G9B08 from the General License Course Section 5.1, Antennas: Theory & Principles
Q. How does the feed-point impedance of a 1/2 wave dipole change as the feed point location is moved from the center toward the ends?
A. It steadily increases
B. It steadily decreases
C. It peaks at about 1/8 wavelength from the end
D. It is unaffected by the location of the feed point
Let’s begin with a simple review of what is meant by feed-point impedance and why it is important, and then we will take a look at the behavior of the AC signal in a half-wave dipole antenna. When you understand a little about how the alternating electric current moves in a dipole this question becomes simple!
The feed-point is the position at which the feed line attached to an antenna. For a half-wave dipole this is (almost always) the center point of the antenna. At the center position of a dipole antenna a pair of ¼-wave elements extend in opposite directions. With a coaxial feed line, one of those ¼-wave elements is connected to the coaxial center conductor wire while the other is attached to the coaxial cable ground-side shield.
Impedance is the opposition to the flow of alternating electrical current, or AC. Impedance in a circuit, including an antenna circuit, is affected by the circuit’s characteristic inductance, capacitance, and resistance. Like resistance (R), impedance (Z) is measured in ohms (Ω). Impedance will vary along the length of an antenna due to AC dynamics, but we are interested in its value at the position of the feed line connection – the feed-point impedance. Why?
Because power may be transferred from the feed line to the antenna most efficiently when the impedance of the antenna feed-point and the impedance of the feed line are equal. When the impedances of the feed-point and feed line are not equal, some power will be reflected from the position of impedance mismatch, and the reflected power will not be radiated from the antenna. Such antenna mismatches drive up the Standing Wave Ratio (SWR) of your antenna system, SWR being a ratio measure of the forward power to the reflected power in the antenna system. So, we’d like to have our antenna feed-point impedance match closely to our feed line impedance to keep SWR low and to keep our antenna’s radiating performance high.
Impedance may be calculated as the ratio of voltage (E) to current (I) at any position in a circuit: Z = E/I. So, if we examine how voltages and currents behave in an ideal dipole antenna we can reach some simple conclusions about how the impedance is varying in the dipole. Consider the following facts about AC within a dipole while referencing the associated figure below:
– The current (I) surges back and forth along the dipole with RF cycles.
– Free electrons are forced by voltage (E) in one direction on the antenna, while positively charged ions are compelled in the opposite direction, and these directions reverse each RF cycle.
– There is no conductive path at the antenna ends for the electrons and ions to continue upon, so they tend to accumulate with greater density toward the antenna ends.
– The accumulation of charge near the antenna ends creates large electric potentials, or high voltages.
– Intuitively you can understand that near these end positions where the charge density is great and there is no continuing conductive path, the opposition to current flow becomes very great.
– Near the end points the voltage (E) is very high and the current (I) is very low. A calculation of impedance (Z = E/I) will result in large values.
– The current flows most readily near the center of the antenna during the RF cycle reversals, and since charges do not tend to bunch up and accumulate here the voltage is minimum here.
– The calculation of impedance at the antenna’s center position yields the lowest value of E/I.
Let’s use a water analogy to think about what’s going on with the current and voltage in the dipole antenna. Imagine the antenna is a long, narrow tank of water. The flow of water is the current (electron flow). The pull of gravity on the water is the voltage. Imagine the tank is repeatedly tilted up and down over a center support, much like a playground seesaw or teeter-totter. The rate of up-and-down tilting is the driving frequency from the feed line. What’s going to happen with the water inside the tank?
Tilt the right side of the tank down and the water rushes down to the right, accumulating at the end of the tank making it heavy. There is a large potential down there now due to the weight of all that water bunched up on one end [as in the high voltages]. It flowed speedily across the center part of the tank in a somewhat evenly distributed flow [high current], but it has nowhere to go at the tank’s end, so the current stops and the water piles up.
Now tilt the left side of the tank down, reversing the direction of force that compels the current to flow. Again, the current flows readily across the middle of the tank without piling up, only to hit the left side end point and repeat its accumulation there. This water act repeats with the up and down tilting frequency, just as the electric charge act repeats in the dipole antenna with the driving AC frequency from the feed line and transmitter.
At some frequency of regular teetering, the back-and-forth flow reversals of the water will occur with great ease, requiring only the slightest tilt angles to keep the back-and-forth flow going strong. You may have had the experience of sloshing an open tub of water at a regular frequency so that the waves get larger and larger until the water begins to slosh right out of the tub! The frequency at which this natural reinforcement of the energy transfer occurs is called the resonant frequency. For electric AC, the resonant frequency is where the greatest efficiency of power transfer occurs, and this is the RF at which your antenna system will perform best.
Now that you have some idea about how the electric charges behave in a dipole, you can easily answer G9B08. Remember, impedance Z = E/I, where E is the voltage or charge potential and I is the current. The lowest impedance (Z) is at the center where E is minimized and I is the greatest. The highest impedance (Z) is at the end points where E is greatest and I is nearly zero. Normally, the dipole feed point will be at this center position of lowest impedance, but some dipoles may be fed “off center” and not have equivalent element lengths. So…
The answer to General Class question G9B08, “How does the feed-point impedance of a 1/2 wave dipole change as the feedpoint location is moved from the center toward the ends?” is “A. It steadily increases.”
Related Questions: G9B07, G9B03, G9B02, G9A04, G9A07 – G9A13