R-F Chokes

When a choke is used to pass direct current and present high impedance to radio frequencies, it may have high r-f voltage across it. High choke impedance at operating frequency is necessary to avoid loss of r-f current which reduces the useful power and overheats the choke. If a single-layer choke is connected to an r-f generator at a given voltage, and if its current is measured as in Fig. 180, the choke impedance is the ratio of voltage to current measured.

Fig. 180. R-f choke impedance.

By disconnecting the choke from the circuit, the tuning reaction may be noted, and from this whether the reactance of the choke is inductive or capacitive. The difference in watts input to the generator, when the coil is removed and the tank condenser is retimed for minimum plate current, is readily observable. This difference times the generator efficiency is the loss in the coil at a particular voltage and frequency.

The impedance of a typical coil, found as described above, is plotted in Fig. 180 against frequency. At low frequencies (a), the curve follows straight reactance line X_L(= 2πfL). At a frequency somewhat below natural frequency b (determined by the choke inductance and effective capacitance), the slope starts to increase and reaches a maximum point at a frequency c of 1.2b to 1.7b. Above this frequency, the impedance decreases until a minimum value is reached at d, which is from 2.2 to 3.0 times b. At higher frequencies, the increase and decrease are repeated in a series of peaks and valleys at approximately equal frequency intervals. The second, fourth, and sixth peaks are of lower value than the first, third, and fifth, respectively. The seventh peak is followed by a flattened slope which suggests a submerged eighth peak. The points of minimum impedance rise in value, so that at higher frequencies the valleys appear to be partly filled in and the peaks to be level off. The watts loss are high at points of low impedance, and they rise sharply at the frequency d.

The change in reactance is shown in Fig. 180. The coil is inductive up to frequency b. From b to c it has no noticeable effect on the tuning and hence is pure resistance, or nearly so. Above c it is capacitive up to a frequency slightly below d, where it again becomes of indefinite reactance. Thereafter, it is capacitive, except for brief frequency intervals, where it is resistive, or only slightly inductive. At all frequencies higher than the fifth peak, the coil is capacitive.

Since a coil has distributed constants it is subject to standing waves at the higher frequencies. The character of these waves may be found by tapping the coil at various points and inserting thermogalvanom-eters in series with the coil at these points. The current distribution is plotted in Fig. 180 against coil length. These diagrams show the kind of standing waves as the frequency increases.

Current distribution is uniform at all frequencies below b. Most chokes are used within the first impedance peak. The useful range for choke impedance of 20,000 ohms in Fig. 180 is 1,700 to 2,800 kc. This choke could be operated at 5,500 kc safely also, but the frequency range is narrower. Also, the safe loss dissipation is less because it takes place over half of the coil surface. Pie-section chokes have similar impedance curves, but impedance peaks following the first are less pronounced.