Antennas 101 / How does an antenna work

okay in this video I want to talk about antennas because we seem to be living in a wireless world we use our cell phones Wi-Fi Bluetooth and Internet of Things is growing in popularity with the Loret system so without the use of a proper antenna system you'll be able to communicate at all so that 10 is an important component in the wireless world now if you've been involved in a radio project you probably seen an antenna like this this is called a helical antenna and they're very popular because they're very short they're shorter than the quarter wave monopole they're vertically polarized and they're welding the helix it's like a spring so when a diameter of this antenna is a lot smaller than operating wavelength it will operate in its normal mode now in the normal mode the radiation pattern of this antenna will be the same as 1/4 wave monopole okay the radiation pattern of a quarter wave monopole is the same as this helical antenna operating in normal mode and we call a radiation pandered omnidirectional equal in all directions as to the shape of a doughnut I don't have it doing it but I have a bagel it's basically the same thing so that would be the radiation pattern of this antenna except in real life it would be a very large bagel now if we look at the top view the horizontal view you can see it's omnidirectional if we look at the vertical view the side view we can see all the powers coming off the sides of the antenna perpendicular to the antenna and we have a null on the top and we have an O on the bottom now if we look at another antenna this one here this one has the same operating frequency that as this antenna so this is a gain antenna this is a 5/8 whip base loaded and so gain antenna so the usually larger than its counterpart because it has a bigger aperture now when we talk about gain antennas it is it does not create power because it's a passive device what it does is focuses the same power density and in a certain direction so if we look at the vertical pattern and again antenna it squeezes the lobe like this to get more power in this direction so it takes power from the top of the lobe and a bottom the lobe and it squeezes it to get more power this direction so we're not creating power we're just focusing in the power okay if you look at the back of your stereo where your speakers are connected you'll see a label they're saying 8 ohms output or 4 ohms output now if it says 8 ohms output you have to hook up a dome speaker or an array of speakers and equals 8 ohms to the output of your stereo and if it says 4 ohms output you have to hook up a 4 ohm speaker to your stereo so we do this to get maximum power transfer from you stereo to your speakers this holds true also for an audio amplifier now if you have an audio amplifier with output impedance of 1k ohm and you hook up an 8 ohm speaker directly to the output of that amplifier you'll get a mismatch and you'll get maximum power transfer they'll actually be shorting out your amplifier so what you could do is add a matching transformer and they look something like this with a primary impedance of one tail and a secondary impedance of 8 ohms so now you can match the output up of the amplifier to the speaker you'll get maximum power transfer now that's what's true for a radio system so if you had a radio that has a 50 ohm output and it's feeding an antenna with a 50 ohm input so we're getting maximum power transfer between a radio and antenna but now the antenna has to match the free space impedance of 377 ohms so basically the antenna is a matching device it's like a matching transformer that matches the 50 ohms to 3 space of 377 ohms and if you can do this you'll get maximum power transfer from the radio to free space so you'll get maximal electromagnetic radiation from the antenna so we'll look at a couple of antennas two common antenna so that I could actually do this so here's our setup we have a radio with a 50 ohm output and feeding a transmission line that has the characteristic impedance of 50 ohms and that's feeding our antenna now the two antennas were to look at or the dipole halfway of dipole that has impede input impedance of 73 ohms and a quarter wave monopole that's impedance of 37 ohms so we're going to get a little bit of a mismatch an amount of mismatch is indicated by the VSWR value that's the amount of RF power that's reflected back from the antenna back to the transmitter now since the VSWR is less than two to one we could accept that so next we're going to look at the monopole antenna and we'll actually see how we could actually build one of these okay I have a piece of coaxial cable this is Archie 58 it's got a characteristic impedance of 50 ohms I have an RF cable on one end and on another end it's open circuit so if I turn on a transmitter all RF power will come up to open open end and all of it will be reflected back towards the transmitter so I'll have a very high VSWR and there'll be no radiation coming out of the cable but if I strip back the cables and bring out the center conductor and the braids like this or I got myself a dipole antenna now I have maximum radiation now the input impedance of this antenna is 73 ohms so have a little bit of a mismatch so 73 divided by 50 is around 1.4 that will be the VSWR and it's less than 2 so we could accept that now because the current in the braid is in the same direction as the current in the center conductor it enables this antenna to radiate okay i reconfigured my dipole antenna into a quarter wave monopole with a ground plane and you can see here my quarter wave radiator and I have four radios which act as my ground plane now the ground plane is actually like a mirror so the antenna see is another quarter wave radiator in the reflection of the mirror so it acts like a dipole now the input impedance of this antenna is 37 ohm but if you droop down the radios at a 45 degree angle you get the input impedance up to 50 ohms so next we're going to look at how it builds one of these that you use in your project okay if you're going to build one of these quarter wave monopole ground plane antennas to replace one of these helical rubber ducky antennas expect about a 60 P increase in signal which could double your range because these helical rubber ducky and tenants are pretty inefficient so what you could do before you start building your antenna this build went out of coaxial cable like I showed you in the last clip now you can like the performance then you can start building your permanent antenna so to start off get yourself a chassis mount female and connector like this one here and that's four holes for mounting it onto a chassis we're going to use those cansada and our radials so then get some electrical wire like this and strip out strip out the copper and cut five pieces of copper the same length and soldering four for your radials and one for your radiator okay here's the formula to calculate the length of the four radials and a radiator so the velocity of propagation factor is built into this formula so I built my antenna for 2.4 gigahertz so you see here the formula I worked out to one point one eight inches for my radials in my radiator now I've been down my radios at a 45 degree angle so I could get a better match at 50 ohms now when you design an antenna for a lower frequency so the lengths will get longer so if they get really long you might have to go into some stiffer wire like maybe welding rods but you can pick up at a hardware store well can you to complete your project you can hook up the transmission line up to the connector on your antenna if you're doing a short run you could use rg-58 cable but if you're doing longer runs you should use some low loss cable like RG 213 or LM are 400 so if this antenna is going outside you should put some hot glue on the base of the radiator to keep the water out down to mount this antenna you get a little short pipe and bring it off over the cable and over the connector and then cut slits in the pipe you can clamp it down with with a stainless steel hose clamp then it gets a vulcanizing tape and some black tape and seal up the whole antenna now the mount this antenna will be very easy now because you have a pipe which you can mount to the side of a building

Channel: 0033mer

Views: 470,193

Keywords: Monopole, 1/4 wave, whip antenna, dipole, ground plane, coax, VSWR, Omni, gain antenna, propagation, RG58, N connector

Id: oiBi9RbNBUY

Length: 8min 24sec (504 seconds)

Published: Fri Feb 10 2017