuring antennas vnas can also be used to measure cable loss distance default filters amplifiers Etc it's also worth noting that single port VNA functionality is sometimes also implemented in Spectrum analyzers by equipping them with a tracking generator and an SWR bridge for separating forward and reflected power let's start with how to connect a VNA to an antenna in many cases a feed line is used to connect a transmitter to an antenna or more precisely to the antenna feed point because antennas generally work best when they're mounted in high or unobstructed locations such as on a tower the feed Point may be very difficult to access and therefore antenna measurements often have to be made at the transmitter end of a feed line there are two methods of connecting a feed line to a VNA the first is simply connecting the feed line directly to the VNA Port the second is using a short highquality DT or device under test cable this is often done for ease of attachment or to avoid strain on the instrument connector measurement accuracy is unaffected by the stut cable as long as calibration takes this cable into account we'll talk more about this in detail in just a couple of minutes minutes configuring a VNA for antenna testing involves three main groups of settings the first is configuring the analyzer's internal tracking generator which provides a sweeping stimulus signal that's used both as the input to the antenna and is a reference when looking at the amount of signal reflected from the antenna note that if the output power of the tracking generator is set too low this can lead to inaccurate results especially when measuring via long or lossy feers the next step is specifying the frequency range over which the tracking generator is swept this should be at least wide enough to cover the intended antenna operating range but is often set somewhat wider than this range in order to better visualize impedance as a function of frequency this frequency range can be entered either as start and stop frequencies or as a center and span the third important parameter is the number of measurement points over the span increasing this number beyond the analyzer's default settings will provide greater detail but more frequency points will also increase the amount of time needed to make a single sweep in addition to these configuration steps a one port calibration is also necessary for accurate antenna impedance measurements the calibration process involves sequentially attaching an open a short and a match or load to the location where the antenna under test will be connected these standards can be in the form of discrete standards or they may be combined into a calibration T in addition to these manually attached standards electronic calibration units can also be used these units switch their internal standards in and out automatically and are controlled by the attached VNA regardless of which type of calibration standard is used this is usually a follow the prompts process in which the VNA will indicate which standards are to be connected in which order and at which times the entire process generally takes only a few minutes with automatic calibration units tending to be much faster than using manual standards note that if the antenna under test or the attached feed line will be connected directly to the analyzer Port then the calibration standards should also be attached directly to this port if a DOT cable is used then the calibration standards should be attached to the end of the the DT cable doing this moves the calibration plane to the end of the DT cable and thus removes the DT cable from the measurement results antenna impedance measurements are typically displayed in four formats standing wave ratio return loss complex impedance and the Smith chart in the remainder of this presentation we will explain and provide examples of each of these formats standing wave ratio also sometimes called voltage standing wave ratio or visir is the ratio of forward power to reflected power as can be seen from this equation an S SWR value of one would correspond to zero reflected power and this is the ideal or best case scenario but an S SWR of less than 1.5 or less than two is acceptable in most applications s SWR can be shown as a value at a given frequency but normally it's plotted as a function of frequency and this is how s SWR is displayed when using a VNA for antenna testing as we'll see in a few moments this plot of s SWR versus frequency can also be used to determine the bandwidth of an antenna let's look at an example as WRR measurement result this plot shows the measured antenna's s SWR as a function of frequency between 450 MHz and 500 MHz and we see a minimum s SWR value of 1.02 at 473 MHz this point therefore represents the optimum operating frequency for this particular antenna over the measured frequency range as mentioned earlier we can also use this graph to determine the bandwidth or usable frequency range of an antenna here we're defining this as the frequency range over which s SWR is less than than or equal to 2 although 1.5 is also used sometimes as the limit from the graph we see that Sr is less than two between 468 MHz and 481 mahz therefore we can specify this antenna as having a usable frequency range or bandwidth of approximately 13 mahz unlike s SWR which is a linear value return loss is a logarithmic ratio of forward and reverse power and this is often a better way to visualize wider ranges of measured values return loss has units of DB and can be calculated from s SWR using this formula for example in S swrf 5 will correspond to a return loss of 3.5 DB although strictly speaking return loss as a positive value it's often represented as a negative value when plotting in 10 imped regardless of the sign greater magnitudes of return loss aren't more desirable because these indicate lower levels of reflected power and therefore a better impedance match let's look at the same antenna again but this time using return loss again we see a minimum in the graph at 473 mahz with a return loss of approximately 40 or minus 40 DB and as before we can specify the bandwidth of an intent an using return loss here we're using minus 10 DB as the threshold and once again we have a bandwidth of approximately 13 MHz between 468 MHz and 481 MHz s SWR and return loss only show the magnitude of the reflection that is they are both scalar values and in many cases this is sufficient however there are some cases in which measuring the complex imped of the antenna is needed a good example of this is when designing matching networks recall that complex impedance Z is a vector value which can be described in two ways one way is as a combination of a resistive and real part and a reactive or imaginary part the other ways as a magnitude and phase angle simple formulas can be used to convert between these two formats on vnas complex impedance values often displayed on a Smith chart the Smith chart shows complex impedance as a normalized value and as a function of frequency in the case of antenna testing the Smith chart allows easy visualization of complex impedance over the measured frequency range and markers can be used to read off the complex impedance at a given frequency although we don't have time to go into detail on the Smith chart in this presentation we can make two broad statements about measured value values first the best impedance match that is the minimum SWR or maximum return loss occurs at the center of the Smith chart and second the farther the line or Trace is from the center the greater the amount of reflected power let's look at an example here we see a plot of complex impedance as a function of frequency over the range of 450 MHz to 480 MHz recall that points closest to the center of the Smith chart represent a better impedance match that is less reflected power and thus our measured antenna's resonant frequency in this example 473 MHz is the point where the trace passes closest to the center of the chart we can also determine the complex impedance at any frequency by placing a marker on the trace at that point and then reading off the displayed value in a variety of different formats let's end with a brief summary Vector Network analyzers or vnas can be used to measure antenna impedance by injecting a signal into the antenna or feed line and then measuring the reflected signal this is referred to as a reflection or S11 measurement and is normally performed over a range of frequencies the results are most often displayed as a plot of standing wave ratio or return loss as a function of frequency and both of these are scaler that is magnitude only measurements in some cases such as when designing matching networks complex values are also useful and these are usually displayed in the form of traces on a Smith chart with markers used to read off specific values and although this presentation is focused on antenna measurements the general measurement methodology settings and results can also generally be applied to many other types of one port VNA measurements this concludes our presentation understanding vna's antenna measurements if you'd like to learn more about antenna measurements vnas or related topics please see the links in the video description thanks for watching