Basic Antenna Theory (HF Dipole)

hi n4h here one of the patreon supporters of this channel asked if i would cover basic antenna theory so let's dive right in okay we we have a transmitter that's our radio it puts out power we call watts now let's talk about that for a second power p there's a formula that we've had to learn for our for our test right uh power equals e for electromotive force voltage times i for current so voltage times current produces power which we measure in watts so you know we we will say our radio is putting out 100 watts but really it's putting out a voltage that is producing a current and when you multiply the voltage times the current or by the way current times voltage it will give you the amount of power that's being produced by your radio okay so you know voltage's potential for the reason we call it potential is it means it's it represents the potential to move electrons in a conductor uh literally ripping them out of the outer shell the valence ring of an atom okay so uh you know just know that what's really happening on our feed line these two lines here we'll we'll say that's our feed line could be coax could be a ladder line what have you parallel wires you know are typically a ladder line and they'll have you know the little the little things holding them together keeping them at the same distance so but that's a feed line all right so what goes into the feed line is actually voltage which produces a current and the amount of that can be measured as watts now why does our feed line not act as an antenna okay why why is it that when we connect that feed line to our radio and it starts transmitting why does the feed line not radiate the power it's because the radio is producing alternating current ac okay not to be confused with the alternating current coming out of your uh wall sockets in your home although that is alternating current but that is a lower frequency depending upon which part of the world you're in it could be 50 hertz 50 cycles per second or it could be 60 hertz now what we mean by cycles per second you know the sine wave that represents alternating current 0 degrees 90 degrees 270 and 360. you know and really it makes a complete circle or a cycle okay and the number of times it does this in one second we call frequency and we measure that in hertz today years ago it was measured in cycles believe it or not it made sense right but in honor of heinrich hurts who did a lot of research in the area of radio frequencies to honor him we started referring to frequency in hertz instead of cycles so you'll hear mega hertz today where years ago you would hear mega cycles okay so in your wall outlet that is only happening about 50 to 60 times per second rf radio frequencies simply means we're up into a realm where it's happening a lot more often per second so for example at 14.2 megahertz you know roughly the middle of the 20 meter band um this is happening 14 million 200 000 times per second that this is completing a cycle okay and that what what's happening with that cycle in alternating current is it's going to go plus and then it's going to go back to zero and then minus and back to zero we call that a cycle so what we mean by the plus and the minus the voltage is going to go up and the current's going to go up now there are 90 degrees out of phase okay just uh just know that but the voltage happens and then the current follows so we'll we'll talk about that more in another slide so just wanted to establish the fact that what your radio is really producing is voltage that's creating current and then when you do the math we get a figure that is watts all right but why does the coax ladder line whatever you're using why does it not radiate our signal like the antenna will and it's because as this alternating current is alternating this for example just look at here this arrow represents the direction of the electron flow on this conductor that represents the air the uh electron flow direction on that conductor they're always equal and opposite in um in current in and in voltage so they cancel one another out whatever the level of current is here and voltage it's going to be the opposite over here and so it cancels out and in fact without an antenna with this feed line just being an open circuit here what's going to happen is um you know waves are being created waves radio waves are being created but they're equal and opposite of one another on each of these conductors so they're cancelling one another out uh let me mention why these waves even come off of these wires in the first place so look at this illustration here that's a wire and this is uh i for current current's flowing in that direction through the wire now literally you know what's happening is it's really a chain reaction if the current's flowing in that direction that means on this side of the wire there is a positive potential a plus if you will and that's going to attract the electrons out of that outer shell of the atoms that make up this wire and pull them along and of course each time it pulls one out of an atom another one comes along and replaces that one but then it gets pulled so it's a chain reaction now if the current is flowing in the wire in this direction and this is called the right hand rule that's a hand you see the thumb the thumb points in the direction that the electron current is flowing you wrap your fingers around the wire and that will show you the direction of the electromagnetism the the what's creating our wave our radio wave okay now if you were to reverse that and the thumb was in that direction then the fingers would reverse and the wave would reverse and what happens is that that is happening at whatever frequency we're on so you can imagine 14 million 200 000 times per second at 14.2 megahertz okay so that's what causes a signal or a wave to come off of that