Hello and welcome back to my blog. I hope that the information that you have seen so far has been helpful. With this post we will be continuing with our antenna series and be discussing Directional Antennas. So lets get to it…
Directional antennas are by a lot of people. As we talked about in the first part of this series, probably the most common use for a directional antenna is your TV. Whether you have satellite or over-the-air broadcasting, you probably use a directional antenna. With over-the-air TV, if you live close enough to the broadcast source, you may only use a pair of “rabbit ears” or maybe you used them when you were growing up. With the digital TV, the new antennas that have been made are also directional antennas. The big antennas that you might have seen outside peoples houses that have about 10 or so elements and are seem to be pointing in a certain direction, yep, directional antenna. The satellite dishes that are mounted to sides of houses everywhere are even directional antennas, parabolic dishes to be exact. Directional antennas are used in a lot of different functions other than ham radio.
What do you say we talk about the ham radio ones though? Yea? OK.
The most commonly used directional antenna is a yagi antenna. A yagi antenna consists of 4 basic elements, the Boom, Driven Element, Reflector and Director.
The above picture is a plan for a 2m Beam. The horizontal piece is called the boom. The element that is on the far left is the reflector, 2nd from the left is the driven element and the rest of them continuing to the right are reflectors. The boom supports the antenna structure and is the the mast is attached to. The reflector is the longest element by approximately 5% compared to the driven element and therefore does at it’s name implies, reflects the RF energy away from itself. The driven element is what the coax/feed point attaches to. The driven elements are approximately 5% smaller than the driven element and basically enhances the radiated power of the transmission and drives it in the direction of the shortest element.
Even though I have been a ham for over 15 years now, I never really dove into the home brew stuff that building antennas is included in so I really didn’t know much about yagi’s, other than the basics. I had planned on including “the formula” or set of them to help you build your very own. Come to find out the math that is involved in designing a yagi antenna is a lot! Any change that you make to the design changes the whole antenna. if you change the spacing of the elements, it changes the whole design. If you change the element diameter, it changes the whole design. I had no idea! Many things that I have read over the past few days has got my mind swimming with information and here I sit trying to dig out what I want. I think what it all boils down to is, rather than trying to do all the math yourself, use one of the many antenna design programs that you can find by doing a simple Google search or using the one I put a link for above.
Parabolic Dish Antennas
So let’s move on for now. Another type of directional antenna is the parabolic dish. This type of antenna is mainly used on the higher frequencies, since the parabolic dish part of the antenna has to be larger than the wavelength of the radio waves used. The dish antenna has some of the highest gains and produces a very narrow bandwidth. This makes the dish antenna very directional! Just a few degrees off center and you might not hear anything. This is one of the major reasons that, to me anyway, dish antennas are probably not a good choice for an antenna for the average ham user. They are, however, a very good choice for things like a relay link for a telephone or televisions system, wifi wan/lan links for data communications and satellite communications. Another use for the dish antenna is for RADAR because the need to transmit a narrow beam of radio waves to locate objects. Probably the most commonly seen parabolic antennas are satellite dishes on the sides of buildings.
There is another type of directional antenna that I learned a little bit about while doing my research and that is the quad antenna. The quad antenna is kind of a yagi antenna doubled, with the double offset 90 degrees. I read several articles, how-to’s, and blog posts about quads and there are two main things that I got out of everything I read. One: a quad and a yagi have basically the same gain and pattern of radiations. Two: a quad is more of a pain to build and maintain. So after reading all this, I’m asking myself, why would you build one over a yagi??? Some people may choose to build a quad over a yagi, and that’s all fine and dandy, but as for me, IF I build a directional antenna, it will be a yagi.
So guys and gals, I think I will end this post here, my brain is a little mushy right now and reading and writing. I hope that you found this post interesting and I ask that you share this post and my blog with your friends whether they are a ham or not. If they’re not, maybe it will help them get interested in the hobby. Our next post topic will be Antenna safety. I hope you will come back and read it as well. Also, please like my facebook page or follow me on Twitter, links to both can be found at the top right of this page.
73 de Curtis, K5CLM