Extra Class Lesson 9.1, Basics of Antennas

I'm Dave Kessler ke 0og your guide through the ARRL extra class license manual for ham radio our topic for today is section 9.1 basics of antennas this section which builds on section 6.1 of the general class license manual provides quite a number of definitions that allow us to compare one antenna type with another we'll talk about antenna radiation patterns gain beam width radiation resistance feed point impedance efficiency polarization and bandwidth that's quite a bit of stuff with these video introductions I'll try to provide background and a brief sketch of each subject to help you understand the material in the text the first topic we'll discuss is antenna radiation patterns let's spend some extra time with figure 9 - 1b which shows the radiation pattern for a simple idealized dipole this is what's called the azimuthal pattern and the numbers along the outside edge of the circle represent degrees of azimuth with north at the top in the diagram you are way up in the sky looking down on the antenna it's sort of like looking at a kind of special map in this diagram the dipole antenna is stretched from west to east note the words direction of dipole element those words go together now let's take a look at the diagram itself the outside marks a circle marked off as though when we're reading a map so zero degrees is to the north 90 degrees is to the east 180 to the south and 270 to the west just like a compass would work things out note that this is different from our usual convention for a circle which has zero at East ninety at north and so on look at the numbers here under the zero at the top the diagram is laid out in decibels it's a signal strength graph zero DB is the outer ring and is taken as the reference point as you move to the interior of the graph the signal gets progressively weaker we see -2 DB minus 4 minus 6 minus 8 and minus 10 note that with each number the distance between the circles gets progressively smaller so this graph is not only logarithmic but also nonlinear we can count down further with minus 12 minus 14 minus 16 minus 18 and minus 20 DB note that these circles go entirely around for example we can follow the minus 6 circle all the way around is shown here in green okay let's see just how we read this chart let's suppose the transmitting ham station is situated on perfectly level ground for example perhaps western Nebraska which is one of the flattest places I've ever seen let us Center the graph over the fictional town of Flat Ville Nebraska okay let's go say a hundred kilometers north of Flat Ville to Northville and put a station there similarly let's put stations at 30 Ville 60 Ville Eastvale 120 Ville and so on until we have flat Ville completely surrounded okay let's make a few assumptions here that aren't realistic but rather ideal each of these outlying stations has an identical antenna and each has a receiver with a perfectly calibrated estimator let us also assume that propagation is entirely perfect and uniform in every direction so here's what happens at Northville our receiving station registers an s9 signal coming from flat Vil our station at South Ville gets the same s9 signal we'll use this as the reference now let's look at 30 Vil here we see that the signal level is about 1 DB less than s9 moving to 60 Vil the signal is 6 DB below s 9 that would be a signal strength of s 8 again note where the receiving station is on the outer periphery of the circle not inside the circle the thick black line inside the circle simply represents the signal strength now note that as we go from 60 Vil to Eastvale along the track shown in blue the signal strength drops dramatically the signal is 12 DB down at about 75 degrees that means an s7 signal and as we nudge toward Eastvale our signal simply drops out of sight in fact Eastvale receives no signal at all you can see that this goes on all the way around the circle with full signal at self Ville and no signal at Westville and then back around where the signal is again strong the key point I want to make here is that this is both a geographic relationship in that the degrees of azimuth are included and also a graphical relationship of the signal in the direction of any given azimuth again I mentioned that this is an ideal case this ideal case never occurs in practice but it gives us something to start with now the point of this particular diagram is to display the radiation pattern of a dipole you can see it has a broad pattern with sharp nulls in practice over real terrain with buildings and trees as obstacles the nulls are not so sharp and the lobes won't be perfectly symmetric and you can imagine what patterns are like at my house here in the Rocky Mountains let's move into three dimensions as shown in Figure 9 - 1a this is the radiation pattern for a dipole in free space which looks like a doughnut or more precisely a torus think of this as the radiation pattern for a dipole on a satellite here I've added the XY and z axes for visualization let's call the antenna axis the x axis the y axis shoots off over here and defines the XY plane or horizontal plane if you slice the torus along the XY plane you get the dipole as a methyl pattern that we looked at a few moments ago in the flat Vil example in fact note that our azimuth degrees lie entirely in the XY plane now a couple things I want to point out the z axis is the elevation axis a perfect dipole radiates equally in all directions in the Y Z plane meaning a lot of the radiation goes straight up and for dipole and free space down to in a practical antenna therefore we must consider that not only does our radiation go forward along the earth but also up into the air the book makes a very brief reference to near-field and far-field all the radiation diagrams you see in this section and elsewhere are for the far field in the far field diagrams you can consider the antenna to be a single point in our flat Vil exercise with the receivers all being 100 kilometres away any HF antenna at flat Vil would certainly be a tiny point by comparison now when you get much closer to the antenna within a few wavelengths the antenna certainly isn't a point in the near field all the