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THE Q OF AN ANTENNA SYSTEM
 
What is the "Q" of a circuit? Q means quality factor. In a circuit or antenna it is a consideration of the reactances (X) and the pure resistance (R) of the circuit.
 
An antenna is a tuned circuit at the end of the coax or feedline. The design and size of the components affect the Q of the circuit.
 
The higher the Q, the narrower the bandwidth or frequency response the circuit has. The lower the Q, the wider the circuit reacts.
 
Q is affected by amount of pure resistance a circuit offers.
 
To get an idea of the Q, it should be measured at the exact resonant point of the antenna or circuit. At this point, either the inductor or capacitor value need to be known. Then, it can be looked up on a reactance chart as to the amount of reactance or resistance it offers at the resonant frequency. It will be the same for either component.
 
The Q equals the reactance value in ohms divided by the series resistance of the circuit.
Q = X/R
 
What does this mean?
 
In a series resonant circuit, if X = 350 ohms and R = 5, then the Q = 70. This will mean that a current of 1 at resonance will drop to .1 in only 8% deviation from the resonant point. At this point the loss is 90% of the current that would be at the resonant point.
 
Does this mean the antenna is a poor radiator? No, at the resonant point it may be good, but a small deviation would mean loss.
 
Antennas that have lower Q's and wider bandwidths will have more resistance to give it the lower number. It will also increase bandwidth. Does this mean an antenna with a wider bandwidth is less efficient?
 
Not always, to get an idea of what Q is doing, you can liken it to the energy passing over the conductor of the circuit the amount of times as the Q. Therefore, if the Q is 10, it passes through the conductors 10 times before radiating. Does this hurt performance?
 
Maybe, to determine resistance, you just do not measure the ohm value of the wire. At RF (HF) the energy only uses about .0006 inch of the conductor diameter due to "skin affect". This drastically reduces the amount of conductor it uses. If this amount of conductor develops 5 ohms, then it is like 50 ohms with a Q of 10.
 
Measuring Q and efficiency can be quite involved and obscure. There are antennas with very wide bandwidths that are very efficient. This is not due to high resistance in the conductors.
 
Therefore, it is not a good analysis to determine if an antenna performs well by a "Q" value it measures. There are other factors beyond the scope of this article and me too. The bottom line is, are you being heard and are you hearing?
 
73,
Ralph WD0EJA
 
11-16
 
 
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