VHF Stability


Posted on Feb 4, 2014

The intelligent discussion of VHF systems requires a feel for VHF systems, and how VHF energy moves through a system. At VHF, wavelength is very short. Wavelength in feet is F/983. 6, a typical 150 MHz system would have a quarter wavelength of 19. 8 inches. This does not include velocity factor of dielectric, or unevenly distributed series inductances


VHF Stability
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or shunt capacitances, which can make electrical distance along a conductor appear much longer than it is. The general rule of thumb is two electrical degree length paths will have a negligible effect on system impedances. While that is 3 feet on 160 meters, two electrical degrees is roughly 1/2 inch on 150 MHz. A 10-inch long conductor, in particular a thin conductor with a dielectric, is just like having no path at all for VHF, yet we see Internet suggestions of adding thin wires from grid pins up to tuning capacitors to beneficially alter the path from tuning capacitor to grid in some amplifier layouts! This is the same false notion Johnson engineers used in the Valiant and Ranger transmitters. In the image below, Johnson used a buss wire to ground tuning capacitors to the 6146 socket ground, which turns out to be a disaster for ground loops and VHF harmonic suppression. A similar fallacy exists in HF amplifiers, where people add an even thinner and longer wire from tuning capacitors to the grid pins of tube sockets. A Kenwood TL922 I worked on had just such a mod, clearly the installer had no idea about wavelengths and transmission line behavior. They used a thin wire several inches long, shown below, in an attempt to reduce tuning capacitor to grid path impedance! VHF paths must be short and very wide, and ideally would be smooth surfaces. A wide path acts more like a groundplane, instead of a transmission line. For...




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