Variable Reciever Bandspreading

An RF coil, such as the one shown on page 10, is designed for minimum loss. It uses a multistrand wire called "Litz" wire. This wire is made by weaving together a large number of fine insulated wires. Its AC resistance is low, and therefore the Q of the coil is high.

Very light cardboard is used for the coil form. The form is varnished both before and after winding, to prevent absorption of moisture. The physical size of coil and form is kept as small as possible. The width of the mass of wound wire is usually about equal to its depth, as shown. The form contains both primary and secondary windings. The primary may be wound near the secondary or directly over the secondary.

A shield covers the coil and shields it from electric and or magnetic fields which might induce unwanted voltages. The shield must be a good conductor to shield against electric fields. Aluminum is usually used because it is light, cheap, and a good conductor. Shielding of coils results in some reduction of efficiency, due to eddy currents induced in the shield by the magnetic field of the coil. These eddy currents produce a magnetic field of their own

Capacitive Coupling From Antenna

Loop Antenna

Capacitive Coupling From Antenna

Loop Antenna

Typical RF Coil with Shield

which opposes the field of the coil. The loss of efficiency due to eddy currents is limited to approximately 15r'f by using a shield with a diameter twice the diameter of the coil. The shield should be well grounded.

RF coils usually have air cores. A few designed for low and medium frequencies have powdered iron cores. The cores are sometimes adjustable. As the iron core moves into the coil, inductance increases and the frequency of the tuned circuit decreases. A brass core can be used for the opposite effect. As the brass core is moved into the coil, inductance decreases and frequency increases. In either case, the circuit can be tuned by moving the core. This is called permeability tuning.

Variable Capacitors

Variable capacitors for RF tuned circuits are somewhat bulky. The plates, both rotor and stator, are usually aluminum, but better receivers sometimes use silver-plated brass capacitors for improved RF conductivity.

The calibration pattern of a variable capacitor depends on the shape of its rotor, as shown on page 11. For calibration, capacitors fall into three classifications straight line capacity, straight line wavelength, and straight line frequency.

With the straight line capacity type, capacity increases directly with the amount of rotation. Since frequency does not increase directly with the decrease in capacitance, calibration puts the upper half of a frequency band in about one-eighth of the dial rotation.

With the straight line wavelength type, wavelength increases directly with the amount of rotation. In this type, the upper half of the band appears on one-third of the dial rotation.

With the straight line frequency type, frequency varies directly with the amount of rotation. This permits linear dial calibration. With this type variable capacitor, a tuned circuit has the same sharpness of tuning over its whole band.

The TRF can use any number of RF amplifier stages, but usually it has no more than three. In each TRF there is one more tuned circuit than there are RF amplifier stages. The extra RF tuned circuit, of course, is in the detector stage. When these tuned circuits are controlled by variable capacitors, it is advantageous to have the capacitors tunable by a single control. This can be done by ganging the capacitors, as shown at the right. This means that the capacitors are mounted in line with their rotors attached to a common shaft. Grounded partitions act as shields. They












Variable Capacitors provide electrical separation between the stator sections of the ganged capacitors.

It is almost impossible to manufacture a set of ganged capacitors of exactly the same capacitance. Similarly, it is difficult to keep each one of a set of ganged capacitors at the same capacitance. Even a slight bending of a single plate of a capacitor will change its value. Still, it is necessary for capacitors to "track" that is, to maintain equal capacitances at each setting of the rotor. To com-

Ganged Tuning Capacitor

pensate for differences, therefore, each of the ganged capacitors is provided with a small additional capacitor. It is screwdriver adjustable, and is called a trimmer. It is connected in parallel with the main tuning capacitor section. (See illustration on top of page 12.)

To make further adjustment possible, the outer rotor plates on each section are usually slotted, as shown in the illust ration of a slotted plate on page 12. The slots make it easy to bend part of a plate and slightly change the capacitance of one section.

Trimmers Used for Tracking

Band Switching

TRF receivers frequently operate over several bands. Switching from one band to another requires switching from one set of tuned circuits to another. Usually the new set of circuits is formed partially out of the old set by substituting different components for either L or C. Since variable capacitors are bulky, it is practical to use as few as possible. Generally, therefore, it is the coils that are changed to form the tuned circuits for the new band. This can be done by use of a series of plug-in type coils, or by use of a switching arrangement. Where a switching arrangement is used, a single coil form may include windings for one, two, or three bands. A rotary wafer switch, such as the one on page 13, is used to switch in the desired set of coils. The switch connects the proper coils to the capacitor section, and, at the same time, grounds all unused coils. As you see, the switch shown has

Slotted Rotor Plate three wafers. When the shaft is turned, metal contacts fastened to the inner portion move from one contact point to another along the outside portion. Each setting of the switch makes a combination of connections for one band.

You can see the electrical equivalent of two switching arrangements at the right. A shows an arrangement for placing different coils into the circuit with one variable capacitor. B shows an arrangement for switching different capacitors in series with one variable capacitor.


In a multiband receiver, each band has a different bandwidth. Still it is desirable for each band to occupy the entire range of the tuning dial. For example, the broadcast band extends from 550 kc to 1600 kc, a frequency ratio of about 3 to 1 and a frequency coverage across 1050 kc of tuning range. When a dial for this band rotates 180°, it covers about 5 kc for each degree of rotation. Shifting to a new band may bring about a situation where the tuning range occupies only a portion of the dial range and where many kilocycles of frequency are covered for each degree of rotation. This concentration of frequencies in each degree of rotation makes dial reading difficult. To correct this, the band is spread until it more closely approximates the full dial rotation.

Bandspreading can be accomplished by mechanical means. The mechanical arrange

Rotary Switch for Band Switching

ment uses gears which coordinate the rotation of the dial with the rotation of the variable capacitor. But this method is expensive and is used only with precision equipment.

Bandspreading can also be accomplished electrically by using trimmer and padder capacitors, as shown on page 14. These capaci tors can affect the minimum to maximum capacitance of a circuit. To increase the band-spread, a variable trimmer, C2, is connected in parallel with the main tuning capacitor. To decrease the maximum capacitance without changing the minimum value appreciably, a padder or series capacitor, C3, is used.

World Band Radio Circuit Board
Band Change Circuits


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