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Old 11th Aug 2020, 4:45 pm   #1
Stuart R
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Default Lockdown Loop Aerials

I've been tinkering with valve radios since my youth in the late 1970s. I try and keep my collection up and running and have also repaired a few sets for friends.

It seems to me that none of them perform like they used to. I used to hear loads of stations in the garden shed on my Murphy TA92 using just 2 or 3 metres of wire as an aerial stretched between the shed and the adjacent garage.

My usual test bench is the kitchen table, with a lead out to the washing line. I always thought this 15m line, about 2 metres in the air ought to do, but stations are lost in general mush, some of which appears to be generated by my PC set-up.

It's hard to convince visitors that these old Radios can still be enjoyed when the constant audible shash is almost as loud as the program.

On a Sunday evening I used to tune my Ekco A22 into BBC Three Counties Radio to enjoy Keith Skue's show which is shared-out between BBC Local Radio stations in the Eastern Counties. He always plays 'a sedate seventy-eight' which suits these radios well, although reception has never been crystal clear. Sadly, Keith's show has been suspended temporarily due to Covid-19 outbreak and also the Three Counties MW Transmitter was shut-down recently. When the show returns, I'll need to tune to the more distant BBC Cambridgeshire, Essex or Norfolk.

I remember reading with interest the articles on Magnetic Loop Aerials that appeared in the BVWS Bulletin magazine in Autumn 2016 and Spring 2018. They promised noise-free reception and the potential to pull-in a few more stations. They have been discussed on the forum before and this particular variant was mentioned as recently as last year, I just thought that someone might like to see my take on it.

After building two, I can totally agree with Gary and David's findings in the Bulletin articles. If you think that your MW reception isn't quite how you remember it and want to maximise the enjoyment of your radios without the aid of Pantry Transmitters or Bluetooth adapters, I'd thoroughly recommend making one.

A few things I would do differently are with the construction of the Indoor power supply. I chose a small ABS box with mounting lugs, thinking it could hang off one of the radio's back panel mounting screws. This design keeps all mains powered items indoors, but means a window has to be left open to get the chunky coax cable through. I could re-design the system, by using two 'local boxes', indoors and outdoors separated by a thin 12V power and signal cables. Or change to using F-type connectors and then make use of one of those 'through window' satellite connector adapters employing flat cables.

The fuseholder seems unnecessary too. I chose a 500mA fuse as the outdoor unit takes around 125mA. When a short circuit developed on the coax cable, I found that the 12V Power Supply unit protected itself immediately, before the thin fuse had a chance to give way.

Pictures:
  • Completed loop with poor-performing washing line behind.
  • Rotates nicely on scrap piece of 20mm tubing found in shed.
  • BNC connector is hidden in 3rd leg of junction box – thin pliers or wireman's 'Apple Corer' required.
  • Indoor PSU - DC input jack, fuse and sprung terminals for A & E to radio
  • Indoor PSU - 'Happy Light' and BNC to outdoor loop.
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Last edited by Stuart R; 11th Aug 2020 at 4:59 pm.
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Old 11th Aug 2020, 4:50 pm   #2
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Default Re: Lockdown Loop Aerials

I was in a bit of a hurry to build mine. The recent repair of a Strad PW461 for a friend was nearly complete, but the sounds coming from it didn't make for relaxing, everyday listening. The deadline for delivery was approaching – fast.

Gary's amplifier PCBs have been made very meticulously on Manhattan style 'tiles' and David has improved the design further with a custom PCB. I didn't have the time, nor the facilities, but further web-searches found the article by George Smart who is also mentioned in the 2018 article. George's website has the complete solution; he has produced a PCB and had a limited number for sale, along with the 'binocular' ferrite cores. The website also includes a full parts list, with other components sourced from RS. Interesting to note that the commercially-made Loop that started George's project off is built on Veroboard, so maybe it wouldn't be a disaster to resort to a more readily available construction technique?

I usually buy from CPC and Farnell, but the RS prices aren't too bad and the quantity/price of copper wire for the transformers gave an instant £15 saving with RS. That, plus my laziness made me open an RS account an order from them. Connectors, cases and accessories came from CPC, mainly as I wasn't convinced the RS BNC socket came with a fixing nut and washer and they were encouraging me to buy a pack of 25 fitting kits, costing £11. In the end, I opted for bulkhead mounting BNC connectors, rather than PCB mount.

