The Principles of the Loop Antenna article was in a newsletter of the MDXC. The next three were from a talk that Mike Bates and James Dale gave to the Northland Antique Radio Club’s Radio Workshop at the Pavek Museum of Broadcasting.
An antenna is known as “directional” if its pattern strongly favors a certain direction. A directional works by concentrating the signal in one direction at the expense of other directions. It is also commonly referred to as the “Beam” antenna. I am going to start with the earliest type of beam discovered, the “Yagi” Beam. This type of beam was discover by Professor Uda but the english translation was done by Hidetsuga Yagi. This design goes back to the 1920s! One would think today there would be better designs. I believe there is, and that’s why I am so interested in antennas!
The Yagi Beam
The yagi is very simple. The basic yagi consists of three elements, as shown in figure 1. The middle element is an antenna you are already familiar with, the simple 1/2 wave dipole antenna. This element is generically called the “driven element”. This is because this is the only element that is connected directly to the radio, it actually drives the whole antenna. The other two outer elements are generically called parasitic elements. One is called the Reflector (some CBers call it the “back door”) and the other one is called the director element. These elements get their name from the job they do. The reflector reflects RF energy, the director directs RF energy. There is no magic circuit located inside the elements, they are simply straight rods! The reflector element is typically 5 % longer than the driven element and the director is typically 5 % shorted than the driven element. How it works. See figure 1. As signal A comes in it strikes all three elements hence generates a current on each element. Remember we said that current on a wire causes it to radiate? Even though the current is very low, this current induced on the antenna actually re-radiates off the antenna again! Ok, back to the action, the signals are re-radiated by the director and reflector and arrive at the driven element in-phase with one another (the two re-radiated signals and the original signal). This basically means, the signals reinforce each other…and make the incoming signal much stronger coming from direction A. Continue reading
This is an area of hot competition among antenna manufactures. I am not going to cover how to mount your mobile antenna (Radio Shack sells everything you need to mount antennas), but the basic mobile antenna designs that most mobile antennas manufactures are using today. If you have just started reading my page and are new to antennas, you are going to be confused. You cannot just jump right into putting together an antenna without learning a few things about them first. You have to read “Antennas Basics”, “Coax Basics” and the “Verticals” sections first before you tackle this section. First we need to introduce a new term that we will be using to rate mobile antennas (gain really isn’t a good thing to use, since mobile antennas generally have no gain), “antenna efficiency”. This is how well the antenna converts your power (watts) to signal instead of wasting it as heat. An efficient antenna puts most of the power out as signal, so the range of 95-99% is a perfectly efficient radiator (all antennas waste some power, none are 100% efficient). Most base station antennas are 95-99% efficient. Say you are using 100 watts, and your antenna converts 95 watts to signal and turns the remaining 5 watts into heat, this is a 95% efficient antenna. Continue reading
4 Element Yagi Building
This article contains discussion of all the different antenna principles previously described elsewhere on this website. A thorough reading and understanding of the other sections are necessary to comprehend all the terms used on this page. It is not necessary to understand all the terms and theories described to build and enjoy this 4 element Yagi however. This particular section is geared towards the “freeband” CB operator – that is one who uses CB channels above (or below) the standard 40 CB channels. These CB “channels” are not legal for use in the United States – but they are quite popular any ways! The frequencies mentioned in this article are legal CB channels in some countries, check your local laws and act responsibly. This article shines little light on finding cheap aluminum or alternative methods of construction materials for Yagi antennas. It is also devoid of information on the physical design of Yagis. Sad but true, useable aluminum tubing in small quantities is expensive. And the physical design of Yagis is a subject beyond my limitations! What this article does do is provide a truly optimized 4 element Yagi design for the 11 meter “freeband” DX operator. Careful consideration was given the operating frequency and “rejection” needs of most 11 meter freeband DXers (CB operators who communicate with other CB operators more than 150 miles away). Continue reading
Building a 4 Element Cubical Quad
