Module 9 - ANTENNAS
- Reading Assignment
- Antenna Waveform
- Radiation Patterns
- Gain & Decibels
- Dipole Antenna
- Ground plane Antenna
- J-Pole Antenna
- Yagi & Beam Antenna
- Vehicle Antenna
- Portable Antenna
- Dummy Load
- Antenna Supports
- Quiz 9
- Recommended Reading
Read Chapter 3 pages 4-5 to 4-16 in your text before you continue.
The most expensive radio is of little use with out an effective antenna.
Antennas are so important that this entire module is devoted to them, and it won't be near enough to do more than scratch the surface.
Hams spend a lot of money on their rigs and a disproportionately lower amount on their antennas.
The truth is you are much better spending your time and money on your antenna and equally important its installation
An excellent signal can be had from a low power transmitter using a good antenna that is properly installed.
I don't care how much your rig costs and how much power you are running, if you have a poor antenna, you will have poor results.
A radio wave is made up of electrical and magnetic fields, thus it is called an Electromagnetic Wave.
Of course, the electric field and magnetic field oscillate at the same frequency as the electromagnetic wave.
These forces cause the electrons in the receiving antenna element to move back and forth causing an RF current and so they can be received, amplified and detected by the receiver
The antenna Polarization refers to the electric field orientation of the electromagnetic wave. The antenna polarization is the same electrical polarization. Therefore an antenna with a Horizontal Polarization is oriented horizontal to the surface of the earth, as in fig M9-1.
An antenna with a Vertical Polarization is oriented vertical to the surface of the earth. Such an antenna would be called a vertical antenna and its driven element is vertical.
When the electrical field of the signal and the antenna are of the same orientation more of the signal is received. However when a radio wave travels through the ionosphere, the signals become randomly polarized.
We are going to spend a little time on antenna patterns because it is important that you understand them to be able to understand antennas.
This figure M9-2 shows the complete pattern around a vertical antenna such as a Ground Plane antenna (see figure M9-5 below).
The pattern is showing the relative signal strength of the field around the antenna. This is called the Radiation Pattern of the antenna.
While this pattern shows the overall 3-D radiation of the antenna, it might be more helpful to show two different views of cross sections of a radiation pattern.
The Azimuth Pattern shows a graphic pictorial of the antennas gain in horizontal directions around an antenna. It is like looking down on the antenna from above it.
Gain & Decibels
The gain of an antenna is the relative increase in radiation at the maximum point in the Azimuth Pattern. It is expressed as a value in dB above a standard. Remember +3db is double the power. The standard antenna is a 1/2-wavelength dipole in free space.
The pattern shown in the Azimuth Pattern in figure M9-3 shows a +12 db gain.
Sometimes the gain ratio can refer to the front to back ratio of the antenna or how much more power is radiated in the forward direction when compared to the backward radiation
Why do we care, well for several reasons. The first is transmitting: a +12 db gain is equivalent to multiplying the power output of your transmitter by 16. Lets assume you have a 100 watt station and the feedline losses getting to the antenna are 20 watts. A 12 db gain antenna would make the effective radiated power 1280 watts! (100 less 20 = 80, then multiplied by 16 or 1280).
Now lets look at the second reason - receiving: A 12 db gain would increase the signal strength you are receiving by a factor of 16 times. If the signal is weak, this is significant, maybe being the difference between being able to understand or copy the received signal. Furthermore, while you are getting a 12 db gain in the direction the antenna is pointing, if you are using a beam on the receiving end, you attenuate any signals coming from unwanted directions. This in addition to any atmospheric or man-made noise being attenuated from the unwanted directions can be significant.
Now imagine if you have a directional beam antenna at both the transmitting and receiving end! You can make a meaningful difference in the overall quality of the signal.
You can price out the cost of an amplifer to increase your power to about 1500 watts and the cost of the electrical power and compare it to the purchase of a directional antenna. I think you will find the antenna purchase much more economical.
Now consider this. If you remember, as the frequency increases, the wavelength decreases and therefore the size of the antenna decreases even more. (most antennas are only 1/2 wavelength)
This means at 50 Mhz (6 meters) and 144 Mhz (2 meters), a very directional antenna will be quite small and easy to construct and erect.
We will explore this more in the next topic. What a seqway, right.
There are so many antennas and there are books available from ARRL that would explain how to build them and how they work, I'll make my antenna presentation somewhat basic.
First lets discuss the Longwire Antenna. It is just what its name implies, at least 1 wavelength long antenna. There is no feed wire and the wire from the antenna to the transmitter is considered part of the antenna.
It is fed in the center of the antenna 1/4 wavelength tied to the center conductor of the feedline and 1/4 wavelength tied to the shield. (This assumes being fed by coax).
The best way to support this antenna is to use Egg Insulators and to rig them like figure 9-4. This way if the insulator breaks the antenna will not come crashing down. You will be able to tell something is wrong because of the SWR (more on that in Module 10).
The radiation pattern of the dipole is horizontal and broadside to the antenna. The pattern look like Figure 9-2, but rotated 90 degrees in line with the horizon.
Both these antennas are mainly used at HF frequencies. So lets move on to VHF antennas, like we will be using as a Technician Licensed Amateur.
Ground plane Antennas
This is aGround Plane antenna. As you know a dipole is 1/4 wave on each side. You can use ground for one of those sides. Yes ground. A quarter wave element mounted at the ground with a good ground radial system (ideally several radials of wire buried in the earth around the base of the antenna hooked together and then tied to the shield of the coax feedline makes a good antenna. But the angle of radiation is high, as it points too high and doesn't give good radiation around the antenna. It is much better to have it up in the air a bit. But then you are too far from the ground.
