Make a 1/4 Wavelength Antenna + Calculations

did I mention I'm wearing gloves because I put my thumb through the table saw hello ladies and gentlemen and welcome back to the current source my name is Derek and I hope you're all doing well amid this corona virus pandemic but today we're gonna make a quarter wavelength ground plane antenna using some brazing wire some brass tubing NSO 239 connector and some aluminum plate so I live in an HOA a homeowner's Associated controlled neighborhoods so I can't have antennas sitting outside so I'm kind of limited in what I can do so a quarter wavelength antenna is obviously a quarter wavelength of the frequency that you're operating on so it allows a nice compact antenna that I can stick up in my attic now recently installed a panel up there with some feed throughs so we'll climb up there shortly and install a couple of these but I made this one this is the 2 meter version and this design is everywhere on the web and it's been in publication in the amateur radio circles for a number of years but they always just show this aluminum panel with connectors coming off but they don't show you how to fasten them so I came up with a pretty decent way I think it uses a couple of clips and you form the metal into like a cup and it pushes down on the brass rod and holds it against the aluminum if you have a loose connection you can change the angle of the radiator and as we'll see this is a way to change the impedance of the antenna itself then when you really bang it down there pretty solid and they're held in place with the friction that this thing provides so I think it's a nice solution you do have to have a method to bend the metal and I'll show you how I do it with the brake but you could do it with the hammer and a vise now we're gonna test the range by using a repeater so I'm going to use my handheld radio here and the rubber ducky antenna on this thing is not all that great of course they serve their purpose but it's not going to be as effective as a really an Intendant like I just showed you we're gonna test this out by trying to hit several repeaters in the Orlando area if you don't know what a repeater is it's basically a radio station that sits off somewhere and it has a you a pretty high-performance antenna now it can receive your signal on a particular frequency and retransmit it using the same antenna on a slightly different frequency with a higher power level and with of course the better antenna so you can reach a lot more people and it's usually like you know several hundred feet to a thousand feet above average terrain so you can really get a wider range just by using a little thing like this but I live kind of far away from these repeaters and I thought it'd be nice to make a couple of antennas are the one I have here for two meters and another one for 70 centimeters which is like the 432 megahertz band so that's what we're gonna do today so I've gone ahead and cut a small piece of aluminum that we're gonna use as the base for our smaller antenna and I've chamfered the corners off here instead of doing the pinwheel configuration like I just showed you we're actually going to come off on the corners directly and the radios will come out like this Center conductor will be with the so2 3:9 connector so we'll drill a hole in the center mount this thing and then we use our little cup friction holders for the radials instead of cutting it out of mild steel like I did for that one I'm gonna make this out of this strap that I got from the hardware store it's kns precision metals number eight seven one six seven if you want to play along at home it's just a stainless steel strap and it is point zero to five inch thickness so I think that should suffice and I'll show you how I cut it and use the break to form the role to hold the radio in place so that's enough of that so let's go ahead and get started on the calculations okay so electrically we're gonna look at a dipole and a monopole a dipole is pretty much one of the simplest antennas that you can make they're basically just two conductors right separated electrically at the center and you're feeding one side here with your transmission line the other side with the other end of your transmission line okay now the free space impedance that means if this antenna were out in space it would be about 72 ohms I believe in free space just as a contrast our monopole is gonna be about 36 35 something like that free space impedance now as we bring those antennas closer to the ground physical earth that's going to change okay depending on the wavelength that you're driving with so it does have kind of an oscillation to the impedance value itself so anytime you install an antenna depending on the metallic objects around or how close you are on the ground you're still gonna have to tune the length of this thing now what determines our impedance well the length that you cut the antenna and the voltage and current distribution along that length okay so we have our current for a dipole is minimum right at the endpoints of the antenna it is at its maximum at the center feed point all right the voltage is maximal at the ends of the dipole and at a null in the center so we've chopped off the bottom half and tossed it in the trash and now we have this quarter wavelength section where a dipole I should have mentioned this before is a half wavelength