There are many reasons you should considering building your own antenna. First you can taylor it to the needs of your communication (local or DX and optimize it for the frequency you talk on) and to the needs of your environment (make it strong if you live in a harsh one where ice and wind is a problem). There is the extra part of knowing you did it yourself. To the best of my knowledge, only one company is producing a commercially available quad (Signal Engineering, Hy-Gain did) in the U.S. Another reason that you might build is you might be able to build a beam more cheaply than buying a commercial one (do not be surprised how much more you might have to spend to complete the project than you anticipated however). I assure you, there is nothing like stomping locals on DX contacts. Until your friends catch on and start building their own too (and co-phase them). Continue reading
Most of us overlook the importance of hooking good earth grounds to our antennas and our radios. Some think if lightning hits, its going to do what it wants to do. Probably right! Lightning protection is not why I am stressing the earth ground. Good earth grounds serve two purposes. One they protect against lightning (by routing current to the ground instead of our radios). Secondly, they discharge stray RF energy. There are a few things this does for us. First, it makes our receiver quieter (less static). Secondly it prevents RF from building up on the station equipment and distorting our audio (So many CBers have this problem!). Have you every heard someone who’s audio would distort when they would talk? The number one cause of this is strong RF currents running on the radio chassis and mic (more power and the problem gets worse!). Also grounding stray RF energy cuts down on interference to TVs, Phones etc. Plus, if you are using a vertical antenna (1/2,5/8 Wave) you can improve the performance by lowering the angle of radiation by using ground rods and radials running on the surface of the earth under your antenna. This is a total must if you are co-phasing verticals.
Angle of Radiation
Most of us think that our antenna’s “shoot” the signal straight off the end (if its a beam), at right angles from the antenna. I have even seen some operators (mainly ones who live in valleys) bend their mast so that their antenna does not sit with the boom parallel to the ground (don’t do it, read on). They hope to shoot the signal up out of the valley. Well, this is not how things work. The angle that the signal comes off the antenna, is mainly controlled by how high the antenna is from the ground. This angle that the signal takes off the antenna is sometimes called the “Angle of Radiation” or “Take-Off Angle”. You can see what I mean by take off angle by looking at figure 1. Most of you probably thought the signal came off at a 0 (Zero) degree angle, right? You would have to have your antenna over 500 feet in the air for this to happen! Continue reading
Co-phasing or “stacking” has long been a way to get high gain from antennas. Co-phasing involves placing two (or more!) identical antennas either side-by-side or one over top another (“stacking”) at a certain distance apart (usually a 1/2 Wavelength or more) and feeding the antennas in-phase. The result is 3 db more than just a single antenna. In my opinion, this is the absolute way to go with beam antennas instead of going with say, 8 elements beams, it would be much better to go with co-phasing two 4 element beams. If you look at the gain figures for a 8 element beam, you see that you will end up with more gain if you co-phasing 4 elements instead. If you are considering co-phasing antennas for your mobile, better check out the “Mobile” section first.
Circular Polarization is a relatively untalked about subject on 11 Meters, perhaps this is the first time you have ever heard of such a thing. Most antennas are oriented to produce linear polarization – either horizontal or vertical polarization. Sometimes, a creative CBer will turn their beam at a 45 degree angle, half-way in between horizontal and vertical polarization. This results in a signal that is weaker than if the owner would have oriented their antenna the same (polarization) as their neighbor’s antenna. This is still a “linear” polarization.
“Omnidirectional” is generic term for an antenna that radiates equally well in all directions. There are several antennas that are considered omnidirectional.
1/2 Wavelength Vertical
Most folks lump all vertical omnidirectional antennas into the same category and call them “Ground Planes”. A ground plane antenna is actually an antenna similar to the vertical dipole.It’s important to understand the difference – especially if you’re looking to invest in a certain type. Whether you’re after antennas or Mobile Broadband deals, misinterpretations usually end badly. Shown in figure 1, you can see the hollow tubing is now instead brought out at a 45 degree angle (and split into 3 sections) out from where it is on the vertical dipole. These rods are usually called “radials”. This type of antenna is really not a very high gain antenna. Continue reading