Dave,
I used 36 turns of #32 bifilar turns on a FT75-50 toroid for the 4 JFETs amp. I designed a new board that has a common mode choke on the back side of?the board, which uses a FT84-75 toroid wound with 18 turns of RG178.
On Wednesday, March 19, 2025, 1:01 PM, W0LEV via groups.io <davearea51a@...> wrote:
Since the last email I read on this loop antenna group addressed an E-Field probe preamp and I've read a number of high-Z preamps on this group, I'll contribute in that thread: high-Z input preamps.
I've recently completed the 4-JFET preamp.? I'll include the schematic as an attachment to this email.? I've used four J-310 N-Channel JEFTs as the active devices instead of the noted devices (2SK932).? While the completed preamp isn't the epitome of neatness, it is proving to be a stellar performer for high-Z E-Field probes at LF and MF (depends on the ferrite material used in the chokes)!!? Just sitting on the bench and powered up with nothing but power leads connected, all the BCB stations are present and accounted for.? I should also add that with the exception of two AM stations, I am not in a strong-signal area.? Just holding a single finger within 1.5-inches of my input disc ceramic blocking capacitor increases the signals displayed on my spectrum analyzer by some 8 to 10 dB.? I'd recommend it to anyone considering the requirement for a high impedance input to a preamp!!
ADDITIONAL COMMENT:? The use of a CMC for the output and isolating DC from the RF should be adopted in many other preamps.? I will certainly use this trick in future builds that require power over coax (new acronym:? POC ?)).? This trick can also be used in the shack to do the same thing.? No need for a multi-component bias tee...
And, yes, the transformers/chokes are wound on 75 material ferrite.? Fortunately, I had a few.? Each has 20 bifilar turns on a 120-75 (1.2-inches OD of 75? material ferrite).?
Also note the output connector (I used BNCs) must be isolated from the main circuit ground plane.? The schematic clearly reflects this requirement.
One last note:? Once the proper bias current is established with the potentiometer, it can be removed, its value measured, and a fixed resistor of the same value can be installed in its place (I had to search a bit deep in my potentiometer collection to locate a 50k pot.).
Dave - W?LEV
Hi Martin
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Have you tried the G4COL design with? transformer in the FET circuit?. Its simple and is supposed to have better large signal handling characteristics?
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I have taken some snippets as pictures.
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I was going to try it to see if it was superior to the Miniwhip design for broadcast band overload.
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Its from a RSGB publication.
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Henry
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Everett N4CY