this is one quarter wavelength okay for the driven element driven element for if you're driving with a coaxial cable is going to be connected to your center conductor and traditionally this ground plane here is connected to your shield so like you said the free space impedance is 35 ohms that's not going to work because our transmitter typically especially for amateur radio is operating at 50 ohms our transmission line I will call it TL is also traditionally 50 ohms why did I do that I don't know reverse umlauts [Music] and our antenna all right we want it to also be at 50 ohms and we do that to transfer the maximum amount of power from the transmitter through the transmission line to our antenna if we left this at 35 ohms as we send energy down here okay if there's a mismatch the power is not going to be absorbed by the load and radiate it out into space okay what's going to happen is a portion of that is going to get reflected back alright and we measure that on I'll do it on the vector network analyzer that is your return loss we want our return loss to be a low value so what we'll see is something we'll see something like this so if we go if we sweep the frequency or the VNA from I don't know 10 megahertz to 500 megahertz we should see our dips somewhere around here because I'm going to tune it for 432 so we'll get a lot of energy coming back out of band and then when we get to 432 megahertz if we tune this cut the lengths of wire correctly we should see minimal energy coming back all right and as we increase the frequency outside of the band we've cut it for we should see energy coming back we should see energy coming back to the transmitter as well now I need to make sure that my transmitter only operates within a certain bandwidth inside of this where the return loss is the lowest that means all the energy is getting out to the antenna which is what we want okay so I just realized as I'm editing this I never explained how I calculated the length so let me go ahead and do that real quick so we're talking about a quarter wave ground plane antenna for the 70 centimeter band that is 420 megahertz to 450 megahertz if we want to be right in the middle of that are our operating frequency needs to be 435 not 432 as I previously mentioned all right so that means if we take the speed of light which is 299 blah-blah-blah-blah-blah meters per second divided by our frequency in hertz that gives us zero point six eight nine meters now you can see why they call it this 70 centimeter band and you don't have to be this precise you're gonna have to make adjustments at so you could just chop off these last six digits and say 300 divided by your frequency in megahertz would give you roughly the same thing I know there are gonna be some people out there you have to take it up to six decimal places that's not true there's going to be much more environmental influence in this than you know taking it out to 32 decimal places it's just not necessary now we can see that at one quarter wavelength we have to divide that number we just calculated by four that gives us a hundred and seventy two centimeters for our radiating element length or center conductor length the radios will calculate in a second they're a little bit different RF travels at different speeds through different materials okay copper happens to be 95% the speed of light that's gonna change the length it's going to shorten it a little bit if we have a wire that has an insulation on it or like coaxial cables they have a dielectric and each different cable type has a different velocity factor so brass I believe is basically the same thing as copper so we're gonna take that 172 centimeters that we calculated before we're gonna multiply that by 95% and that's gonna give it give us an actual real-world length of one hundred sixty three point four centimeters for our center conductor okay now our radials okay this is my antenna there's any ain't she a beaut our radials are going to be the same as this this radiating element here multiplied by a 12% increase in length now that comes from the test book I don't know if that's historically like found empirically or what but I'm gonna stick to the rules on this one I just say 12% plus what we already calculated added back to that is 183 centimeters now I'm a new this is a tube so on the top I'm gonna actually install a brass brazing rod that's a little smaller diameter and I'll solder in place once we get it all tuned up you don't have to fine tune it going straight off the calculations should be good enough to get you a minimal return loss it should be perfectly acceptable to get you liked and WR of 2.0 or less than that but I'm nitpicky so I'm gonna tune this on a vector network analyzer when I'm done and I'll show you the process so for the dipole we have a current distribution along its length we have a voltage distribution we don't have the same thing here so the ground plane acts as a kind of electrical mirror so that we make up the other part of that voltage distribution and that current distribution now this determines our impedance now I need to bring this impedance 250 ohms so what I can do is I can bend these radials down and we'll see on the VNA that it brings it from 35 to 50 ohms and in the text it always says bend them to 45 degrees I've found that my particular height above ground and my configuration with all the metal