[Boatanchors] transmitting attenuators
gzook at yahoo.com
Mon Jul 18 20:11:14 EDT 2011
I finally got around to calculating the impedance of the Johnson design 6dB transmitting attenuators that I have been making for use with grid-driven amplifiers, etc. The impedance calculates out right at 350 ohms! No wonder that they won't work with anything that has a fixed 50 ohm output!
Therefore, I have completely redesigned the attenuators to work with 50 ohm impedance. I can still make them for 350 ohm impedance if someone really wants one. The new attenuators will be available in either 6 dB or 10 dB versions for 50 ohms and 6 dB for the 350 ohm versions.
The original Johnson design used a light bulb in addition to resistors for the "high power" (100 watt) version. The new design attenuators use only resistors and have 150 watts of dissipation in the resistors. Also, a heavy duty heat sink has been added. Although the resistors are rated at 150 watts total, I still suggest running no more than around 100 watts, especially on AM. Of course, I definitely rate things conservatively.
Two versions are available, one with a bypass switch and one without.
The price for all of the attenuators is more than the old "low power" version and are less than the "high power" versions. Just about midway between the two.
The original Johnson design had a definite input side and output side. These new attenuators are symmetrical. That is, either jack can be used for input or output.
These attenuators can be used for other things which require a reduction in transmit power as well as for use with grid-driven amplifiers.
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