TheRadioBoard

I have a rudimentary understanding of resonance in parallel tuned LC tanks: if the tank is excited with an electrical pulse, electrons slosh back and forth between the inductor and capacitor at a certain frequency, creating a "ringing" effect.

How does this work in a resonant dipole? Does it "ring"? Do electrons slosh back and forth between L and C? Can we conceptualize dipoles as LC tanks? And how does a capacity hat work - it doesn't seem like a capacitor in the traditional sense?

In attempting to find a good short dipole design I realized I don't even really know what a dipole is. A magnetic loop antenna, OK - I can think of that as an LC tank. But the fundamentals of the dipole elude me. Is there a simple and intuitive way to understand electron flow, resonance, and radiation in dipoles?

The thing that got me started wondering about this was a statement I read that a short, inductively loaded, capacity-hatted dipole is essentially a straightened out magnetic loop antenna! That was a very fascinating idea, and I'm trying to understand how dipoles relate to LC tanks and magnetic loops.

I think of a dipole wires as capacitor plates with inductance.

One wire goes negative other positive, then oscillator alters the polarity... all the time the inductance tries to counter this change, by creating a higher impedance (add a coil) somewhere on the dipole we will make the antenna look electrically longer, this as the coil will slow the electrons advance,

with added capacitance basics tell us more more electrons can run the gauntlet before capacitor has reached its limit, this is what a capacitance hat does.
I guess you already know that a capacitance hat is adding several parallel wires all electrically connected and same length.

One can get a compact antenna with decent performance by combining the two things above, a capacitively and inductively loaded dipole antenna.

EDIT
if we add the coil on the ends it will not make much of a difference but if it is added near middle it most certainly will. if you add several you can make antenna resonate at several frequencies, and not necessarily harmonics, true it will never be as efficient as a full dipole but...

You won't see L and C in the equations that describe antennas. A quarter-wavelength conductor (half of a dipole) represents 90 degrees of waveform (360 / 4). So, when voltage is highest at the feed point of one element of a dipole, it is lowest at the far end of that conductor. At that moment, though, the voltage at the feed point of the other element is lowest, meaning the voltage at its far end is highest. Current flow is maximized and we get good radiation.

Rather than thinking of antenna elements as lumped LC, it's probably wiser to see them as sections of transmission line. LC circuits are a way of synthesizing the characteristics of transmission lines with components; we're so accustomed to using them we've forgotten the relationship.

Your magnetic loop antennas are sort of a combination of transmission line and lumped-circuit elements, as are inductively or capacitively loaded dipoles. Now it doesn't hurt to bring what we know about practical LC circuits back into the discussion. Cheap crystal sets used the self-capacitance of the tuning inductor to resonate the circuit; there was no tuning cap. The elements of your dipole work similarly. Check out an antenna-modeling program to see how they break the elements down into little pieces. Each little piece could be considered as a discrete LC circuit. The entire element then is a series of LC circuits strung together. (I should say LCR, as there is always resistive loss.)

Crystal sets without tuning caps, that rely on the inductor's self-capacitance, aren't selective. That's a negative for a receiver, less so for an antenna element (assuming the lack of selectivity isn't overwhelmingly due to resistive losses). Rebuilding the set with a better inductor and adding a tuning cap can be equated in an antenna by replacing some of the inherent LC circuits making up the elements with lumped components: loading coils, capacitance hats, or both. Those lumped components will likely have higher Q than the LC circuits we imagine constituting the elements of a full-size dipole, and bandwidth decreases.

It's tempting right now to say, Oh, higher Q means better efficiency! In the case of an antenna, not necessarily. Remember, there's no free lunch. Inductors are lossy. That's why we want to put them out near the ends of the elements where current is lowest. But then we need more inductance, which may be mechanically impractical. NFL. Capacitors are theoretically lossless, the only losses being in mechanical junctions. If they're placed at the low-current tips of the elements, they should have less negative impact on performance than inductors, but they have to be physically large to do much good. NFL.

73,

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Informative thread - thanks.

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YES

A "resonant" antenna, dipole or otherwise, will "ring".

Electons will oscillate back and forth!

In the case of a dipole, electrons will move from one end towards the other end "bunching up" at that end. Then they will reverse direction going towards the other end.

I dont remember just now if it was Heinrich Hertz who experimented with ring like loop antennas with a gap in them. The gap acted like the Cap, and the Ring itself acted like the inductance.

Whoever it was he was "ringing" his "rings"!

With a dipole some of the electrons will actually go off the end into the air/atmosphere causing "end effect", which changes the wavelength equation somewhat.

73
kb0lxy