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Old 21st Jun 2017, 7:22 pm   #21
Al (astral highway)
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Default Re: Broadband transmission line transformer design ?

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Originally Posted by jimmc101 View Post
How about Motorola AN267 'Matching Network designs with Computer Solutions' for a brief outline of common matching networks.

Hey Jim, that looks really helpful, thank you. Even just skimming it, I can see that it will go a long way to provide me with ways to solve my conundrum.

The software you point to, that looks useful, too, but can it run on a Mac?
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Old 21st Jun 2017, 8:34 pm   #22
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Default Re: Broadband transmission line transformer design ?

Note that in the case of your original loop antenna with the capacitive tap you are only interested in measuring at resonance. So you can exploit the nature of the modulator circuit to give a pretty good indication of the resistance looking into the actual antenna. This is because the modulator is a current source at RF. As long as you have a decent scope with a correctly compensated x10 probe you can get the resistance looking into the loop from:

(Vpkpk on scope with no modulation)/ (current source Ipkpk)

The Ipkpk of the original circuit was 12mA so if your scope showed 12Vpkpk at the capacitive tap with no modulation then the resistance looking into the loop at resonance will be

12/0.012 = 1000 ohms.

But you need to make sure the modulator is built correctly and has sufficient RF drive to run the differential stage to 12mA pkpk and that your scope and probe is OK and also make sure you have found resonance correctly
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Old 21st Jun 2017, 8:50 pm   #23
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Default Re: Broadband transmission line transformer design ?

The other way to measure the loop at 1MHz would be to use a sense resistor and a function generator (sine wave) and use a dual channel scope.

This would give a reasonable indication of the resistance of the loop at the capacitive tap at resonance if you don't mind entering a few numbers into a calculator or spreadsheet to find the resistance. So in the case of the loop antenna I'm not sure you actually 'need' a VNA unless you want to individually measure the impedance of each discrete component used in the loop.
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Old 22nd Jun 2017, 1:48 am   #24
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Default Re: Broadband transmission line transformer design ?

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Is your analyser idea a concept or does it exist in a replicable form?
Many thousands have been built/ They see to have become a bit of a standard in the QRP world. Kanga did kits for them and I was told the BBC bought a bunch of them for the people servicing medium wave fill-in transmitters.

It's not an analyser. It measures forwards and backwards flow of power between your transmitter and ATU. It doesn't measure phase (Though I did a version for myself with phase metering and return loss computation) So you need a VNA if you want to measure true complex impedances to use the results in calculations. This thing will give you an indication of how close you are, so sequentially twiddle ATU knobs to minimise reverse power indication and it will get you there even though it doesn't tell you where there is.

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Old 22nd Jun 2017, 2:59 pm   #25
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Default Re: Broadband transmission line transformer design ?

Jeremy, thanks for these ideas; they are resourceful and I can see we could get a long way using this method. However, a few changes in assumptions:



I'm no longer using the single turn loop, since discovering its inherent , mathematicallly proven limitations compared to a multi-turn loop. So if I use a loop at all, it will be a 20 turn job that I've already rewound to replace the disappointing original . I can see that your method would still work, with adaptations for there now being no centre tap.

I'm also moving from 1MHz to 1.4MHz,

I'm evaluating a base-loaded antenna instead of a loop The loop may end up as a receiving loop, not decided yet...

I'm considering an additional stage of RF amplification for research purposes only.

The vector analyser will help me work out complex impedances in multiple input / output configurations , so a really powerful diagnostic and learning tool.
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Old 22nd Jun 2017, 3:25 pm   #26
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Default Re: Broadband transmission line transformer design ?

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I'm no longer using the single turn loop, since discovering its inherent , mathematicallly proven limitations compared to a multi-turn loop.
I guess a lot depends on what your requirements are for things like range. For me, the single turn loop was all about providing near field energy storage in a kind of magnetic field 'bubble' and the idea was that the receiver could harvest some of this stored energy within the confines of a typical house. I think that was Charles' design goal.

Your mathematical analysis is based on far field radiation, i.e. energy 'leaving' the system and radiating away forever as per a regular antenna design. So your analysis looked at radiation resistance as a key design driver.

I'm still a bit confused how you managed to get a 2.5uH single turn loop to resonate at 1MHz with just two 5nF capacitors in a tap configuration. That would resonate at the second harmonic at 2MHz and would give very poor range at 1MHz and would also cause fairly high spurious levels at the third harmonic at 3MHz. I think you did report seeing spurious at 3MHz?
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Old 22nd Jun 2017, 7:11 pm   #27
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Default Re: Broadband transmission line transformer design ?