wire uh electromagnetic wave all right and that's how we communicate the strength by the way of this wave is determined by the amount of current that is flowing in the wire at 100 watts it's some amount of current at 1 500 watts it's going to be a lot more current flowing in the wire and these waves get stronger stronger and stronger and you know they can be picked up hundreds and even thousands of miles away all right let's go to the next slide and we'll talk about the basic antennas called a dipole two poles two wires well how do we get a dipole um you could essentially you could take your your transmission line your feed line and strip it back and fold it out like that and and you you would wind up at some point you would have a usable antenna but we generally you know we'll have a center point a balance or a you know a balanced to unbalanced um trans uh transformer we call a ballon um we would have that in the center although you don't absolutely have to have a balance in certain situations so let's just talk about a strict dipole cut for one band the impedance at that center point you see here i have 73 ohms it's approximately 73 ohms so the reason you wouldn't absolutely have to have a ballon is if you were pretty pretty precise in the measurements you made for each of the wires on the each side of this dipole again dipole 2 poles because if you were precise enough then the current flow in each of these is also going to be equal and by the way opposite but since they're not running parallel they create a wave that doesn't get canceled out now you see here i equals max the middle point of the antenna here the feed point that is where most of the current is flowing so it's actually true that most of your signal is coming from near to the middle of the antenna so the ends you can see here it tapers off the current flow begins to drop and drops uh to you know virtually zero on the ends of the of the wires so that's why it's important to get get at least the middle of your antenna up high i mean if you put the ends up high well high that's also good um and with a dipole cut for uh a particular band and if you get that center point and the ends up at least a half wavelength above the ground so you do your math for a full wavelength for example here look at this um now i've already taken into account end effect in this calculation if you're not familiar with end effect let me give you a quick synopsis our normal formula is 300 meters per second which is 984 feet if you're doing your if you're doing your calculations in america and you're using a ruler or tape measure that is in inches and feet then we don't you know we won't necessarily use the 300 we will convert it over to feet and it's really 984 okay but if you take 984 multiply it times .95 point nine five that will give you nine roughly nine thirty five is nine thirty nine thirty four point eight and what that's doing is that's adjusting your formula for end effect realizing that electrically this wire is going to look about five percent longer than it really is honestly when you put an antenna up a dipole up surrounding objects are going to affect your match as well so there's always a little bit of adjustment if you use 935 um and and then you you know give it a couple of inches of slop in there then it's it's easier to trim to get it precise where you want it like in the middle of the band than uh than it is to add wire okay but 935 will get you pretty close uh divided by the frequency in megahertz now you see it says it's 65.8 feet that's the answer to that well that means that a a full wave see this is half of a sine wave right here this is a half wave dipole imagine that that continues down in the you know in the negative direction um so this 65.8 foot figure represents the entire length of the wave half wave dipole is only going to need to be a half that and really it's going to be two quarter wavelengths so if we take that figure divide it by four we get 16.5 feet so that means that we would cut this antenna wire to 16.5 feet per side okay 16.5 feet now again once you get the antenna up surrounding objects um can affect it a little bit and so uh you may need to trim the ends a little bit that's why i say you know make it just a little bit longer than it than the math says it should be okay now by the way if you do erect the antenna as what we call a flat top where the ends in the middle are at the same elevation and you get it up at least one half wavelength okay so if you take 65 really really what you want to do is use the 984 okay to get the the real number uh 984 divided by 14.2 that's the height cut well cut that in half let's let's do the math on that i'll pull up a calculator here so what we want would want is we would take 984 divided by 14.2 0.2 69 almost almost 70 feet now we can divide that by two higher is better okay but what you want to shoot for is at least in this case 34 roughly 34 and a half feet 34 35 feet for a 20 meter dipole if you get the middle and the ends up at roughly 35 feet i'll tell you what's going to happen the antenna is going to be able to produce for you somewhat of a figure 8 pattern off the broad side of these wires so let's say we're looking down from an airplane or a helicopter at our antenna and the wires run this way all right there's the center point our and our feed lines going straight down to our radio what's gonna what the signal is gonna look like coming off of this from a helicopter looking down is more like a figure eight uh broadside to the wire so you can see here not a lot of signal uh here now i'm exaggerating this a little bit this this would be more spread out okay all right but you don't get as much signal on the ends why because look the current on the ends drops to virtually zero so not much signal in these directions with a horizontal dipole that's at least a half wavelength