The trivia question for the Cascades Amateur Radio Society’s Sunday Sunshine net, held on the Jackson, Michigan 146.880 MHz repeater (pl 100 Hz), concerns the dipole and a standing wave. Almost one year prior to this date, I gave a presentation on the dipole to the Society at one of the monthly club meetings. The notes for that presentation are found here: The Mighty Dipole Notes for Distribution 4-21-2020. Discussion concerning tonight’s trivia questions is found in this document.
The objective of these trivia questions is to stimulate the licensed amateur radio operator to learn what is inside the FCC amateur radio exam pool questions rather than to merely know the answers to them. Knowing why the answers are what they are will enable the amateur to launch into innovation rather than being stuck always looking for somebody’s cookbook on building and doing amateur radio activities.
I hate cookbooks and you should too. Cookbooks always have typos and other problems. I will never forget as a youngster having a subscription to Popular Mechanics. Each month there was a build-it-yourself electronic project. There were two occasions when I attempted to build one of these projects and neither one worked. In each case, the magazine later published corrections to those projects. I hate cookbooks and you should, too.
The trivia questions appear below. These are probably too numerous for a roundtable net but one or two of them will stand out to you. Please address those.
QUESTION NO 1a: The voltage at the ends of a resonant center-fed dipole antenna is a maximum while the current is a minimum. Why? If you understand the why and wherefore on this question, you will be able to easily answer the rest. HINT: What physical law can your answer be based on?
QUESTION NO 1b: If we were to make this an end-fed antenna instead of center-fed, what would be the end-fed input impedance? Why?
- Maximum or
The astute reader will recognize that question 1b answers question 1a.
QUESTION NO 1c: What difference, if any, is there between the two voltages at the two antenna ends? Explain your answer.
- No Difference in magnitude
- The magnitudes are identical but 180 degrees out of phase
QUESTION NO 2: The voltage at the feed-point of a center-fed resonant dipole antenna is a minimum while the current is a maximum. Why?
- Maximum or
All of the above questions may be answered by Ohm’s Law. Let’s consider the antenna an electronic circuit that we can draw a schematic for. From the most basic of basic electronics, what is the voltage at an open circuit? The answer, of course, is the source voltage or a…maximum voltage.
Continuing, what then must the current be? The current in an open circuit is zero.
So, if the voltage is a maximum and the current is a minimum, what then is the impedance? For the answer, we return to Ohm’s Law. According to Ohm’s Law, resistance or impedance is equal to the voltage divided by the current. What is ANYTHING divided by zero? The answer is infinity or, in this case, a maximum. Therefore, if we were to move the feed-point from the center to the end, our new input impedance would be a maximum or just simply, high.
But the dipole antenna, regardless of the feed-point, has two ends. We know that the voltage at the ends is a maximum, but what difference is there between the two ends, assuming resonance. The magnitudes are equal though out of phase by 180 degrees. This is because the dipole, by definition, is a half-wavelength end-to-end.
All of these same answers (applicable to an end-fed antenna) may be applied to a center-fed antenna. While the voltage at the antenna ends was a maximum (current a minimum), with 90 degrees phase difference (half of 180 degrees) the voltage now becomes a minimum and the current a maximum. What is ANYTHING divided into zero (or in this case a minimum)?