Making the Outdoor Loop

A trip to B&Q sourced some 20mm diameter electrical conduit, 3.1m long and a round, 'T-piece' junction round box. I'd watched a YouTube demonstration of someone using a heat gun to bend this into loops, it seemed worth a go.

I made a rough former with wood screws on chip board for the first loop. There are a few pitfalls with this method:
  • When heated, the conduit stays solid for quite some time then suddenly becomes flexible. At which point the cross-section can accidentally be distorted with light finger pressure.
  • The weight of any hot conduit hanging over your work surface will cause it to droop. For the second loop, I worked within a smaller circumference of my former so the warmed conduit was always supported. A nearby sawing horse also helped.
  • The second loop was made during warmer weather, the conduit became pliable much more quickly.
  • The former's screws make indentations on the conduit as it is bent around them. I reduced the radius of the 'screw circle' and added a strip of cardboard to stop this.
  • Whilst bending the conduit into the circle, the centre of this bend tries to move outwards, creating another kink. It's a constant game of spotting these and putting another screw in on the outside of the curve to stop this happening. I wrapped the screws in gaffa tape or wedged the tube in place with a rounded cardboard tube to stop more 'notches' appearing.

Pictures:
  • Materials gathered
  • Circle sketched out on chipboard, not enough screws in position
  • Adding more screws, but still get a 'stepped' finish with notches
  • Cardboard Jacket around screws to reduce notches in heated conduit
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Last edited by Stuart R; 11th Aug 2020 at 4:58 pm.
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Old 11th Aug 2020, 4:51 pm   #3
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Default Re: Lockdown Loop Aerials

Sounds good and I’m very tempted. Which one did you build? My old bulletins are not to hand to look up the articles.
Thanks,
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Old 11th Aug 2020, 4:52 pm   #4
Stuart R
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Default Re: Lockdown Loop Aerials

The Outdoor Loop – further thoughts....

Before bending the conduit I dropped some string through to act as a draw-wire, but found this not to be necessary.

The conduit seems quite forgiving and mistakes can be re-heated and re-formed, I used a cloth to rub over the re-heated conduit to remove the worst of the screw indentations. Heatguns are very hot (even when switched-off). Be careful!

I think I made a better job of the first loop as I was working more slowly.

I cut a circle (as best I could) from a sheet of scrap plastic and used this to mount the recessed BNC connector in the central 'arm' of the junction box. The trick is to score the 20mm circle and drill-out the BNC hole first before cutting and shaping the final circle. The completed plastic 'bung' was glued in with two-part epoxy adhesive.

A couple of M4 set screws, washers and Nylocs were used to stop the conduit rotating in the junction box 'arms' and therefore keep the loop upright.

The joins and set screws were wrapped in self-amalgamating tape before the system was sent up to its new owner. Maybe some silicone sealant would have looked neater?

Version one was made with a loop of single stranded 'hook-up' wire. I had read that some experimenters with AM passive loops get improved performance when using Coax cable in a Möbius loop, I couldn't hear a noticeable difference.

Pictures:
  • Sagging conduit where heated and un-supported
  • Support on Sawing Horse.
  • The PCB will fit if the corners are filed away
  • BNC bung, recessed in third 'arm' for protection
  • Completed Junction Box
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Last edited by Stuart R; 11th Aug 2020 at 4:58 pm.
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Old 11th Aug 2020, 4:53 pm   #5
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Default Re: Lockdown Loop Aerials

Sorry I added before seeing your “part 2”
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Old 11th Aug 2020, 4:58 pm   #6
Stuart R
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Default Re: Lockdown Loop Aerials

Electronics

George's website gives very clear construction notes. It's best to start with the two tiny transformers made with the enamelled copper wire. I wasn't sure at what pitch to make my bifilar windings, so used his sketch as a very rough guide, I cut two wires, each of 30 to 40cm in length for each bifilar winding. My first transformer failed with a fractured winding, I had twisted the wires around a match stick and continuously twirled this around between my fingers whilst holding the other end. This resulted in some nicely-packed twists, but they got progressively tighter towards the matchstick end, which I think contributed to the break.

