Antennas 2, The Ground Plane Vertical (Bob, Week 6)

This week we introduce the other most common type of antenna. It goes by many names but they all essentially refer to the same thing. Here are some of the names:

• Vertical
• Ground Plane
• Quarter-Wave Ground Plane
• Monopole

Not all vertical antennas are a quarter-wave of course and they are not even all ground plane antennas, but the names above usually refer to the quarter-wave ground plane antenna and that's what we will discuss today.


HOW DOES IT WORK?
Let's recall the dipole from last week. That antenna is center-fed and is 1/2 wavelength long overall. So each radiating half of the antenna is 1/4 wavelength. Further, this is a balanced antenna where each side is being fed a voltage and allows a current to flow. The voltage and current is opposite going to each half.

The quarter-wave ground plane antenna is one half of that dipole and the ground plane acts like a mirror to electrically give the appearance of the other side of the dipole. Most people have seen at least a photograph, if not in person, where a mountain is reflected off of a very still lake or pond. It appears that there is a mountain exactly like the real one but directly below it and upside-down. The same idea applies to the ground plane antenna. The ground plane is the still pond and the vertical monopole is the mountain. To a receiving antenna it looks electrically like a vertical dipole with the bottom half of the dipole being the reflection.


MORE ABOUT THE GROUND PLANE
What make this antenna work the way it does is the ground plane so let's talk a bit more about that. The ground plane acts as a reflector but it doesn't have to be mirror smooth. In fact it doesn't even have to be solid. A typical ground plane is 3 or 4 wires extending more or less horizontally from the base of the vertical element. Because of the size of wavelengths we are dealing with this is sufficient to give the "mirror" effect. Also it's important to note although it may seem obvious that the ground plane is actually kept at ground potential. That means no voltage and no driven current. (There probably is induced current but I haven't studied up on this and I'll leave it for another time.) It also means that this is an unbalanced antenna. Unlike the dipole where we are driving the two elements in equal and opposite directions, with this antenna we are only driving the vertical element and not driving the ground plane.


WHAT DO YOU MEAN "MORE OR LESS HORIZONTAL"?
I didn't say that because we don't care if it's horizontal or not. Although it's true that the ground plane will work even when the elements are not exactly horizontal. But we do care what the angle is because changing this angle changes the feedpoint impedance. This allows us to create an antenna that closely matches the standard 50 Ohm impedance of the unbalanced coax feedline used by amateurs. It turns out that if the ground plane itself is up at least one-half wavelength off the ground (earth, the ground you are standing on) then a horizontal ground plane has an impedance of 22 Ohms. If you tilt the ground plane elements down 45 degrees, the antenna will have an impedance very close to 50 Ohms.


A SIMPLE HOMEMADE QUARTER-WAVE GROUND PLANE
By putting together the elements described above you can make a simple home-made ground plane antenna for 144 MHz out of nothing more than a female UHF connector and some heavy solid copper wire. This was my second homemade antenna (a wire dipole was the first) and I got some very good reports reaching a long distance on low power. You get a UHF connector designed to be attached to a flat plate. It will have a square plate attached with four holes, one at each corner. You simply solder the vertical element which is a measured length of wire to the center conductor of the connector and make small loops on the end of the four ground plane wires, use screws and nuts to hold them in place and solder them as well. The plans are in the ARRL Handbook and the Antenna book and must be online somewhere. I will look for a link and add it.

As promised here is just the first link that came up when I googled. There are doubtless many more.
2 Meter Ground Plane Project

Antennas 1, The Dipole (Bob, Week 5)

This week we begin an open-ended series on Antennas. Antennas are the one part of our hobby left where it is common and worthwhile to build your own gear. It's true that you can still build your own radio, amplifier, power supply, etc. and many do, but with the advent of modern electronics this is now much less common than it used to be. But antennas haven't changed too much over the years. There have certainly been some new and effective designs but the basic function and design is still as valid today as it was in the very early days of radio.

Today I start with the Dipole, also known as the Half-Wave Dipole since the length of the antenna is roughly one-half wavelength of your operating frequency. More on why it is only "roughly" one-half wavelength later. The dipole is good for illustrating some of the antenna basics and is a good and effective do-it-yourself antenna if you have the space and can get it high enough off the ground.

So, what is it? and why half-wave? A dipole at it's simplest is a long straight conductor that is broken at the middle into two separate halves. Each half is connected at the center by one of the two conductors of your feedline. Because of this way of connecting your feedline to the antenna we say that a dipole is a center fed antenna. As the RF current passes from the feedline to the antenna it is stopped by the endpoints of the antenna. So the current at the endpoints is zero and it is at a maximum at the feedline connection. The reason a half wave is chosen is so that the antenna will be resonant.

RESONANCE
Resonance could be an entire topic on it's own and maybe someday it will be but I'm not prepared to go into it in depth today. Instead I will give you a musical analogy. When you blow across the top of a pop bottle you hear a tone of a given frequency. Let's not worry about harmonics just now. The reason you hear the tone you do is because the bottle is resonant at that frequency. When you blow across it, the sound you make isn't of any given frequency. That sound travels down to the bottom of the bottle and back up again in a certain time and reinforces the sound you made. The time it takes to go down and back determines the frequency of resonance. In a similar way, the resonant frequency will depend on how long it takes a radio wave to travel the length of the wire antenna and be reflected back. It turns out that the distance to do that is one-quarter wave. Since you have two of these quarter-wave pieces, end to end, you get a half-wave antenna.

WHY 'ROUGHLY' HALF-WAVE?
When we talk about a wave-length we are talking about how far the wave would travel in the time it takes to go through one cycle. In general when we talk about the speed of radio waves we say that it is the same as the speed of light. But inside a wire it is not. It is actually a bit slower so we have to apply a factor to the calculation. So in effect the antenna is a half-wave long but it is a half-wave at the speed it travels in the wire.

BALANCED OR UNBALANCED?
I will fill this in more later...