Wednesday, June 25, 2008

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.

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.

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.

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.

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

1 comment:

Chris Altwegg said...

A couple of manufacturers of ground plane antennas for 2 meters.

I have an Arrow dual band GP and it's wonderful. This antenna is so well built of such sturdy materials, I could will it to my grandchildren.

I got an Arrow dual band J-pole too, same great materials and milling.

Chris - K7BRK