Compression Tuning

Every radio amateur, experimenter, and technician needs high-quality coils for use in resonant circuits, transformers, discriminators, or filters. While fixed and permeability-tuned inductors can be obtained from radio supply houses, they may not always have the high-Q factor needed.

The best coil for VHF applications is the air-core solenoid. This simple and inexpensive coil has a high Q factor, typically greater than 200, at frequencies from 30 to 300 MHz.

To construct an air-core solenoid, wind heavy wire (No. 17 or better) on a cylindrical form. The coil can then be removed from the form, as it is rigid enough to support itself. Coils with a larger diameter and heavier wire will have a higher Q factor.

The inductance of a solenoid is proportional to its cross-sectional area and the square of the number of turns. It is also inversely proportional to its length.

One way to vary the inductance of a coil is to alter its length by squeezing or stretching it. As the length is increased, the inductance decreases, and vice versa.

A simple way to achieve compression tuning is to use a screw and nut made of a dielectric material, such as nylon or polystyrene. The screw is threaded through the coil, and the nut is used to squeeze or stretch the coil.

There are several possible mounting arrangements for compression-tuned coils. One end of the coil can be soldered to an eyelet in the base, while the other end can slide through a contact in the base, be bent to form a toggle joint, or be connected with a flexible lead.

Other types of air coils can also be used for VHF applications. For example, conical-shaped coils can be flattened to a pancake when adjusted.

If spring-type wire is used instead of soft copper wire, the coils will return to their original shape when the adjusting screw is loosened. This type of coil can be tuned many times without permanently deforming.

Variable inductors can be used to replace variable capacitors. They are less expensive and can be tuned over a wider range.

Tuned transformers can be constructed using two air-core solenoids. The coupling between the coils can be adjusted by moving them closer together.

Air-core solenoids can also be used in VHF filters. The high-Q factor of these coils results in sharper cutoffs and steeper attenuation curves.

When constructing VHF filters, it is important to carefully consider the layout of the coils to avoid undesired coupling. Adjacent coils should be placed at right angles to each other.

Air-core solenoids are stable and can withstand harsh environmental conditions.

Air-core solenoids have been used in a variety of applications, including VHF filters, tuned transformers, and variable inductor

Air-core solenoids are a versatile and inexpensive type of coil that is well-suited for VHF applications. They have a high Q factor, are stable, and can be easily tuned.