above the ground and again this even this pattern assumes that there aren't objects nearby that could skew it a little bit you know metal objects gutters uh buildings uh like like like a storage shed you know things like that that will skew it just a little bit yes and again that's why it's always good to get your antenna as high as possible so you want to shoot for a minimum of a half wavelength above the ground beneath the antenna and again that dipole will give you more of a figure eight pattern now there's also something called an inverted v where you actually intentionally droop the ends rather than putting them up at the same elevation as the center point and what that does is that skews this pattern a little bit more so it's more like almost like a circle not quite but you know it's again surrounding objects will affect that but when you drop those ends like that it it will skew that figure eight pattern and you get a little bit more even coverage now this is very crude but i want to show you a little illustration here here's that figure eight when you skew it and make it a little bit more circular you see that you don't get quite as much distance as you did with the figure eight in those two directions though right but you get signal in those areas where you had no signal before okay that's very very generalized and crude but that's the advantage of a of inverted v if you've ever heard someone talk about an inverted v a little bit more omni-directional than a pure dipole and by the way again this is assuming that the antenna is cut for just this band there are multi-band antennas out there and they have these what we call lobes like this this figure eight these are called lobes and the multi-band antennas which you know it's one and one antenna length but it's working across multiple bands these are different for each band what directions you have good lobes in what directions you have nulls will vary okay but back to our discussion on how do we get a signal into the air so again the current you see the little uh arrows here uh down up right the light is when it's when this side is positive the darker blue is when this side is positive so it's going back and forth back and forth imagine being those electrons being pushed back and forth pulled really pulled toward the plus but then the plus switches over to here and then they go back here and then the plus switches over here and they go back in that direction and again chain reaction it must be frustrating for the little electron all right but again the idea is to get these two sides cut to approximately a quarter wavelength per side now our coax is generally 50 ohm impedance and the antenna feed point is 73 right the output of the radio is 50 ohms in order to transfer all of the power from the radio into the feed line the feed line needs to be 50 ohms as well and ideally the antenna would be 50 ohms a dipole is not exactly 50 ohms it's approximately 73. so look up here the antenna is 73 ohms the coax is 50. that is approximately a 1.5 to 1 standing wave ratio that's actually considered acceptable that's any if you can get a 1.5 or less uh go with it you're you're absolutely fine with that but that's the characteristic impedance of a dipole two wires two poles and then the feed line is not going to radiate because the currents are equal in opposite directions on it all right that's how the radio wave sets up you know it just in this case they're not canceling one another out and we get this half wave off the two quarter wavelength pieces of wire all right so let's move along to the next slide this one i want to discuss safety here that's the real purpose of this slide remember the current is at a maximum in the in the middle so you get more of a wave more signal strength here at the in the middle of the antenna at the feed point the voltage here on the blue line that is at a minimum there but watch what happens out on the ends remember the current goes to virtually zero the voltage is the highest on the end so the ends are dangerous this could electrocute really it causes an rf burn i've i've had that a few times and it's not pleasant um the 90 degrees well what's happening is if you see here the voltage is zero here and maximum here the current is at maximum here and zero here and that that is if you think about it that distance there represents 90 degrees of the sine wave okay so what's happening is the voltage is at a maximum on the end of the wire where the current is at a minimum so that could shock someone so what my advice to you is if you're doing a horizontal dipole it's usually no problem because you're gonna you're gonna try to get that thing up you know a half wavelength above the ground with an inverted v you're going to droop the ends you're going to let them slope and ideally by the way you want to be sloping somewhere between 90 and 120 degrees at the apex up here at the midpoint you want that angle to be between 90 and 120 but i will tell you you know you can get outside of those boundaries a little bit and it'll still function but at the ends you don't want to let those ends get close enough to the ground that someone could touch them because they could get injured and so what i my general rule is try to get them about 10 feet above the ground at least eight feet above the ground on the inverted v and achieve that 90 to 120 degree angle at the apex so you remember with an inverted v if your antenna is going to be more like this so the there's the feed line going up to the middle and here are the ends you want these ends to be at least 10 feet off the ground just for safety's sake so a tall person couldn't reach up there with a finger tip and touch it somebody going through your backyard even if they're trespassing and see that they might get curious and there you are in the shack transmitting so just know that