radiation is sorting itself out here's conceptually how it works each point on our dipole acts as an isotropic radiator the circles are the wave form emanating from that point sort of like ripples in a pond let's add another point a small distance away the radiation patterns create both destructive and constructive interference and you can see that the energy goes mostly up and down now let's add another point and we see the radiation pattern focusing even more as we add a final point here you can see how all this constructive and destructive interference makes for a mess close to the antenna but as we get further away from the antenna things sort themselves out into a proper wavefront now actually we have an infinite number of these and the radiation from each will either interfere with or strengthen that or the other points so we'll mark the near field area in green here and the far field areas in blue note that there is not a sharp distinction between the two it is possible to put some math on this but doing so is well into the realm of sophisticated antenna engineering just think of the far field as the point past which our antenna pattern stays the same and the near field as the area closer to the antenna where all that power is sorting itself out the distance from the antenna to where the far field sorts itself out is a function of the frequency and the antenna gain now the isotropic radiator I just mentioned is a theoretical antenna that radiates equally in all directions without any law is if 100 watts goes in 100 watts of RF comes out since there's no gain or loss and the pattern is uniform we say that the isotropic radiator has unity gain meaning a gain factor of 1 in every direction note that the log of 1 is 0 so in DB terms it has a gain of 0 DB in every direction now there is no such physical antenna but it can be handy as a point of reference now here you see two diagrams from the book the one on the left is the radiation pattern for the isotropic radiator as shown in Figure 9 - to be the radiation pattern is a little hard to make out in the book so I've drawn it here in green the diagram on the right is for the free space dipole note and this is very important these are both drawn on the same scale so we can compare them directly recall that on the dipole diagram where the pattern touches the edge it's an S nine signal but on the isotropic diagram shown in the same scale the Green radiation pattern is a circle it never touches the outer circle so it's never s 9 in any direction on the periphery of the circle in fact in every direction is 2.15 DB lower so now given that the transmitter is connected to the isotropic radiator and the dipole provide the same amount of power how do we get from the isotropic radiation pattern to the dipole radiation pattern well if we squeeze the isotropic in here meaning we don't radiate in an east-west direction that will provide power in other areas on the upper half we can Rob power from the green areas to create power for the magenta area the green areas look like they're bigger than the magenta area but that's just an artifact of work was a log scale we can do the same for the power from the blue areas going to create the yellow power so we say that either to the north or the south a dipole provides 2.15 DB gain over an isotropic radiator but that is only true in the north-south direction in any other direction the dipole gain is less and so how do we squeeze power in one or more directions well that brings up the whole subject of antenna design all of which is aimed at putting our transmitter power in the direction we want it's just like this flashlight the reflector in the flashlight aims light mostly in one direction it's pretty dim in areas where it is named but when I point this light directly at you you can see it very well indeed as an aside let me mention something called reciprocity an antenna that has gain while transmitting also has gain while receiving let's look here at the case where this station on the left is transmitting and the station on the right is receiving let's say that both have dipoles and the receiving stations receiver says s7 now let's move to the case where the station on the left are transmitting station installs aayegi with 12 DB gain relative to a dipole the receiving station still with its dipole will receive an s9 signal now let's flip that over on top we see that we get an s7 signal when both stations have dipoles just as before but on the bottom note that the far station still has only a dipole but the station on the Left receives an s9 signal why because if the antenna has 12 DB gain when transmitting it also has 12 DB gain while receiving and it's 2's units okay another aside this directly addresses the question of whether to add an amplifier at your station versus improving your antenna adding an amplifier only helps the other station hear you better however it does not help you receive the other station better but an antenna improvement helps both ways so improve your antenna system before you invest in an amplifier now if we can squeeze power in one direction more than another we can easily conceive of an antenna that squeezes most of its power in one direction there are lots of antennas that do this for example Yogi's in Figure 9-3 the radiation pattern is quite directional note that no matter how hard we try we cannot squeeze all the power in a single direction that's because in the near field as all that power sorts itself out some will head out in other directions we'll learn more about this in the next video which will discuss practical antennas now a lobe is a curved section of something sort of like an earlobe looking at this radiation pattern in Figure 9-3 we have a main lobe into which most of the power goes and some minor or smaller lobes also very important note the very deep nulls in between the lobes this actually is quite important although it's not possible to direct all energy in a given direction it is very much possible to construct an antenna that radiates no energy in a particular direction it also receives no signals from these directions these deep nulls can be handy when trying to reject an unwanted station now when we measure