COAXIAL TRAP DESIGN

de VE6YP, Tony Field

tony.field@logicnet.com

This programme computes the construction parameters for coaxial traps.

The Coaxial Trap Design window is divided into two sections.  The top section contains the basic design parameters (i.e. dimensions of the coil form that is used as well as the characteristics of the coaxial cable used).  These items must be provided to perform the design.  The second section, below the design parameters, displays the calculated coil design.

Provide “Design parameters”  information:

Frequency:  The desired resonant frequency of the trap

Form Diameter: The diameter of the coil form used.  In the above example, the outside diameter of the black PVC pipe used as a form is 1.625 inches.

Coax Diameter: The outside diameter of the coax cable used.  The above 0.195 inches is the outside diameter of  one of the Belden RG58 coax cables.

Capacitance: The capacitance per foot (or meter) of the cable.  The above 29.667 pf/foot is the capacitance of one of the Belden RG58 coax cables.

Typically,  use RG58 or RG59 for power levels over 500W and RG178 or RG179 for power levels in the 500W or less range.  Obtain the characteristics of your cable by direct measurement or from data sheets provided by the manufacturer.

Press the ENTER or TAB key to move between enterable data cells.  After each field is modified, the coil dimensions are given.  The units of measure may be alternated between British and Metric by pressing the radio buttons.

The calculated coil dimensions are the following:

Turns:  The number of turns required for the trap

Coil Length::  The total length of the coil.

Coax Length:  The coax should be cut to this length

The indicated length represents the coil portion of the trap plus one additional inch of dressing. The extra inch accounts for  ½ inch of the coax is inserted into the coil form.

 End  Sensitivity:  Sensitivity to coax length.

This figure indicates approximately how much the trap frequency will change if the coax ends are trimmed by one inch (or one cm).  This accounts for capacitive effects only and applies to the ends of the coax that extend into the coil form

 Turn Sensitivity:  Sensitivity to changing the number of turns.

This figure indicates approximately how much the trap frequency will change if the number of turns in the coil are changed by one inch (or cm) along the circumference of the coil form.  This accounts for capacitive and inductive effects and can be used as a guide to tuning the coil.

Length/Diameter: The ratio of the coil length to coil diameter.

The optimum band width of the coils is achieved when the length to diameter ratio is 0.45.

The computed inductance, capacitance and reactance at the design frequency are also displayed.

NOTE:

I have found that the calculations are “very close” - but not quite exact because of stray capacitance and inductance when the trap is constructed.  This is more pronounced when the Length/Diameter ratio is 1.0 or greater.  Also, the accuracy of the standard formula for inductance calculations is not perfect. I have had to shorten the coax length by about 2%.  For example, a trap constructed for 40 meters required the coax to be shortened by  1 1/4 inch (or 3 cm).

The QST article (referenced below) indicates that the miniature coax cable such as RG178 was suitable up to power levels of 1KW.  I found that the traps would heat up and become “quite warm” after  one minute of key-down CW with 700W of power. I suspect that a maximum power level for miniature coax might about 500W if CW is the intended mode of operation or if the traps are used in very hot climates.

Trap Construction

The trap construction is usually of the following style:

Typically,  use RG58 or RG59 for power levels over 500W and RG178 or RG179 for power levels in the 500W or less range.

Tuning the Traps

The exact operating frequency of the traps may be determined with a grid dip meter.  If desired, a signal generator and detector may be used as illustrated below:

The signal generator may be a normal bench signal generator or other RF generating device such as one of the MFJ antenna analyzers  - or even a transmitter set to the 1 watt or less output level!.

Tune the signal generator through the trap frequency.  The oscilloscope or volt meter will indicate a minimum at the trap frequency.  If an oscilloscope is used, it is quite possible that the second harmonic of the signal generator will become the visually dominant signal since the attenuation at the trap frequency is “very high” and the signal reaching the oscilloscope at the trap frequency becomes “very small” compared to the strength of the second harmonic which is passed through almost unattenuated.  As a result, the RF probe and volt meter might be more desirable.

The trap frequency may be lowered by increasing the number of coil turns or increasing the length of the shielded coax protruding into the coil form.  The trap frequency may be raised by reducing the number of coil turns or by decreasing the length of the shielded coax protruding into the coil form.

Small changes in frequency (10’s of kHz) may be made by bending the connections in the coil centre closer to the inside surface of the coil form to cause a small change in the effective capacitance and inductance. This effect is most pronounced when the length/diameter ratio is 1.0 or greater.

References

For technical analysis of coaxial trap antennas, refer to the following:

    1. Trapping the Mysteries of Trapped Antennas

         Gary E. O’Neil (N3GO)

         Ham Radio,  October, 1981, page 10

    2. Optimizing Coaxial Cable Traps

         Robert C Sommer (N4UU)

         QST, December, 1984, page 37