those ends of your antenna are high voltage and they could hurt imagine if you're producing 1500 watts from this radio the amount of voltage that would be present on the end of that wire so let's re recap right quick the reason our feed line does not radiate is because the current end voltage is always equal in opposite polarity and they cancel one another out all right otherwise these waves of force here would radiate off these wires they and they do but they cancel one another out and again what we're really outputting is voltage that creates current and then we measure the result of that in watts p for watts e for voltage electromotive force and i for current all right and our signal is alternating current so again here in this example i'm alternating current it my alternating current frequency is 14.2 megahertz 14 million 200 000 times per second it is cycling these electrons are going back and forth at a at a blazing rate all right but they still cancel one another out until they get out here and the open circuit is no longer there it now has a wire to travel into and they are not parallel anymore so they're not equal and opposite and it sets up this wave the wave is the strongest in the middle so it gets you the middle of your antennas should be at the highest point if you're going to do an inverted v just get the middle up at least a half wavelength above the ground beneath the antenna and again remember the feed point impedance approximately 73 ohms with 50 ohm coax you will have a 1.5 to 1 swr and that is plenty plenty good for amateur radio all right and again the math remember i'm using 935 instead of 984 to get my antenna wire lengths so i can know how what length for each side of the dipole and essentially you take the result of this divide it by four and it tells you how long to cut each of these and remember you're going to cut them just a little bit long and maybe an inch or two and then check your swr if you're really wanting to be you know at the lowest swr at 14.2 megahertz and you cut it a little bit long you know you may be the it may be the lowest swr is at 14.15 megahertz just trim a little bit off the each side by the way make sure you trim it equal on each side and if you get that close you might want to just trim in half inch increments okay and then finally there's the safety aspect of this remember the voltage is the highest on the ends and that could hurt someone so just be mindful of that when you install your antenna okay i hope you found that helpful and informative and hey i want to thank my patreon who suggested this video and i really appreciate all of my patreons speaking of patreon supporters i want to name a few names here these are some of the newest patreons who have joined as a supporter of this channel we have andrew dennis daniel scott william harold and alex i do appreciate your support there's a time commitment to doing this and some expense and so your monthly donations you know help offset some of the cost and you know it motivates me to continue doing this so if you appreciate the video content of this channel please consider becoming a supporter there are three different levels of support via patreon there's the associate level for five dollars a month there's the executive level at ten dollars a month and then vip at twenty dollars per month and uh you know every little bit helps believe me but for the executives and the vips there are some perks um there are some documents and other content up there that you might find helpful i'll mention a couple there's the glossary of amateur radio terminology it covers a lot of the you know terms that you'll hear thrown around uh when hams are having a conversation uh covers the q signals and what they mean and gets into the various controls on your radios your the buttons and knobs on a modern transceiver and uh gives a a brief definition of what each of those is for so that would be something that executives and vips would have access to and if you happen to be into portable operation um i created something called a sota primer some it's on the air where we climb mountain tops and make contacts and it's a point system you get awards i created the soda primer it serves as a type of quick start guide to help you get into summits on the air gives you some insight into how to get set up with that so those are just a couple of the items available exclusively to executive level and vip level patreons so if you would like to become a patreon supporter of this channel and help me continue this mission go to www dot patreon p a t r e o n dot com forward slash n for h n h that's patreon p-a-t-r-e-o-n dot com forward slash n4h and you'll see the three different levels and uh you know whichever one you're comfortable with please uh consider supporting the channel and of course it costs you nothing to like this video and subscribe to the channel which also helps our standing with youtube uh may drive some more traffic this way we can grow the channel and my my hope is to grow the channel to the point where not only do we have some influence over the manufacturers but maybe they'll even come to us and ask our opinion of uh things that they might be considering in a future product so their strength in numbers 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Channel: N4HNH Radio

Views: 676

Rating: 5 out of 5

Keywords: Amateur Radio, Ham Radio, HF antenna, basic antenna theory, how an antenna radiates, right-hand rule, why doesn’t coax radiate, feedline, coaxial cable, transmission line, feed line, Antenna impedance, how antennas work, how an antenna works, HF dipole, dipole, how does a dipole work?, Lobe, Lobes, radiation pattern, inverted V, electron flow, valence ring, BalUn, ladder line, Can coax radiate?, Can ladder line radiate?

Id: dM9Ww37afYs

Length: 25min 54sec (1554 seconds)

Published: Sun Sep 05 2021