For tonight’s trivia question I carefully word-smithed a question on the mighty dipole to align with some select FCC amateur radio testing pool questions. If you could answer tonight’s trivia question, then you are probably knowledgeable enough to accurately answer maybe a dozen of the FCC amateur radio pool questions without memorizing them beforehand.
So here we go with some prep-info: A center-fed dipole antenna consists of two electrical wires joined together at the middle by an insulator. The insulator creates a mid-point gap less than the width of your hand where a feed line will eventually be attached. We want to build a dipole antenna that is to be resonate at some named frequency.
Our trivia question for tonight’s net was:
What length must each of those two wires be in terms of fractional wavelengths and other incidentals, if any?
Obviously, many of you thought that was too simple. While that is the actual trivia question, here are more questions that will largely answer the trivia question.
- The insulator creates a mid-point gap. Does this affect the length that each wire must be cut to?
- Yes, subtract half the gap length from each wire. But while true in theory, in practice the correction is probably too small to matter.
- The length of the antenna needs to be a half-wave length. Is that end-to-end including the gap?
- Yes, absolutely and that without question.
- But in practice, the correction is probably too small to matter when you consider other slop built into the fabrication of your dipole antenna.
- What would happen if you shorted the two wires at the gap, or feed-point, with another electrical wire?
- It might make big difference or it might be small.
- If you doubt that look at the J-Pole.
- Note that the J-Pole is just one big, fat, happy wire with the feed-point taken across a tiny little piece of copper wire. Some might call this…a short.
- What role, if any, does a velocity factor play in the wire lengths?
- The speed of light is slowed in a wire relative to free space. Therefore, shorten the wire by a factor equal to the velocity factor of that wire that was used.
- A common velocity factor for copper wire is 0.97. Therefore, for a wire that would otherwise be 1 meter. you will need to cut 30 centimeters from it to allow for a degraded speed of light through it.
- Velocity factor is one place where theory very much affects practice.
- There is theory and there is practice. What distinctions would you call out here between theory and practice?
In conclusion, I need to re-iterate my standard claim that I am new to amateur radio and don’t know noth’n about no amateur radio, no-how. So don’t expect too much out of me.