One transformer has 10 connections to the PCB and ideally, you want to get these fine wires through the holes before soldering other components which sit very close by. I did manage to extract the transformer, re-wind and replace it by making it with extra long tails.

For the second attempt, I secured the far ends of the two cables in a bench vice, then holding one cable in each hand kept crossing them over, like plaiting hair. It's about 100-150 twists, but quite easy to get a feel of how far to pull to get a uniform plait. Definitely a better job than the first attempt.

Both George and Gary's articles contain mention of an error in the transformer wiring on the diagram, but if you follow the colour codes and pin numbers on the transformer sketches, everything works out well.

It's then an easy job to solder in the rest of the components, starting with the smallest first. On version 2, I forgot to set the pre-set potentiometer to mid-way before fitting it. This needs to be adjusted carefully once assembled to get the correctly balanced current through the two transistors. My multimeter has some HFE sockets, so attempted to choose two components with closely matched values for version 2. I didn't bother with version 1, but it seemed to work well enough.

These PN2222 transistors do get very warm and are fitted with clip-on TO-92 heatsinks, This makes the completed PCB quite a tight fit depth-wise in the junction box.

I don't have much test equipment here, so apart from the balancing adjustment, I did ensure the 12V was reaching the outdoor PCB with the correct polarity and also that it wasn't present on the outgoing aerial connection and onwards to my vintage radio.

In Gary's Bulletin article, he described various encapsulation and protection treatments for the outdoor PCB. I'm afraid at this stage, I've not done any such work, apart from some small condensation drainage holes around the BNC connector. I'm just enjoying some loud, clear stations while 'soak testing' my second unit. Gary also mentions protection from nearby lightning strikes and additions of Transient Voltage Suppressors are something I should look to in the future.

I don't post here often and when I do it's a bit of a long rant. Not trying to show off my technical ability – I can't take any credit for the design. Just a word of encouragement to those like me, who may not have the best technical capabilities and/or workshop facilities. But this is something that has made a real difference to the performance of my sets, which I feel I should share and say to others, “Have a go”.

I've recently seen a circuit design for SDRs that keeps all the signal and power wiring on a shielded twisted pair cable, rather than Coax. I wonder how this would perform to bring decent AM reception through structured Cat 5 wiring? Would adjacent cables carrying network traffic make a complete mess of this? Something to try....

Pictures:
  • Bifilar winding - not tried this before, but second attempt works better
  • T1 Complete
  • Both Transformers Mounted (More complicated T1 is Rearmost)
  • Complete PCB (Version 2)
  • Bias T PCB (note leads extended on capacitors to fit sideways)
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Last edited by Stuart R; 11th Aug 2020 at 5:08 pm.
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Old 11th Aug 2020, 5:06 pm   #7
Stuart R
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Default Re: Lockdown Loop Aerials

Hello Peter,

You snuck-in there whilst I was mid 'cut and paste'.

This version is from Geoge Smart's website, his design is compared with Gary's design in the Spring 2018 Bulletin.

https://www.george-smart.co.uk/projects/wellgood_loop/

It's also been discussed on the forum here:

https://www.vintage-radio.net/forum/....php?p=1142437

This one has been going since around 10:00 this morning, so hoping it's a successful build. Those transistors do get warm and my small, black (camouflaged) enclosure can't help matters.

Regards,

SR
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Old 11th Aug 2020, 5:44 pm   #8
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Default Re: Lockdown Loop Aerials

Loops do make MW/LW/SW a joy to listen to again. Nice build, good as a bought one as they say. Mine is on a rotator (fast, 360 degrees in less than 5 seconds, in essence a large RC* servo controlled from the kitchen) makes nulling out unwanted stations easy.

*Radio Control.
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Old 11th Aug 2020, 8:14 pm   #9
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Default Re: Lockdown Loop Aerials

Stuart, have you tried running the amp at lower voltages? I have built both Gary's design and George's Wellgood and it seems to me that neither need more than about 5-6 volts on them (they actually kick in at less than that) and do not perform any better above that. Moreover, at that voltage there's no concern with transistors getting warm.

I am so "Lockdown numb" and "over cooked" that it took me some minutes to figure out how a Lockdown loop differed from a magnetic loop, then finally realising that it refers to the era rather than the technology

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Old 11th Aug 2020, 8:58 pm   #10
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Default Re: Lockdown Loop Aerials

Excellent Stuart - very neat is all respects!

