Radiation Patterns of Mobile Antennas: What are those? Which antenna is best?

Radiation Patterns of Mobile Antennas: What are those? Which antenna is best?

If you don’t like reading, jump down to the Conclusions. Or keep reading and impress your friends!

A few principles:

• In antennas, “radiation pattern” or “gain” is nothing more than taking RF power from where you don’t need it and putting it where you do. It’s not more power. It’s the same power, redistributed. It’s like a balloon full of water. You can squish into different shapes but it’s always the same amount of water. Gain in one direction must be at the expense of another.
• RF radiation (electromagnetic wave) is created by accelerating (wiggling) electrons. The more wiggling electrons (higher current), the more RF. You pinch somebody, they yell. You wiggle an electron, it spits out a photon. The photon is RF radiation. It’s that simple.
• The most basic antenna building block is the half wave dipole. We usually, but not always, feed half wave antennas in the middle which is the point of highest RF current so it follows that it’s the zone of highest RF radiation. There isn’t much current at the open ends so they don’t radiate as much there.

Knowing all that we’ll discuss three of the most common antennas used on vehicles for 2 meters: a quarter wave vertical, a half wave vertical, and a 5/8 wave vertical.

A “quarter wave” vertical for 2 meters is (duh) about a half of a meter high. The vertical wire is half the dipole. The ground plane or radial system under a quarter wave vertical is the “other half” of the dipole. A quarter wave vertical is fed at its base which is the point of highest RF current. We call it a “current fed” antenna. But since it’s sitting on an “RF mirror” a lot of the radiation shines upward rather than outward. Visualize a hemisphere with a big dent in the top. That’s the approximate “radiation pattern” of the quarter wave vertical.

A “half wave” vertical for 2 meters is (you guessed it) about a meter long. While you can feed a half wave vertical in the middle, on vehicles they’re usually fed on the bottom end. Since ends of half wave antennas are zones of low current but high voltage, you need some form of step-up network to create a high-voltage feed from your low voltage coax line. We call this arrangement “voltage fed.” Half wave verticals on cars are generally used with glass mounts because they need little if any ground plane. (Or on fiberglass Corvettes!) The high current zone is half way up the antenna, away from the skin of the car. Since the high current zone is up in the clear, its maximum radiation pattern is mostly straight out from the antenna. Imagine a doughnut-shaped radiation pattern. There is very little high angle radiation and almost nothing straight up.

A “5/8 wave” vertical is about 1.25 meters high. My wife calls it a “flagpole” when it’s on the roof of my car. Think of it this way: it’s a half wave vertical sitting on top of an eighth wave vertical. As you might expect, the characteristics are a mix of the two. It’s a current fed antenna but since the high current zone is higher on the antenna than our quarter wave vertical, the ground plane is still needed but a smaller one will do. This antenna has both high and low angle radiation.

Conclusions: Which mobile antenna is best? As usual, it depends:

• Quarter wave: Needs a good ground plane. Fits nicely through your garage door. Good for hilly or urban terrain because it can shine upwards to launch your signal up and out of the gulch. But not the best for long distances.
• Half wave: Little or no ground plane needed. Good for glass mounts and non-metallic surfaces. Low angle radiation is good for long distances over moderate to flat terrain. Not the best for steep or cluttered terrains.
• 5/8 wave: Needs a moderate ground plane. Bangs the garage door and makes it easy to find your car in the parking lot. Good mixed-use antenna with high and low angle radiation. My pick for best all-around IF you can deal with the height.

Take your pick and get on the air!
73,

Chuck, NA3CW