How do antennas work? How specifically do antennas like loops and cubical quad antennas work, when they appear to be a direct short??
To understand how antennas work unfortunately requires a bit of AC theory (sorry) and why AC at different frequencies acts differently than DC.
So, yes if you use a Volt Ohm Meter at the feed point of a loop antenna it would show a short. But that's only true for DC. When fed with the correct frequency AC, the loop acts as a huge transformer to create the RF field which in turn propagates through the atmosphere.
An AC circuit is constantly changing, not just on and off, but up to the max, back to zero, down to the max, back to zero. This is why it is called alternating current. DC is not , it is direct current it only travels in one direction and there are no waves. No waves mean no RF is created.Your transmitter puts out RF Radio Frequency or different wavelengths of AC current. AC and DC behave quite differently. Again, if you were to take a loop of wire and connect DC to the ends, there would be little to stop the flow of current. If we connect an ohm meter to the loop, the meter would verify that there isn't much resistance and the ohm value would be very close to 0. In DC, all we have to impede current is resistance caused by the wire or aluminum. Due to a direct connection, so much current would flow that bad things could happen. The loop would be a short circuit to a DC source. But not with RF or high frequency AC.
So now we take and connect a high frequency AC source to the loop and find that not much current is flowing. Instead of the loop acting like a short, it's now acting like a resistor (Impeding the flow of current). AC travels in two directions, alternating back and forth.......in your house it does this at 60 hertz...or 60 times per second. This is not fast enough to create magnetic waves as we know them in the amateur radio HF world, in order for that to happen you need your radio to speed up the alternating process to 28 million times a second (on 10 meters) and 14 million times a second on (20 meters), and so on.
Why?
The answer is because High Frequency AC that is created is travelling in a wave it is affected by another form of resistance to the flow of current it is also slowed down as it travels. This is called reactance. Reactance affects current flow like a resistor in an AC circuit. There are two forms of Reactance, Capacitance and Inductance. The two are opposites and can cancel each other out. Capacitance being the high of the wave and inductance the length of the wave(like a coil). So Alternating Current traveling in waves, has another obstacle to overcome besides just the wire's resistance to get back to it's source, The combination of those two properties is called Impedance. It acts like a resistor limiting the current to the point of not acting like a short is in place.
This why the cubical quad loop or a loop antenna is not a direct short. Because the current that flows through it Is RF (high frequency alternating current).
SO:
Direct Current or DC does not have Impedance. We have only the resistance of the wire.
At AC high frequencys or RF we now have Impedance. This is measured in Ohm’s, any value above 0 ohms is not a direct short.
The reactance (capacitance and inductance) and the wire resistance are added over the length of the wire or aluminum tubing, so that the Ohms go up and the current goes down at different points on your antenna, as you are at different points in this wave. The current in these waves is now expended by making a magnetic field emanating from the wire. AT 28MHZ, that means the electrical circuit is turning on and off 28 million times per second, creating waves as it moves down the wire or aluminum. This wire or aluminum carrying AC current creates an electro-magnetic field surrounding it. This field around the wire can be manipulated using fairly simple shapes like coils or loops.
So a loop is not a short, and a dipole does not act like an open circuit, and the simple answer is that at high frequency AC behaves differently than DC or even 60 Hz AC does. At high frequency there are waves rapidily pulsating waves of AC that are called RF….Meaning they are creating radio waves of electrical energy.
So then how would a dipole work, where it looks like and open circuit with no connection?
As AC energy travels down each side of a dipole it gets to the end, it then reverses and heads back towards the center, when both sides reach the center of the Yagi or dipole, although they are separated, the AC waves moving at 28MegaHertZ have created waves of equal electromagnetic fields around them. As they come together in the center they combined to create one electromagnetic wave of energy. RF
So when your hear Cubical Quad and Loop antenna owners say, they're antenna is quieter then a comparable Yagi, its technically accurate. The Quad or Loop circuit is closed and all of the electromagnetic waves and have been created and manipulated through a closed circuit. The Yagi depends on two separate electromagnetic waves combining in space to create one electromagnetic wave. What’s in space? Other electrical charges. Static. And many other kinds of electromagnetic waves are encountered and absorbed into the aluminum ends and then combine with and travel with the AC waves current moving through the aluminum…this shows up at the receiver as noise. Not always but sometimes, depending upon 100's of factors no one can fully explain. So any Yagi old timer who says Cubical Quads are not quieter than Yagi’s doesn’t understand the properties of how RF is created. I touched upon the Loops versus Dipole noise issue on the Quad page.
A ham radio creates the RF by creating a rapidly changing electric current. You can do that by rapidly connecting and disconnecting a battery, or with something called an Oscillator, you can create a sine wave with just a few electronic components -- a Capacitor and an Inductor = an Oscillator which can create the sine wave, and a couple of transistors can amplify the wave into a powerful signal. Basically this Oscillator can create AC from DC by generating a waveform and If this current is applied and removed over and over for a period of time, a series of waves is propagated at a RF frequency. Oscillators store power then discharge over and over again. This phenomenon is the basis of electro magnetivity and basically describes how radio waves are created within transmitters.
I promise this is not a test, but hang in there and you will get through it.
The capacitor receives energy into it , it stores then releases this energy and it will then start to discharge through the inductor. As it does, the inductor will create a magnetic field. Once the capacitor completely discharges, the inductor will keep the current in the circuit moving, which then goes into the front of the capactor again so it will charge up the capacitor again. Once the inductor's field collapses, the capacitor has been recharged (but with the opposite polarity), so it discharges again through the inductor. This oscillation will continue as it is self sustaining. It will oscillate at a frequency that depends on the size of the inductor and the capacitor.
In a Receiver : Thousands of sine waves from different radio stations hit the antenna. The capacitor and inductor want to resonate at one particular frequency. The sine wave that matches that particular frequency will get amplified by the resonator, and all of the other frequencies will be ignored. The tuner knob on your Amateur Radio Transciever is a variable capacitor that’s all, that’s changing the frequency of waves it wants to hear.
Buts lets say you had a machine (your radio) which has the power to make that super ball bounce 28 million times per second (28MHZ). Would it make it the end of the tube with enough energy? Of course it would. And as it hit the top and bottom of the metal tube, it would certainly resonate or make a sound possibly a hum and the faster or slower the ball was sent through the tube the different the sound would be 10 million times per second or 10 MHZ would sound different then 28 MHZ. As the ball hits the top and the bottom it loses some energy due to the sound it created while hitting the surface. That’s RF . And the sound represents (loosely) the different characteristics that different HF frequencys have with one another.
With DC the energy does not oscillator or bounce, you would roll the ball and it would get to the other side straight through with no oscillations or bounces and therefore no RF is created and no distinct RF characteristics are made.
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