In a recent thread on simple series circuit the question was asked about why the current in each component of a series circuit HAD to be equal. Lots of good answers given. But it also raised a good question about how you explain a simple dipole antenna (assuming at perfect resonance) where the current is NOT equal in each segment of the wire, but follows a classic sinusoidal current distribution.
Now, most of us old R.F. geezers grasp this pretty intuitively....we immediately recognize that any time you DO have a circuit like this that doesn't seem to follow ohm's law, you have an antenna....almost by definition. We know this, and it's pretty much second nature, but it still doesn't explain where the alternate paths actually exist...by ohm's law (or even KCL), the currents leaving each section of wire have to equal the currents entering each section of wire...but we know that they don't....we can measure this "aberration" with an r.f. current meter or light bulb. There HAS to be a parallel path somewhere...or actually MANY of them. What gives?
We know that the mysterious entity Radiation Resistance is actually modeled by a single resistor in series with the antenna's reactive components. But this equivlalent circuit is actually misleading.
Radiation resistance actually manifests itself as an infinite series of resistors IN PARALLEL with the "real conductor', i.e., the wire. These resistors are progressively smaller in value as you approach the center of the antenna from either end. We actually DO have multiple parallel paths, but these paths are space itself.
Interestingly enough, if the antenna is self resonant, we will find that the current and voltage at ANY point along the antenna are precisely in phase! In other words, the power factor at any point on the antenna is 1, even though the phasing between voltage and current, relative to DISTANCE is 90 degrees.
It's stuff like this that makes me wonder if we should be teaching antenna theory BEFORE teaching lumped constant theory.....the latter will be very easy to grasp afterwards.
eric
Now, most of us old R.F. geezers grasp this pretty intuitively....we immediately recognize that any time you DO have a circuit like this that doesn't seem to follow ohm's law, you have an antenna....almost by definition. We know this, and it's pretty much second nature, but it still doesn't explain where the alternate paths actually exist...by ohm's law (or even KCL), the currents leaving each section of wire have to equal the currents entering each section of wire...but we know that they don't....we can measure this "aberration" with an r.f. current meter or light bulb. There HAS to be a parallel path somewhere...or actually MANY of them. What gives?
We know that the mysterious entity Radiation Resistance is actually modeled by a single resistor in series with the antenna's reactive components. But this equivlalent circuit is actually misleading.
Radiation resistance actually manifests itself as an infinite series of resistors IN PARALLEL with the "real conductor', i.e., the wire. These resistors are progressively smaller in value as you approach the center of the antenna from either end. We actually DO have multiple parallel paths, but these paths are space itself.
Interestingly enough, if the antenna is self resonant, we will find that the current and voltage at ANY point along the antenna are precisely in phase! In other words, the power factor at any point on the antenna is 1, even though the phasing between voltage and current, relative to DISTANCE is 90 degrees.
It's stuff like this that makes me wonder if we should be teaching antenna theory BEFORE teaching lumped constant theory.....the latter will be very easy to grasp afterwards.
eric