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Getting an HF antenna to work (Part 3)

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KK4CGM
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Getting an HF antenna to work (Part 3)

What follows is Part 3 of Ralph Bilal's email communication relating to antenna theory.

Last time resonance was mentioned and minimum SWR determined that point. Now we can go into the 2 components of resonance.

Start with inductance. Normally a coil of wire that produces a magnetic field when we put an electrical source into it. This magnetic field oscillates the same as the frequency that we are using. For example, at 7 Mhz, the magnetic field is oscillating at 7 million times per second. When this magnetic field increases, decreases and reverses it causes an electrical current in the coil opposite to the current we sent into it. This opposite field battles with the incoming source from your transmitter, offering a resistance.

The larger the coil in diameter, and number of windings determine how much resistance it produces. This resistance is called "INDUCTIVE REACTANCE". Like a regular resister, it is measured in ohms. You will often see it referred to as XL.

The reason we call it reactance is due to how it acts in the circuit. The resistance varies with the frequency of electrical energy you put into it.

The reactance increases with an increase in frequency. It decreases with a decrease in frequency. If you want to get fancy the formula is.

XL=2p fl

XL = INDUCTIVE REACTANCE (in ohms)

 pi = ABOUT 3.14 

 f =  FREQUENCY 

 l = INDUCTANCE IN HENRIES

 

Since the reactance varies with frequency, inductors are measured in Henries. With this value you can go to a reactance chart and know what the resistance in ohms will be for any frequency.

What we want to keep in mind, is inductive reactance increases with an increase of frequency and decreases with a decrease in frequency.

Next time we will offer information on its partner, "CAPACITIVE REACTANCE".

73,

Ralph WD0EJA

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