Good to hear that you're impressed with the performance.

A very enjoyable project to build at little cost, which shows that there's rather more on MW than many believe, with the added benefit of low background noise.

Dr George Smart's PCB is very neat and takes all the hassle out of building the amp and it has to be said that it's rather neater than the stripboard Wellbrook amp. Gary Tempest has a Wellbrook loop, which he's had for some years, and that's what inspired him to experiment to see if he could design and build a homebrew one with similar or better performance. Hence, he was able to 'benchmark' his version against the Welbrook in the same location. In 2017 Gary kindly gifted me one of his first versions which used five transistors, and in 2019, I built Gary's 2-transistor version which featured in the Spring 2018 BVWS Bulletin, which is simpler to build and performs just as well as the first version.

As to transistors, David – ‘Radio Wrangler’ - had kindly gifted me several premium grade Motorola 2N3866 devices. All with gold plated legs, all marked with "4-247" which is the shortened version of the HP part number 1854-0247 so I decided to use those and fitted heatsinks. (On George Smart’s site there is debate and discussion about the merits of various alternative transistors). The 2N2222s have the merit of being physically small, so more suited to George's compact PCB.

Hopefully your efforts might encourage a few more to have a go at building one.
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Old 12th Aug 2020, 12:32 am   #11
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Default Re: Lockdown Loop Aerials

Quote:
Originally Posted by merlinmaxwell View Post
Loops do make MW/LW/SW a joy to listen to again. Nice build, good as a bought one as they say. Mine is on a rotator (fast, 360 degrees in less than 5 seconds, in essence a large RC* servo controlled from the kitchen) makes nulling out unwanted stations easy.

*Radio Control.
Merlin, do you have a picture of that servo set up?

B
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Old 12th Aug 2020, 8:32 am   #12
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Default Re: Lockdown Loop Aerials

Quote:
Merlin, do you have a picture of that servo set up?
No, I must get round to it. Version 1 used a winch servo, they go round more than 360 degrees, normal servos go 60 degrees. They are easy to control with a 1 to 2ms pulse.
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Old 14th Aug 2020, 8:59 am   #13
Stuart R
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Default Re: Lockdown Loop Aerials

Thanks for your kind and helpful comments.

A rotator would be an added luxury - I had rather disappointing reception of Radio Caroline at first, but then realised the tree where version 2 was hung for testing is planted at the wrong angle.

I haven't tried a lower PSU voltage, but will. Some cooler transistors would make me happier.

Maybe the Version 2 that I have kept for myself isn't quite right as the washing line aerial still outperformed the loop on SW (Sobell 516 during daytime), so more investigations required. This was a slightly different build using the Möbius coax loop instead of stranded wire - I only made comparisons of those technologies on MW.

Regards,

SR
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Old 14th Aug 2020, 9:43 pm   #14
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Default Re: Lockdown Loop Aerials

I've probably missed something, but am I correct; is this an unscreened loop ~1m dia?
How does the performance of this compare with a screened loop?, [ie, I turn in a circle of metal tubing split opposite the feed point]

And what is a Moebus coax loop?

The amplifier configuration is also used, in a single ended configuration, in the Lowe HF225 rx as a wideband whip preamp, using a ZTX327, and in various things at Racal...
ZTX327 was AFAIRR the same chip as the BFY90- so I would not expect the 2222 to work well at higher frequencies; you need a 2N5109 [now unobtanium] or better for HF
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Old 14th Aug 2020, 11:12 pm   #15
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Default Re: Lockdown Loop Aerials

John, I wonder if you are familar with George Smart's webpage https://www.george-smart.co.uk/projects/wellgood_loop/regarding his investigation of the Wellbrook antenna?

Comments made on his site are as follows;

The transistors used in the original design were ZTX327, and so this board is designed to fit those. These are now obsolete and are quite expensive to acquire from sources such as eBay. A ZTX653 has a matching pinout with suitable characteristics, and would be a direct drop-in replacement for the ZTX327. Preliminary tests were done with an MPSH10, but the pinout is different, and so require some tweaking to fit to the board. The original ZTX327 transistors had a hFE-min of 15. It is important not to have too high forward current gain (hFE) on the transistors, as this will disturb the correct biasing of the transistors. The pinouts are shown below. Dave GW3WCV writes in (see Feedback section below) to say that he had success with PN2222A devices, which improve the higher frequency performance, too. I have since been using PN2222 devices with improved results and it is pin-compatible with the ZTX327. The PN2222 is my preferred choice.