The ground plane antenna is the answer to that. By putting at least 4 radials sticking out from the bottom of the antenna that are approximately 1/4 wavelength long, you make a false ground or Counterpoise. This is a very common unity gain antenna used in VHF communications because it is Omni Directional or all directions of horizontal radiation are of equal strength.
The ground plane can also be a car roof or trunk lid or other metal surface will suffice.
You can calculate the length by using this formula [Length (in feet) = 234 / Frequency (in MHz)]. Or you can use the 1/2 wave formula of 468/frequency in Mhz and cut it in half for a 1/4 wave. That way you don't have to memorize 2 formulas.
There are some variations of the ground plane used a 5/8 wavelength antenna that shows gain because although it is not directional, the ball pattern around the antenna is squashed down from the top so it is stronger horizontally and less signal is directed vertically.
This is an especially good antenna for a repeater, because of the gain and the low angle of radiation. This is because the repeater is usually higher than the surrounding area and looks down on the service area.
This is an antenna that anyone can construct in little time and with little effort.
The J-pole antenna is easily constructed using 1/2 inch copper pipe, a few fittings and a couple of clamps. It can be fed using standard 50 ohm coax cable.
Do a Goggle search for J-pole antenna to obtain the dimensions. It has approximately 3 db of gain, is omni-directional and vertically polarized.
It is relatively small in size and does a good job on 2 meters. An excellent starting antenna, probably the simplest to make. Physically simpler than the ground plane and with very common parts.
Yagi & Beam Antennas
Next you see a Yagi Antenna, which is a type of Beam or Directional Antenna. The Yagi can be constructed of many elements or just a few. First your have the Driven Element, the element the feedline is connected to. This is 1/4 wave on each side and feed by coax like the dipole (which is how it starts).
Now the magic. If we put slightly longer Parasitic Elements, which are place behind the driven element and called Reflector Elements. These reflectors are increasingly longer than the driven element. ie each progressive reflector is longer than the preceding one.
Now, lets put some more parasitic elements in front of the driven element. We call them Director Elements. Each progressive director is shorter than the proceeding one.
The more reflectors and directory used, the more directional the antenna. The figure shows a 7 element beam, as there is 1 reflectors, a driven element & 5 directors. More elements equal more elements.
There are other configurations of beams in your text book, but there are not any test questions on them, so lets save them for the General Exam.
A directional antenna would be used with a rotator to point and use the antenna in one narrow direction, while an omnidirectional antenna would be used to communicate in a 360 degree pattern.
Lets look at a mobile antenna. It is typically a 1/4 wave antenna and used the roof or trunk lid of the car or pickup roof as the counterpoise or ground portion of the antenna.
Antenna mounting: Your antenna position and mounting is paramount to the mobile functions properly. A good ground plane (metal surrounding the base of the antenna) is a must for good antenna function. The trunk lid on a car or the pickup roof is a good spot to mount the antenna.Don't try mounting the antenna on a gutter, rear-view mirror or a lumber rack. If you do you are severely compromising the ability of the antenna to function properly.
You will have a heck of a time trying to tune this antenna, because it only part of an antenna. Put it on the roof. They make nice little rubber plugs to cover up the hole when you sell the vehicle.
There are also variations of this car antenna in a 5/8 wavelength with a coil at the bottom to make the antenna electrically longer that exhibit gain just like its base station ground plane brother. The radiating element is also longer and therefore radiates better.
Sometimes we want to test and not radiate any signal to the outside world.
A Dummy load is an artificial load used to provide an alternative load to an antenna when testing a transmitter or amplifier. Ideally, this dummy load provides a SWR of 1:1 at the impedance under test. Typical dummy loads have 50 ohm inputs and provide a resistive load.
They are usually a simple device consisting of a high wattage 50 ohm resistor immersed in a one gallon paint can filled with oil for heat dispersion.
A final antenna we will examine is the antenna for your hand-held or portable radio. It is a Rubber Ducky Antenna. This antenna is a compromise antenna that is physically shortened and electrically lengthened to make the HT convenient to handle.
A rubber ducky antenna will not give as long a range as a full base antenna or even close to a mobile roof mounted antenna. It will especially be degraded if used inside a vehicle or building. The use of repeaters makes this an acceptable compromise most of the time. But it is not always enough to make the signals workable. You can try moving a few feet as there are multiple paths for the signal to travel.
The ones of most importance to the Technician Licensee are the Ground Plane and the VHF roof mounted Beam Antenna
They are easily mounted on a small tower or on a TV antenna mast. There are many mounts available from the major suppliers.
Always check for zoning restrictions or Covenants before erection of an antenna, especially towers.
The J-Pole or a Ground Plane on VHF frequencies is fairly small and should not be a problem. There are various stealth antenna designs to keep pesky neighbors from complaining.
Lets learn by doing the quiz and learn what we need to reinforce by our answers to the questions. Ok, lets do some practice.
You can take the quiz as often as you wish.
In this module you have been introduced to antennas. What a large subject to master. Don't be intimidates because antenna experimentation is one of the facets of Ham Radio that is the most fun. Look at the recommended reading and do some internet searches to gain more knowledge on the subject.
You might find this USMC book on antennas helpful.USMC Antenna PDF
You will find several books available from the ARRL here ARRL Antenna Books
View the complete Technician Question Pool in Chapter 11 of your Ham Radio License Manual.
Review the Term Glossary in Chapter 10 of your Ham Radio License Manual
Practice Exams are available at www.arrl.org/exam-practice and there are links to many other test practice sites also.