crap around me I've got to pin this down to about 35 degrees okay so we're gonna design our systems so that we can loosen the screws and adjust them without actually bending on the things okay [Music] okay so now it's time to put on the center conductor I'm gonna grab my fattest roll of solder solder for those who don't like the way I say it I understand the meaning is a little bit different in Europe in the UK my apologies all right we're gonna preload this what I'm gonna do just put a length of solder I know why I have a really difficult sign a difficult time pronouncing that L and put this inside the tube so that when I flow the solder it will melt to the center pin we got my iron cranked up on Macs and I'm gonna go ahead and put a little bit of flux getting yeah everything's falling off my bench I'm having some allergy issues today I do not have coronavirus as far as I know but you never can tell can you so let's load that back up in there now that we have some flux going on I'm just gonna kind of leave it I'm actually on all the way let's heat up that center pin get everything flowing I'm gonna push it down in there applying heat to the brass tube I start to see a flow out of the bottom and then I'm gonna try to write it so that it's completely vertical and let it cool there we go that looks pretty good doesn't have to be exactly perfect but the more vertical it is the better off you're going to be and there we have it our very own 70 centimeter quarter wavelength ground plane antenna let's stick it on pole okay above my desk and we'll take some measurements we're starting to describe any antennas cube it looks like a little spaceship how about that alright so here is the Nano VNA that I'm using to make these measurements right now sorry it's upside down but it's got everything's plugged in just when you're gonna buy one of those watch out because there are a lot of fake Chinese ones out there okay and I'll probably do a separate video on this because it's a pretty cool little device and it's not a full-blown VNA yeah but those are very very expensive all right so I have calibrated out the coax any connectors that I have in line and an open short and load calibration at the antenna base without the antenna attached of course all this crazy stuff you see on the screen here the calibration offsets that are compensating for the feed line so let's go ahead and sweep it so we're scanning we're scanning from down here 50 kilohertz all the way up to 900 megahertz and let's put our marker on the screen and see where we are at so we are 43 ohms and resonant at 441 megahertz which is pretty good so if I want to decrease my resident frequency I need to increase the length of the antenna so I'm gonna pull out that telescoping section hang on one second all right so I pull that telescoping section out a little bit let's scan it again and you can see the frequencies decreased a little bit down to 432 so I need to push the telescoping section in a tiny bit more maybe another millimeter let's let's change the bandwidth here let's start at it started 250 megahertz then we'll go to 550 so we're right in in the sweet spot where I want to be so a little bit of fiddling and we got right on the money so I'm gonna go ahead and screw this into the wall up there in the attic and I'll take another final measurement do some tweaks that I'm not gonna show on camera so anyway it looks like a successful build here check it out and I flipped this around you got the antenna mounted here to a 2x4 using an RCA clamp okay so just got a I got a hose clamp that's squeezing the PVC - but cut some slits in it and it's mounted up there coax is coming through the PVC I'll put it through the feedthroughs and we'll test it out all right I got my HT over here connected the SWR meter most of you are leaders connected to the antenna so that's just see the KSW are actually is circuit no less than 1.5 to 1 which is pretty good I might do a little bit more pruning but I'm gonna call it quits it's really hot up there all right I'm connected directly to the antenna now by passing list of the our meter and one of the stations 4:44 0.075 I was only hitting an s 2 on the meter now you can't see it but you're gonna have to trust me so we're gonna see if we can get a better signal so if when I hit the repeater and it comes back it should come back with from Morse code and identify itself so all right let's see if we can hit it mm-hmm Victor x-ray hotel testing it beeped and I got better it's seven on that one so improve from an s 2 and s 7 that particular repeater is 17 miles away so let's try another one we had all right let's go to 4305 zero and I don't know where this one is but I was hitting that 7 we should peg in this 9 by now or better antenna Victor x-ray Hotel testing that's 9 all right it works so that wraps it up for today now you know how to make a quarter wave ground playing antenna and the calculations and some of the theory behind it not all because you know this video is already gone on long enough who I hope somebody gets something out of this and you know subscribe cuz there's gonna be more RF stuff more general electronic stuff and it's been fun so stick a fork in me have a good one

Channel: AmRad Podcast

Views: 45,656

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Id: haizSAYquUg

Length: 20min 8sec (1208 seconds)

Published: Tue Apr 14 2020