what we call gain we are measuring the amount of power radiated in the direction of the main Loeb with respect to the amount of power that would be sent in that same direction by reference antenna that reference antenna can either be a dipole or anisotropic if we use a dipole for reference we measure the gain in decibels referenced to a dipole or DB D similarly measuring with respect to an isotropic antenna is measured in DB I let's look at our squeezed diagram again the difference between an isotropic antenna and a dipole antenna in the direction of the main lobe is 2.15 DB note that's expressed to three digits of precision in real life I would say that a good dipole and a half wife length above the ground will be about a couple DB or so better than the theoretical isotropic but be that as it may the test questions use the theoretical gain of 2.15 DB so here's how it works let's let the x-axis represent gain with higher gain to the right an isotropic radiator has Unity gain meaning it doesn't add or subtract to the signal in any direction and all RF power is converted to radio waves with no losses unity gain means gain of zero dB a dipole has some gain in its strongest direction about 2.15 DB compared to an isotropic radiator now let's put a yaga here with 10 DB gain over a dipole or in other words 10 DB d but if we measure the yagi from the isotropic we say that the antenna has a gain of 12.15 DB I there are a couple of examples of this in the text now let's take a look at figure 9 - 4 to see how beam width is described how wide would you say this lobe is well by convention the beam width is the in degrees between the half power points half power is negative 3 dB the diagram didn't have the 3 DB line drawn in so I've added it here is the red line I've also made note that the left hand line going through the 3 DB point comes out at 3 45 degrees and on the right hand side at 15 degrees so the angle between them is 30 degrees but note something 3 DB is a half a s unit if we tolerate going down just a single s unit we see that an s8 signal is to be had at 40 degrees but we've got to use a generally agreed upon convention for beam width and that's at the 3 DB point but I do note that for antennas most commonly used in amateur service beam width is only a general indication of how much your signal is concentrated figure 9 - 5 appears in some test questions and is a rather arbitrary antenna pattern with the main lobe back lobe and 2 side lobes note here that there are no sharp nulls let's read this diagram closely as is conventional everything is measured from the peak of the main lobe so all the DB references are referenced to the power in the main lobe which was off here to the right and note here that a more normal convention for circles is followed in the degrees are positive from the x-axis going counterclockwise the circles in the diagram are in dB it shows negative 3 6 12 and 24 I went ahead and drew in the negative 18 DB line in red auerbach lobe is halfway between minus 12 and minus 24 so we can estimate it as being about 18 DB down relative to the front lobe by the way that's a front to back ratio of 18 DB which is actually pretty good a normal HF yagi might eke out 10 dB anyway the side lobes are between 12 and 24 DB down it's halfway between minus 12 DB and minus 18 DB and the book says to pick 14 DB which as a rough estimate is good enough now the book provides three ratios front to back which is widely used front to rear which is something I've never encountered in 40 years in ham-radio and front to side which works in this case this diagram but in the case of a real antenna you'll probably be dealing with multiple side lobes and I've not seen this particular ratio used before either now let's move to the topic of radiation resistance the book defines it as a virtual resistance that if it existed would dissipate the amount of power that in fact is actually radiated by the antenna the equation given is simple there's the wire resistance called ohmic resistance and everything else is radiation resistance yes well real life is not that simple let's take a look at an example in real life in this case found in my own backyard the antenna in the foreground is my butternut hf9 V vertical you can see the various traps that are used for different bands note the trees which by and large are also vertically polarized and they will absorb some of the radiated energy there's a great big metal propane tank not far away in a chain-link fence around the dog run that's close enough to affect the antenna pattern a bit but the biggest item interfering with the hf 9 v is the 22-foot long aluminum antenna mast holding up my large horizontal loop it's close enough to react rather strongly with the hf 9 v i plan to replace this mast with one made of fiberglass at some point to see how much i can improve the performance of my vertical my point in showing this is to say that everything around an antenna and I do mean everything can affect antenna performance the easiest way to look at feed point impedance is by looking at a dipole this is quite reasonable given that the dipole is the basic unit in nearly all antennas this diagram shows the dipole and the current and voltage distribution along the antenna with the current in green and the voltage in magenta no current can flow at the ends of the antenna simply because there's no place for it to go so the current is zero at the ends the current is maximum at the middle the voltage is highest at the ends and lowest in the middle in theory approaching zero at the midpoint we recall that the feed point impedance equals the voltage divided by the current the impedance is lowest at the middle given low voltage and high current and it rises as you move toward either end so the point where it is theoretically infinite at the ends the feed point impedance depends on many factors for example it tends to go down if the antenna is at less than an ideal height in real dipoles it Heights common for ham radio operators the impedance at the middle is on the order of 50 ohms and the impedance at the end is on the order of a few thousand ohms by the way given that the highest voltage is at the ends of the antenna you