Antenna Design
Antenna Design Software

# Behaviour of a Tuned Trap Inserted in an Antenna Wire

Author: R.J.Edwards G4FGQ © 9th October 2004

A trap is a parallel L and C tuned circuit operating at or near to its resonant frequency. Its function is to isolate the wire beyond the trap from the wire which precedes it. It cannot do this perfectly even at the trap's exact resonant frequency but there is a band of frequencies over which performance can be considered to be satisfactory.

This program calculates the input impedance of a trap as seen by the 1/4-wave length of antenna wire which precedes it and when terminated by the wire open-circuit length which follows it. The length of wire beyond the trap depends on the two basic frequencies at which the antenna is designed to operate.

It is required that the trap's input impedance should be as high as possible over a relatively narrow band of frequencies which depends on the L/C ratio of the trap. But the L/C ratio is pre-determined by the ratio of the two basic working frequencies of the antenna. Only the inductance needs to be entered.

For correct working the trap input impedance should be at least 10 or 15 times the impedance Z which it follows. Z is the end impedance of a 1/4-wavelength of wire which is resonant at the higher of the two basic operating frequencies.

The impedance at HF at the end of a 1/4-wave antenna wire is an uncertain impedance of the order of 2000 ohms. Therefore the input impedance of a trap should be at least 20,000 ohms, either reactive or resistive, over the amateur band for which the trap is intended. This is affected by the stray impedances between the antenna wires and between the wires and ground. If trap impedance is too low the 1/4-wave section is detuned, trap loss increases, and the supposed isolated length of antenna wire begins to radiate.

Program Operating Notes
Whenever the trap resonant frequency is changed via the keyboard the test frequency is automatically set equal to it. The test frequency can then be swept over the trap operating frequency range by using keys 1,2 (Fast) and 3,4 (Slow).

Coil Q is estimated by the program and is displayed. Capacitor Q is assumed to be 2000. Low values of Q decrease the impedance presented by the trap.

Computed frequency deviation is the difference between trap frequency and test frequency. Operating bandwidth between the band edges is twice the deviation.

The ratio of coil wire diameter to winding pitch is assumed to be a typical value of 0.7 diameters of the coil and both antenna wires are non-critical.

An obvious use of this program is to check the performance of a trap in an existing two-band trapped antenna. It is necessary only to know the length of the wire beyond the trap, trap resonant frequency and coil inductance.

It also calculates the input impedance of the length of wire which follows the trap up to a length of about 1/3-wavelengths at the trap frequency. This may be of use in other unrelated applications.

Used in conjunction with program TRAPDIP it may assist with choosing suitable trap capacitor values from a restricted range of practical values and types. It is impractical to trim capacitors. Whereas a trap coil is more easily adjusted to a precise resonant frequency - the most critical design parameter - which can be easily checked and measured with a fixed value capacitor in situ.

TRAPDIP and TRAP3 results may not coincide exactly because modeling differs in detail. E.g., coil and capacitor Q are differently estimated. Coil length is, or is not, included in overall antenna length, but the accuracy of both programs is sufficient for the intended purposes.

This program is self-contained and ready to use. It does not require installation. Click this link TRAP3 then click Open to run from the web or Save to save the program to your hard drive. If you save it to your hard drive, double-click the file name from Windows Explorer (Right-click Start then left-click Explore to start Windows Explorer) and it will run.