G8JNJ

A couple of weeks ago I received a private email about this circuit, and decided to make a copy, as the person who wrote was having problems with it, and I couldn't figure out how the circuit could perform as described.

I found that the IMD performance was poor and it had several other issues which couldn't be easily fixed.

If you want to build or buy an amplifier of this type, my advice would be to take a look at the DXE RPA-1 circuit instead.

Regards,

Martin - G8JNJ

The 2N2222 seem to provide the lowest value of input resistance from the types that I have tried. 

For the output pair, almost any NPN transistor with a power dissipation of approx 1w and a FT greater than 1GHz should be fine.

Regards,

Martin - G8JNJ

It's basically two Norton lossless feedback amplifiers connected in a push pull arrangement.

The winding in the emitter circuit provides negative feedback and the ratio of the windings sets the amount of feedback and therefore the overall gain of the amplifier stage.

I'm not at home at the moment so I can't access my list of web links, but try a Google search for Chris Trask's papers on lossless feedback for more information.

When you reduce the supply rail it may reduce the noise floor, and also possibly reduce the gain, which also may give the impression of reducing the noise floor. But below about 3v the bias point of the transistors is way off and the output power capability (and therefore IMD performance) is seriously reduced. What you are observing may actually be a secondary artifact resulting from something else changing which is in turn 'fixing' another problem such as FM BC interference, amplifier oscillation or some other out of band parameter which is impacting the overall performance.

One final thing - it may be that George's 'reverse engineering' is not 100% accurate, especially regarding the construction and winding of the feedback transformer. I suspect that the two amplifiers maybe should be wound on two separate halves of the binocular core but that the input winding is common to both. This is how Trask has implemented some of his augmented push pull amplifiers, but I can't confirm this is how the Wellgood design works.

If you have an opportunity I'd suggest that you also try the LZ1AQ amplifier design http://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm as a comparison. It's actually easier to build if you leave out the regulator and just feed the output into coax rather than using CAT 5 / 6 cable. 

I found that the LZ1AQ performed at least as well, if not better, than the Wellgood clone. But as I stated earlier this may be actually be due to an error in the reverse engineered circuit.

The Yahoo Groups Loops forum is other very useful source of information and is frequented by Trask and Wellbrook. There has been some discussion regarding the Wellgood copy and Wellbrook don't seem to be that bothered that the design is now out in the open. This could be because they have now moved to a new design, or it could be that George's Wellgood circuit isn't correct and they don't want this to be known.

Regards,

Martin - G8JNJ

OK - if that's a problem you need to use much bigger value capacitors :-)

Regards,

Martin - G8JNJ

I think you could either center tap the input transformer on the loop side (prefered method), or add two 1K Ohm value resistors (or similar) from either side of the loop / transformer connection to ground.

Regards,

Martin - G8JNJ

My guess is that you have inadvertently scraped some of the enamel insulation off the transformer winding(s) at some point, as it's very easy to do this.

I use Kynar insulated wire (the type sold for wire wrapping) wherever possible, as this is much more resilient. However this doesn't work with very small cores, as the wire is too thick, so in this case I just have to be extra careful when pulling the enamelled wire through the holes in the core.

Regards,

Martin - G8JNJ

The LZ1AQ loop amplifier kit uses CAT6 for power, control and signal.

CAT 5 & 6 has a characteristic impedance of 100 Ohms per twisted pair. You can build an impedance matching balun or use two pairs connected in parallel and a 1:1 balun. However if you use this method beware, as some CAT cables use different numbers of 'twists per inch' on the different cable pairs in order to reduce cross talk between pairs. This results in some pairs being electrically 'longer' than others., which may cause problems if you connect them in parallel to halve the impedance.

Regards,

Martin - G8JNJ

I've now spent a bit more time playing with this application.

It's not the easiest thing to setup or initially use, but it does have some quite impressive features such as the ability to import various online databases and parse .xlsx files such as the 'Numbers and Oddities' and 'ITU monitoring' databases into a set of local databases that can be searched, sorted and indexed.

When driving a local radio or the Twente WEB SDR (or others of that type) it's very handy for click tuning, ID'ing or scheduling stations.

Definitely worth spending a bit of time with to get the hang of it, plus it incorporates some really good ideas and useful features.

Regards,

Martin - G8JNJ 

I think the loop is still unbalanced.

The split is to avoid a continuous 'screen' (Faraday screen to minimise E-Field pickup) around the loop. But if one end of the center conductor is connected to ground you may have a problem.

Check signal levels with the loop normal and 180 degrees as suggested previously.

A 1:1 transformer on the loop input may help improve balance. Try six bifilar turns through a BN73-202 core as a starting point. One winding as the (balanced) loop input (primary) the other as the output (secondary) unbalanced connected to the amplifier input, with one end of the winding connected to ground. This will also provide galvanic isolation.

You can also try a few more turns on the secondary as an experiment as this may improve reception on the LF bands.

If you made a balanced loop amplifier with a much lower value of input impedance (LZ1AQ or Wellbrook copy) you wouldn't need the loop outer screen to minimise E-Field pickup.

Regards,

Martin - G8JNJ

I recently found a broadcast database application that allows control of various receivers (including the Twente WEB SDR).

 http://www.df8ry.de/htmlen/csvub/features.htm

Although it's a bit of a pain to setup, it does have some 'interesting' features that may be of interest for KiWi admins and users.

Regards,

Martin - G8JNJ