Rob's web

Coaxial neutralizing capacitor

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - Coaxial neutralizing capacitor

During the construction of my linear amplifier I came up against the familiar problem of obtaining a neutralizing capacitor. I needed a capacitor of 10 to 15 pF with a voltage rating of 2000 volts. Now RG-58/U has a capacitance of 28.5 pF per foot and a voltage rating of 1900 volts (r.m.s.), so I figured that six inches of it would do the job.

My procedure for making a coaxial neutralizing capacitor is as follows. First determine the length of coax required to provide sufficient capacitance and add 4 inches. Call this length A. Measure the distance between the plate and grid circuit of the amplifier, taking the route the coax will take. Call this length B. Cut the coax to length A or B, whichever is the greater. Remove ½ inch of vinyl insulation and braid from both ends. Strip the dielectric from one end and pull the inner conductor through the dielectric so that the dielectric protrudes beyond the other end of the inner conductor by about ¼ inch. Now slide both center conductor and dielectric through the braid for about 2 inches. Then slide the outer insulation over the braid to expose inch of braid at the other end. Fig. 1 shows the relative positions of the various parts of the coax when the capacitor is ready for installation. Connect the braid to the grid circuit and the center conductor to the plate. The capacitance can be varied by sliding the inner conductor together with the dielectric back and forth inside the braid. In the case where the routing of the coax requires a longer length than calculated, the braid and outer insulation should be trimmed to a length 3 inches greater than that required to provide sufficient capacitance. After the amplifier has been neutralized the inner conductor can be trimmed to suit the lead dress, providing the length inside the braid is not disturbed. Depending on lead placement, stray capacitance between the outside of the coax and the plate circuit may reduce the calculated length of coax by an inch or two.

Fig 1
Fig. 1. Cross section view of RG58/U coaxial cable capacitor.

Alike P. Hughes, VESEIH/W5.