Tuesday, 15 February 2011

Switchable 10 dB transmining inline attenuator




Any QRPer knows how much fun operating with low power can be. A decrease of power from 50 Watts to 5 Wans is -10 dB, or about 2 S-units. The increase in fun is also about an order of magnitude. Usually, decreasing power to a tenth of normal power can be effected simply by turning the transmitter's drive control down, if you have a commercial rig. But there's a point of diminishing returns, and it's frequently impossible to lower the power much less than a fav Watts, and even harder to know what that level is.

With simple QRP rigs, the drive is often fixed, and the power thereby set to 2-5 Watts. Wouldn't it he great to have the means to readily and easily decrease that QRP power by a factor of ten, thus making QW even more fun? And for about 5 or 6 dollars? Well, that's the basis for this project, a switchable 10 dB transmining inline attenuator. Simply place this device inline between your transmitter and antenna tuner (or other 50-Ohm load).

With the switch in the "Bypass" position, full power goes to your antenna normally. A flip of the mini-toggle switch to "-10 dB", and your output goes to one-tenth ofits original power. In either position (assuming your antenna tuner is properly adjusted with high-power applied), your transmitter sees 50-Ohms.

The attenuator schematic is shown above. I used eight 100- Ohm 1-Watt 5% metal-oxide resistors, available from Radio Shack for a mere 25-cents each (RS 271- 152). The DPDT micro-toggle switch (RS 275-626, $3) was selected for it's small form factor and it's excellent ratings. Two UG- 1094 BNC jacks provide my favorite means of connecting RE Ifyou do the math, you'll discover that the attenuator isn't really 10 dB (it's 9.6). This means that for 5 Watts input, you'll get about 550 mW out

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