However, I don't recall whether or not George makes any comment about frequency coverage. It does seem to be the case that a lot of people investigating magllops are only interested in low frequency (MW/LW?) whereas I'm playing aroun on 80m (using 2N3866). A number of people have reported using either 2N5109 or 2N3866 successfully in the amp, but these physically larger transistors are a bit of a squeeze on that PCB. There was also some reason to think that some 5109's which were on sale online in the UK were bad, with gains of only ~40.

I'm beginning to loose track of how many threads there have been on the Forum on magloops this last ~3 years (5 or 6?). I wonder if it would be a good idea to have a single on-going thread?

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Old 20th Aug 2020, 9:41 pm   #16
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Default Re: Lockdown Loop Aerials

"The transistors used in the original design were ZTX327,.... A ZTX653 has a matching pinout with suitable characteristics, and would be a direct drop-in replacement for the ZTX327"

I cannot agree. The ztx653 is a much slower device, Ft 175 MHz, than the ztx 327 of late and lamented memory, Ft 800MHZ, it may work at LW/MW but is seriously lacking in speed for HF.. the PN or 2N 2222A is better, Ft 300 MHz

The field of discrete RF devices has changed - so many common useful bits are no longer made - 2N3866, 4427, 5109, BFY90, ZTX327 ...and very difficult to find equivalents for.. There are many newer devices, much faster, but lower voltage/power, aimed at cellphones, I think. If anyone knows different, speak up!! Please!!!

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Old 21st Aug 2020, 12:20 am   #17
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Default Re: Lockdown Loop Aerials

BFW16A TO39 has an SMT equivalent, BFQ17P
The old favourite Si J-FETs for low noise RF applications have been supplanted by SMT types with improved performance. There is one particularly useful one designed for use in car aerial pre-amps but I'll have to search for the packet to remind me of the type number
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Old 21st Aug 2020, 10:56 am   #18
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Default Re: Lockdown Loop Aerials

Somewhere on the web is an article (i'll see if I can find it) discussing the advantage of using transistors which have really good linearity for magloop amplifiers. I have to admit that I cannot recall why this was claimed to be the case. I think the author had worked his way through quite a few candidate transistors, testing them on an HP curve-tracer. Among the ones which I can recall him picking out were the 2N's 2222, 3866 and 5109. Of course, the successful LZ1AQ amp design uses 2222's.

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Old 22nd Aug 2020, 6:09 am   #19
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Default Re: Lockdown Loop Aerials

Is this paper by Chris Trask the one which you recall?
https://web.archive.org/web/20200109...Evaluation.pdf
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Old 22nd Aug 2020, 8:10 am   #20
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Default Re: Lockdown Loop Aerials

That's a useful paper, and Chris Trask is pretty thorough.

It shows that the 2N3866 is best kept below 50mA, which the mag loop amps on the go do.

One thing he misses saying is that on the plot of the family of curves, you should consider a resistive load line, representing what the collector of the transistor will see. This load line for the NPN polarity devices will be a straight line from a point towards upper left to a point towards lower right. So you can now see why the region where the saturation current starts to curve off to the right becomes important because 'bent' operation encroaches into a larger and larger fraction of the available load line length.

So the obvious way out is to accept that you need to bias-up the transistor to not go above, say, 50mA and to rig the load line impedance to match this. But there is a second escape... use more of the space to the right by using a higher supply voltage.

The more curved saturation line can be seen in those curve tracer plots of transistors with higher Ft and plenty of those with lower noise figures. It is mostly due to the doping recipes used to get those high Ft values.

Things like 2N3866 and 2N5109 were shaped by the needs of the cable TV industry for small distribution amplifiers. In this application, linearity is the crucial need. But in Trask's curves, these devices do not look amongst the best. The answer to this conundrum is that these amplifiers were designed around 24v power supplies.

Transistors give their worst linearity in their low voltage-saturation region so keeping the voltage up and out of that zone is the direct fix. And it's a fix that works well.

