Posts

Capacitor Materials in HF amplifiers

  For the Heathkit SB-200 (572B tubes) and SB-220 (3-500Z tubes), the critical capacitor positions are the RF input/output pi-network tuning and the grid bypass capacitors (200 pF in the SB-200, which are specifically chosen to resonate with the tubes' internal grid leads near ~70 MHz for VHF parasitic suppression).   This makes dielectric choice far more consequential than in a typical circuit. Heathkit SB-200 SB-220 amplifier capacitor dielectric comparison View all Ceramic Dielectric Comparison for These Amps Dielectric Class Stability Loss / Q Suitability in SB-200 / SB-220 C0G / NP0 1 ±30 ppm/°C Very low (tan δ ≈ 0.0015), high Q Only recommended ceramic. Used at 1–2 kV rating for RF tuning and grid bypass.   Preserves the grid-lead resonance and pi-network Q. C0H 1 ±60 ppm/°C Very low Technically fine but rarely stocked; no practical advantage over C0G in these amps. P100, N33, N75 1 Compensating Low Designed for temperature-compensation networks, not ...
Post a Comment
Read more

ADDING 160 METER COVERAGE TO HF AMPLIFIERS

  ADDING 160 METER COVERAGE TO HF AMPLIFIERS   (updated original manuscript, article published in January 1989 *QST*) by Richard L. Measures, AG6K, April 1988 WHY? If you have ever tried to use the 80 meter phone band for local communications on winter evenings, you probably experienced situations where under 200Km propagation was almost nonexistent. The group of friends that I talk with on 80 meters was plagued by these "winter doldrum" conditions so a few of us put up 160 meter antennas to see if the situation could be improved. We discovered that, when the 80 meter band was essentially useless for short-haul propagation, the 160 meter band provided good communication. There were tradeoffs to using the 160 meter band: Most of us had to put up compromise antennas that would fit on a city lot -- like the end-fed Marconi . These antennas were not as efficient as full-size dipoles. The only practical way to compensate for the lack of efficiency was to increase power. Another pr...
Post a Comment
Read more

A Balanced Balanced Antenna Tuner

  A Balanced Balanced Antenna Tuner By Richard L. Measures, AG6K   The published version of this article appeared in the February 1990 issue of QST Magazine. This is the pre copy-edited version, updated in May, 2003. ........................ JPG of a remote controlled, balanced L-network tuner built by Mike, WA1SEO. Antenna tuners are like shovels. It takes more than one kind of shovel to be able to efficiently perform a variety of jobs. For example, a snow shovel isn't suitable for digging holes in hard ground. A tiling spade could be used to shovel snow, but it wouldn't be very efficient. Similarly, no single antenna tuner circuit can do everything extremely well. A balanced load tuner should be designed, from the ground up, for the job that it is intended to perform. This article describes a circuit that does a good job of feeding an open-wire transmission line [ladderline]. It is not intended to be used for unbalanced loads such as a coaxial transmission line or for end...
Post a Comment
Read more