Re: Guy Atkin's Independent Testing of PL-380 7.5" Loopstick


jim_kr1s <jkearman@...>
 

> I went on your website last night, and am in awe. I'm afraid my technical knowledge isn't up there for designing circuits. I just love to tinker. I can see myself trying to duplicate your final working circuits, though. It looks like a lot of fun.

The MW regen would be easy to duplicate, except for the 3-section variable, which is hard to find. No other strange parts. You could use J310 FETs throughout without changing more than one resistor. The first version, which didn't last long enough for a photo, used a single-section variable and a 240-uH coil, but it was hard to sneak up on peak gain at the high end of the band, without it jumping over the threshold into oscillation. You could skip the first three stages of audio filtering and just use the last op-amp stage to drive the LM380. It pumps out lots of audio, more power than my old SX-100. A lot of times that's all you need to hear weak signals. The noise level isn't the problem, it's just that a weak signal doesn't make enough audio. Putting an rf preamp up front can work against you. As this receiver has no AGC, I have to remember to back off the audio gain when tuning around, lest I tune onto a powerhouse.

> Ben Hester was quite a joker, so he set up his Japan Radio 515, I think it was, and then took a bunch of wire in a tangled mess and dropped it on the porch under all the longwires coming in. His tangle heard everything we heard

Sometimes it pays to be late! Commercial and military stations use slightly more elegant versions of the same idea. One antenna feeds a distribution box. To keep the receivers' input circuits from interfering with one another, there's usually an active stage isolating each output port. The isolation amps are usually broadbanded so they never need adjustment. Back in the Cold War days, the U.S. had listening posts all around the Soviet Union, with dozens of R-390s running off common antennas. The radios were racked, and line-level audio went to the listening position (often in a different building), so the intercept operators never knew what frequency they were copying. I imagine NSA has something similar now, with more-sophisticated stuff we'll never see. I imagine Silicon Labs has a Skunk Works where they make the really good DSP chips for the Feds. We can only hope none of them finds its way to China!

73,

Jim, KR1S
http://qrp.kearman.com/  

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