Some transistors on a curve tracer will start to show the 'flat' part of their curves bending upwards at the right extreme. Yup, this is another source of non linearity, but it tends to be most strong at higher currents, and the load line tends to keep the transistor away from the high voltage and high current simultaneously condition. So it's less of a threat and Trask doesn't bother showing it.

Now, there is another effect on the go. It doesn't show up on curve tracer plots, and it is significant.

Notice the spec snippets include Ft, the transition frequency and the Miller capacitance Ccb - the base-collector feedback capacitance. These things are a related.

That capacitance is that of the collector - base junction, essentially a reverse biased diode where you could think of transistor action as a big leakage problem ( ) controlled by carrier injection from the nearby forwards biased base - emitter junction. But you should still see it as a reverse biased diode junction.... As Bruce Forsyth said, "What do points make? points make prizes!" So what do reverse biased diodes make? Reverse biased diodes make Varactors.

Oh b*gger! varactor diodes are variable capacitors, controlled by the reverse voltage across them. And they are FAST. They have a long history as VHF and microwave frequency multipliers as well as parametric amplifiers. This doesn't sound good. Frequency multiplication is harmonic creation and that suggests non-linearity.

In an amplifier there is usually voltage gain, so the collector, being the output, swings around more than the input. With a common emitter amplifier, the collector - base capacitance is multiplied by the Miller effect making it more significant still. And as the collector swings around, Ccb swings with it. So the high-frequency roll off point, and the phase shift associated with it ALL swing around with the signal voltage.

So if you have a strong signal, it will pump Ccb and this will amplitude and phase modulate all the other signals. Inter-Modulation is the word.

So there is an RF intermod mechanism which is off of the curve tracer charts. This effect gives an advantage to those devices whose Ft is comfortably far above the wanted frequency range. So the high Ft devices, with the bendier saturation region on the curve tracer plots actually work a lot better in active antennae than you would expect from the plots. Using higher supply voltages and higher collector voltages also puts you onto the lower capacitance and less bent part of the varactor curve of Ccb.

Next!

Just because you're only interested in receiving a chosen frequency range doesn't mean that signals intercepted by your loop stop there. Oh, no! In fact a small loop becomes a progressively more efficient interceptor of signals at growing frequencies. From the point of view of an amplifier attached straight to a loop, it will see everything on the go in the RF environment. It's remarkably difficult to arrange filtering between loop and amplifier without spoiling the bandwidth of the loop, so amplifiers screwed directly to loops it has to be, then.

So low bandwidth transistors get to really flaunt their HF and VHF non-linearities, pumped by signals that are likely outside you range of interest. They don't understand you're not interested in them, so they don't know you want them to swerve around and miss your loop!

This is where resonated loops with remotely controlled variable capacitors (Mechanical ones, please, for god's sake not varactors!) come to the fore. A remote servomotor or stepper will do the job. I got given a number of really nice small steppers intended for very long life.... would you believe for driving the spools in one-armed bandits? They were microstepped for smooth rotation, and even the joggle as the spool halted to show you a lemon was faked by software.

A way around the Miller multiplication of Ccb effects... the reduction of frequency response as well as the less well known modulation by big signals is the cascode circuit. It pays dividends here.

Some time ago I wass designing myself a hifi amplifier. I wanted to get intrmod products more than an hundred dB down even at the top of the audio band and at high (1dB below clipping) levels. Ccb modulation turned out to be a major factor and the eventual structure I developed was heavily cascoded. It did the job. Besides, A colleague and myself were having a bit of a laugh. He was designing himself a very minimalist amplifier and I decided to design one using as many transistors as possible as a feat of creative p***-artistry. Of course, I wanted every transistor to do something useful... something I could point to as giving an advantage.

Could I hear this? most probably not, not even at that age. But I was having fun.

Anyway, Trask's paper shows how a curve tracer makes a mockery of a simple Hfe tester. The same goes for valves. You see the crazy prices the audiophile toob-rolling world has pushed AVO VCMs up to, well consider how much better the Tek and telequipment curve tracers are at providing you with information. But remember the bit above, about RF non linearity not appearing on curve tracers.

The one really nice bit about valves is they lack the varactor effect. They don't have depletion regions whose thickness gets modulated. The bad bit is you don't get 'PNP' complements. Maybe I have the seeds of another April the first audio article?

Enough typing for now. Time to rustle up some breakfast.

David
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