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Getting an HF antenna to work (Part 6)

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Getting an HF antenna to work (Part 6)


Last article a Series Resonant circuit was described. This is where an inductor and capacitor are wired in series. When the 2 reactance's become equal, we are at the resonant frequency. We also have a minimum Z or impedance of the circuit. At this point the impedance of the circuit looks like a resistor, with no reactance showing.

On small circuits, such as in your radio, the impedance or Z is close to 0 ohms. This is good for radio circuitry, but not for antennas.

Making the circuit physically larger causes another affect. It is called Radiation Resistance. As the physical area of the antenna increases, the Radiation resistance will also. This is in respect to a Series Resonant circuit with nothing else added. This value will increase only so far. It will also vary in respect to the design, shape and location of the circuit.

Ideally you want 50 ohms. Most antennas will come close to this. Dipoles can go higher since they are floating above ground. This is good however. A higher Radiation Resistance means the antenna is operating at a higher voltage. This means less loss from the antenna.

50 ohms matches our transmitter for a 1:1 SWR. If the antenna is above this value, matching with a Tuner will work well, with no noticeable loss. If the antenna design makes it lower than 50 ohms, the current is higher in the circuit and can create losses.

Now we have 3 values to our antennas that we need to look for. RESONANCE, which has Inductive and Capacitive reactance and the Radiation Resistance. Actually there is a 4th element. The pure resistance of the parts of the antenna. Normally this is low enough as to not have much affect.

How do we use this?

First we need to remove any type of reactance. That means we find the resonant point first. This will offer a pure resistance and leave us now with only the Radiation Resistance.

If the SWR at minimum or resonant point is 2:1, we have two values, but only 1 is right. Either the antenna is at 25 ohms, or it is at 100 ohms. SWR values that are higher at resonance means the Radiation Resistance value is either lower or higher in value than 50 ohms.

How can we take the guess work out of tuning an antenna? The next article will start to address this.


Ralph WD0EJA


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