Martin, just a remark, in your wire numbering scheme of the input transformer two wires of different windings carry the same number "4".

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Regards

Fred

I found another JFET matching circuit at:

It is slightly more complicated but would be easier to use.

The phasing dots of the transformer are a key piece of information in the circuit diagram and crucial for proper functioning of the amplifier.

?

regards

Fred

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On the off chance there is interest in matching JFETs for a clone of the Wellbrook loop antenna, I've posted a PDF of a simple and effective unit.

Layout isn't critical, I constructed mine on vero-breadboard with 0.1" centers and used point to point wiring and mounted it ~1/8" above an unetched PCB for a ground plane.

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Note 1: I did not design the unit, I built one late last night and tested a bunch of 2N3819.

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A lifetime ago, 1975, I was given several hundred prime TI 2N3819s. Sadly the 2N3918 isn't the most useful JFET but it was nice to test 50 and find 3 that matched extremely closely.

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YMWV

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Note 2: The 2n3819s aren't for sale.

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The file name is

JFET-matcher-rullywow.com-Build-Doc-v1.01.pdf

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It can be found in the files section.

Mine added too, derived separately, useful for comparison purposes.

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?

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On Sun, Jul 21, 2024 at 04:26 PM, Mike M wrote:

Steve,

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Schematic added:

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Also, I notice that the 2SK715 had four ranks for IDSS and the 2SK932 has two ranks for IDSS.? Any idea of which rank was used by Wellbrook?

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Thanks again.

--

===================================================================
Mike M

Hi Martin,

?

Well when you have a separate 50 ohms coupling loop ( your excitation loop ) coupled with a tuned loop:?

1. More separation betveen both loops - lower the magnetic coupling K coefficient, less induced power from excitation loop into the tuned loop.

2. Less separation, the coupling coefficient rises, induced power goes up and the resonant frequency of tuned loop goes up insignificantly in accordance with:? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? f= 1/2xpi x sqrt (( La (1-K^2)x Ca)) .

?

Raphael

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Steve,

?

Thanks for doing this. It would be a shame to lose these designs now that Andy is gone.

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I added a folder and two files illustrating T1:

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Could you take a look and see if I have it correct?

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Thanks!

--

===================================================================
Mike M

I'd like to see that Russian verswion. Could you forward it?

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Chris

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When the going gets weird, the weird turn pro

- Hunter S. Thompson

First, disconnect the anttena, Is the red light on?? My antenna interface is like yours, has a resettable fuse too.

?

Ckeck your power supply, and check the interface output with a voltmeter

I will repeat Chavdar's commercial email address (he produces the amplifier and interface board under the active-antenna.eu brand name) as before.

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?

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On Sat, Jul 20, 2024 at 06:27 PM, KQ4NUO wrote:

Hi Todd,

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Measure the DC current being drawn by the loop amplifier, somewhere along the coax feeding the loop.

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This will determine that power is actually reaching the amplifier, and if the devices are OK.

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The current will ?most likely be in the region of 100 to 150mA if it is working correctly.

?

It is possible the previous owner has accidentally transmitted into to loop interface and caused damage. This has happened to the majority of faulty ones I have been asked to look at.

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Regards,

?

Martin

Hello all,

?

I am trying to troubleshoot an issue with a used Wellbrook antenna.? When I set it up the day before yesterday, it was working very well!? Then today I noticed a problem with an intermittent LED on the antenna interface not lighting up (as if no power was getting to the unit).? I found some troubleshooting steps on this forum and checked the following and found to be good:

1.? co-ax is ok (using new RG58)

2. Power is at 12 V to the unit

3. Red LED is now lit up.

?

However, now there appears to be absolutely no signals being picked up, it's completely deaf.? Is there something else to check?? The only other thing I can think of is checking the connections to the loop at the amplifier (they are tightly taped by the previous owner, so I suspect they would be good).

?

Thanks,

Todd

Some additional power supply notes.

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I mentioned there is a severe problem in the way most circuit designs for the uA723 have an error. I said I thought 100 ohms was needed in series with the current sense, it is 1,000 ohms. Most fried uA723 are the result of not using that 1,000 ohm resistor.

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Re noise from batteries.

Mr. Werner Ogiers is a severe audiophile. While I don't necessarily agree with severe audiophiles, I mean oxygen free cryogenically treated copper speaker cables strikes me as a bit unsane, there are some very smart and dedicated people in that clan who actually understand measurement.

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Mr. Ogiers has a series on power supplies and page 4 has noise graphs from chemical batteries.

I doubted his data and performed some tests of my own. [forget the "Some people prefer the sound of the TL431 operating in non-unity-gain" on page 3, even very smart people have their delusions.]

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I used Rod Elliot's Low Noise Test Preamplifier

at https://sound-au.com/project158.htm

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My Dataq data acquisition unit with the Low Noise Test preamplifier with SpectraPluse revealed that "Yyes chemical batteries do produce noise."

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Some batteries produce a lot of noise. And I mean A LOT of NOISE!

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https://www.tnt-audio.com/clinica/regulators_noise4_e.html

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Take it for what it's worth.

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I use Charles Wenzel's Reducing Voltage Regulator Noise

for all of my active antenna power supplies.

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For really critical, low current, needs I add a capacitor multiplier to the output. The noise is so low I can't measure it.

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Something all too many RF designers ignore is "rectifier commutation noise."

https://www.microsemi.com/document-portal/doc_view/14617-rectifier-reverse-switching-performance

[rectifier commutation noise is the term my favorite EE professor used.]

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I won't get down into the weeds over proper rectifier commutation noise snubber design but this can be a serious problem. By serious, I mean so bad as to render a very well designed linear power supply useless for RF work.

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One final note, there is no point in lowering your home grown EMI lower then the ambient RF noise floor, which, if you have AC power, telephone, cable TV, and neighbors within ~300 feet, will be rather high. Such is life and why my wife and I try to escape to a nature preserve so I can do some serious DXing.

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Send an email to Chavdar.

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?

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On Sat, Jul 20, 2024 at 04:21 PM, KQ4NUO wrote: