How to Build a Yagi Uda Antenna

How to Build a Yagi Uda Antenna

Transcript of the video: Yagi Uda antennas are quite popular mostly in VHF and UHF bands,  because the size is reasonable in those bands. It is also used in HF bands like 20 meter and 40 meter. Then in VHF, the antenna can be used both the 2 meter, 4 meter and six meter. But most often you make a VHF Yagi Uda. The Yagi Uda antenna has multiple elements. The most important is the driven element which is usually either a usual dipole or folded dipole. And then, behind that there will be a reflector, which is slightly longer about five percent longer and in front you have the directors.

This is the schematic diagram of a Yagi Uda antenna. The structure on which all the elements are mounted is known as the boom, and the driven element which is usually dipole. This is a usual dipole, half wave dipole, one half will be, of the half wave dipole will be, half the wave length of the required frequency. Quarter lambda, this is also quarter lambda, together they will form a half wave dipole antenna. The feed points are seen here. A usual dipole antenna, you will not connect the feed point to the boom, it will be insulated. This will be insulated and similarly the directors in front and reflectors in the back can also be insulated from the boom. Or sometimes in certain designs they are connected by drilling in the center of the boom and that is on the side and you just connect it with a screw,  because the RF voltage in the center will be zero. So there is no harm in connecting in the, to the boom also, but there will be difference in the length of the elements depending on whether the elements are isolated from the boom using usually a hydraulic clamp in case of VHF and UHF antenna or some other method in other types of antennas. So this is the driven element typically a half wave dipole with a quarter lambda on either side. And this is the reflector element. That is the signal from a half wave dipole will go in both directions, in this direction as well as in this direction. But not much will go in this direction. So a half wave dipole has a gain of about 2.15 decibels compared to a isotropic, compared to an isotropic antenna. That is actually the gain is expressed as dBi, compared to an isotropic antenna, which radiates all around spherically. So in this direction there is hardly any radiation, so it has the radition in both these directions. So we want to prevent the backward radiation and make it as a beam antenna which will beam only in this direction. That is why this configuration of Yagi Uda antenna is used. So just like a mirror this reflector will reflect the radio wave signals coming in this direction and it will go here and these will be something like focusing. Each director will focus the beam so that there will be a high front to back ratio, that is backward radiation versus forward radiation. A good multi element Yagi Uda antenna can have a front to back ratio of about 20 dB that is reverse signal will be 20 dB less than the forward signal. Similarly an antenna can have up to a gain of about 20 dB. But it is all depending on the design and also the number of elements. Adding a reflector will give almost a five percent increase and one director might give one percent increase. So the number of directors as they increase it is not a linear increase it increases in a diminishing way and at a point it will be not worthwhile increasing the number of directors. But since the length is small for a UHF antenna the number of elements are usually much more compared to a VHF antenna. When you come to a HF antenna it will be much lesser because the length will be too high to handle because if you have a beam antenna usually you need a rotator also, antenna rotator. Otherwise you can’t work stations in other directions. So having a 40 meter beam up there with multiple elements and an antenna rotator is quite a strenuous job and the rotator everything will have to be very powerful. So most popular these are on VHF and UHF. In UHF I have seen people having 20 elements or more number of elements for UHF antennas. This is the folded dipole which I mentioned as one of the possibilities for the driven element. Two quarter wave lengths, with an additional half wavelength added as a loop above it  and this is the feed line. This can also be used as a driven element for the Yagi, you would have seen in the yester years before cable TV became very very popular a lot of beam antennas for television used to have folded dipoles. The difference is that the feed point impedance will be around 300 that is 280 I would say so that in those days the feed line used to be a ribbon tape for TV that will be having a impedance of about 300 ohms, while the usual dipole with this without this additional Loop will have an impedance of around 70 ohms. So that will fit or compare with the impedance of a TV cable which has an impedance of 70 ohms. So folded dipole you would use a feed line which is parallel line that is it was usually a ribbon tape while simple dipole, TV cable used to be of about 70 ohms impedance. That is the difference between a folded dipole and a simple half wave dipole. Now you would have heard of gamma mats when you implement a connection between a coaxial cable and the driven element of the Yagi Uda antenna. The schematic diagram is shown here. This is the driven element. The advantage of using the gamma match often is that I have seen, I have shown you that the driven element will be cut into quarter wavelengths. But if you use a gamma match you can use a full half wavelength driven element which can be even mounted on the boom and connected to the sleeve of the coax. This portion is a zero current region. Then you need a capacitor to tune out the inductance of this segment, connecting segment. That will be about seven picofarads for one meter length of the antenna so you can imagine depending on the length of antenna whether it is VHF or UHF what will be the capacitance. Here this region is known as a gamma rod. Typically it has to have half the diameter of the driven element but here both are shown as same, simple for simplicity. Then you have, you have a shorting bar which connects the gamma rod to the driven element. Here it is connected electrically, here it is connected electrically, but there is no connection here from this central portion of the coax, there no connection, you have to have a capacitor here. So what is usually done is that you calculate the approximate capacitance needed based on seven picofarad for about one meter length and usually what is done is you place a variable capacitor here tune the antenna for the correct frequency and then you find out the capacitance of this capacitor and then you can replace it with a fixed value capacitor.  That is one method of tuning the antenna. This is implemented usually for HF frequencies but it can also be used for VHF. The length of the gamma rod will be around five percent of the length of the driven element and the spacing between the gamma rod and the driven element will be about 0.7 percent of the length of the driven element. Sometimes instead of using a capacitor, a coax may be used for the gamma match. Here you can see this is the connecting stub and this is the gamma rod. In this region what is done is that this is an aluminum pipe and inside you can see a piece of coax without the in outer covering. So the outer covering of the coax is removed and the insulator and inner conductor are placed inside this gamma rod and the sleeve of this feed cable is connected directly to the boom while the inner conductor of the feeder is connected to the inner conductor of the coax kept inside this tubing, and this tubing can be slided for tuning. So that will change the capacitance of the combination of inner conductor of the coax within the tubing so that this will act as a variable capacitor. Tuning is done by moving this stub this way or this way depending on how you want to adjust the feed point capacitance here. So this is one method of implementing a gamma match without using a variable capacitor.