want to keep these up and away from anyone that might touch it this is particularly important if you're making an inverted V every part of the antenna should be higher than a wandering neighbor might touch can you feed an antenna somewhere other than the middle certainly you can feed it anywhere even at the end the key of Cour is to match the transmission line impedance to the antenna impedance in ham radio we use lots of coax that has near 50 ohm impedance such as RG 8 RG x RG 58 LM are 100 LM are 400 RG 2:13 and so on which leads us to want to feed dipoles in the middle there are other types of transmission lines available with different characteristic impedances depending on the situation then of course the feed line impedance needs to be matched to the transmitters output impedance nearly always 50 ohms in amateur practice if there is a feed line impedance mismatch at the antenna it can change the apparent impedance that the transmitter sees that's where we insert an antenna tuner my 80 meter full wavelength horizontal loop definitely doesn't have a 50 ohm feed point I use a balun to help translate the feed point impedance into 50 ohms then I run the coax down to my antenna tuner which connects to my rig let's talk about polarization polarization refers to the plane of the antennas electric field which is sometimes called the e field remember we talk about radio waves as electromagnetic waves so in addition to an electric field there's a magnetic field which is at 90 degrees sometimes called the H field in this diagram from the ARRL handbook the direction of propagation is in the direction of the x axis the e field is parallel to the ground so this waveform is emitted by a horizontal antenna shown here we use the right hand rule to find the direction of the magnetic or H field we point our right thumb in the direction of propagation then our fingers point in the direction of the magnetic field just as the point of clarification waves don't wiggle through space like this they travel in a straight line but the graph gives a representation of intensity note that the E and H fields are in synchronization except for the 90-degree orientation difference now and this is important a vertically polarized antenna will have a vertical electric field and the magnetic field will then be horizontal so by and large you can determine the electric wave polarization by looking at the antenna elements a dipole provides a horizontally polarized electric field and a vertical antenna produces a vertically polarized electric field here's an example of a vertically polarized VHF antenna you can tell the polarization direction by looking at the element orientation now I want to look at figure 9-6 the book talks about this being in the H field direction that means that the antenna in question is horizontally polarized this drawing which appears on the test shows the elevation profile for the antenna pattern meaning how much the gain changes with changes in elevation it's like looking sideways at an amateur station and watching the beam reach into the sky on the figure we see that there are four lobes pointed in the forward direction at seven and a half degrees twenty-five degrees 45 degrees and 73 degrees and three tiny lobes pointed backward there may be side lobes but we don't see them in this diagram because we'd be looking at them and on now this antenna is typical of yagi one key definition is that the elevation from the antenna providing the most powerful lobe is the antennas effective elevation angle this diagram shows clearly that there's also lots of energy in the other forward lobes why is this important because the power from each lobe will hit a different part of the ionosphere and be reflected to different places the power in the lowest low will likely travel the furthest but not always a key takeaway from this diagram is that antennas usually do not propagate directly horizontal to the earth but rather the energy tends to travel up a bit that leaves us with one more topic that of an antennas bandwidth let's take a look at this antenna meter which shows SWR as a function of frequency by definition bandwidth is the width of the frequency spectrum that provides an SWR better than a given reference the book uses one point five to one as a starting point now let's look at this almost all transceivers can handle up to a two to one SWR we see here that the two to one bandwidth is wider than the one point five to one and many modern transceivers with built-in antenna tuners can handle up to a three to one SWR and the three-to-one bandwidth is even wider needless to say there are myriad ways to compare different kinds of antennas there's a lot of folklore out there on the subject some of it true and some of it's silly generally the best antenna for you is the one that works the best for you which could be entirely different from someone else's setup you may have to consider the amount of space you have which can cramp your antenna installation perhaps you can put a dipole but can't get it very high in the air every antenna installation all of them are compromises of some sort and real antennas seldom perform the same as their theoretical cousins this is why antenna experimentation is so popular the ARRL s magazine qst devotes an issue every year to the topic of antennas a couple rules any antenna is better than no antenna height helps full size helps beyond that world of exploration lies before you and that's it for this lesson the next lesson focuses on practical antennas this is a good time to review section 9.1 and study the test questions if you have comments questions concerns or spot an error please comment either on youtube or on my website you can find a complete list of these extra class videos at WWE net / extra if you would like to support the making of these videos you can do so by clicking on the circle with the little eye in it until next time I wish you the best of luck in your studies and 73

Channel: David Casler

Views: 306,216

Keywords: antennas, antenna polarization, isotropic antennas, Dipole Antenna (Invention), feed point impedance, Amateur Radio (Hobby)

Id: n8_iSL4xKj8

Length: 35min 16sec (2116 seconds)

Published: Tue Apr 07 2015