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I'm still a bit confused how you managed to get a 2.5uH single turn loop to resonate at 1MHz with just two 5nF capacitors in a tap configuration. That would resonate at the second harmonic at 2MHz and would give very poor range at 1MHz and would also cause fairly high spurious levels at the third harmonic at 3MHz. I think you did report seeing spurious at 3MHz?
Hi Jeremy, you seem to want to use any method under the sun to challenge my rationale to bin the single-turn loop, including, now, discrediting my reasoning. I'm not offended, just saying that's how it comes across.

For the record, I clearly state in this thread :https://www.vintage-radio.net/forum/...3&postcount=75
"I used 2x10nF and made up a 0.5uH inductor from two precision hand-wound coils. I put these in series with the antenna, which in my case was made of a hexagonal loop 2.02 metres in circumference. Total inductance: 2.5uH, resonant frequency bang on 1MHz"
I see that earlier in that thread, I mistakenly cited using 5nF in the divider, probably because I was posting on the hoof and because each 10nF in the divider was made of twin paralleled 5nF's. I'm pretty sure I picked up on that and explained it at the time.

To be clear, I'm not knocking Charles's design, whereas you seem protective of anything pertaining to it. I think the four transistor circuit, with integrated crystal oscillator, diff amp and modulator, is economical in component count and produces a nice, well-modulated signal with plenty of potential to be a good pantry transmitter.

But I can't agree that it has anything like the coverage he implies. Even your tests show that it's only good to around 6 metres (and that's using higher drive than the original design) Plus that's if we're operating towards the top of the AM band, so no chance if we're down at 300m.

He's talking about going through rock walls and being useful over a much larger area
"...this loop transmitter generates a magnetic field that can cut right through the thickest walls. I live in an old rock home filled with concrete and reinforcing wire mesh and this transmitter's signal can be easily picked up in the bomb shelter at the opposite end of the long house. I'm sure it can be received in the cave next door!"

That's patently difficult to justify. My results show that empirically. So do yours. Other forum members (Argus25, for example) have reached the same conclusion using a calculator.

Quote:
Originally Posted by G0HZU_JMR View Post
I think you did report seeing spurious at 3MHz
Yes. That was at the unloaded collector of the output transistor, with a 22uH choke as directed. The problem disappeared with a 220uH choke.

I really am convinced that the single turn loop is just not the way to go. I've given it a good chance and I've moved on to other possibilities. These retain the good things in the original circuit, with some small mods. My personal jury is still out on whether to use another version of loop based on my 20 turn version, or to use a base-loaded antenna via a suitable matching network that has become the focus of this current thread.

For that reason, I'm delighted to be able to use MM's vector network analyser. Very much looking forward to seeing what I find out!
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Old 23rd Jun 2017, 2:57 pm   #28
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Default Re: Broadband transmission line transformer design ?

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To be clear, I'm not knocking Charles's design
Come on... you were knocking it and so were a few others. The words 'shonky', 'hopeless' and 'mess' were used to describe it.

I'm happy for you to bin the single loop antenna and swap to whatever antenna you like and whatever power you like and design whatever matching networks you like and transmit for 200 metres or more, that's all fine by me

I'm just trying to give a balanced opinion about the merits of Charles' design. If I hadn't seen his name at the foot of the article I would have left these threads alone. In this case a respected designer was effectively getting slated for shonky work and producing a messy/hopeless design. You don't agree but maybe your apartment and garden is very large and you expected too much.

I went out today and found some copper plumbing pipe and made a copy of Charles' antenna to his dimensions and with his capacitor values. This antenna is slightly bigger than my skinny/lossy wire prototype. I made my first skinny prototype smaller to be similar to your version.

I also turned the current down to the standard 12mA. It was resonant at just under 1.6MHz and it was no surprise that it worked better than my skinny wire version even with the standard 12mA current. Not much better but at about 6 metres it was now the strongest signal on my Trio receiver. The S meter was full over rather than just under full over. So obviously, it is going to work at greater than 6 metres with decent S/N ratio with a decent receiver.
My little Flirt receiver is surprisingly immune to overload for its tiny size but it is asking a lot to expect it to receive stations at night with no subtle overload noises on a busy band of big signals at night. So at the same 6m range the Flirt wasn't noise free the other night when I held the receiver to my ear listening for 'any' noise between speech gaps.

In terms of total range on my little portable Flirt receiver it (today) managed 46 long strides up the road after 8 feet through the house and a thick masonry wall. It was a noisy and weak DX signal by then but I stopped when the speech was still just legible but very noisy and weak. Another 10 paces and I think it would have been totally gone. But it seems fine for casual listening to talk stations at 10 metres on the little Flirt as long as you don't expect a huge S/N ratio. Put this in the centre of a loft in a typical UK house and I'd expect it to give decent reception in every room as long as you rotate the receiver loop antenna to the optimal position and this assumes your house isn't full of interference from plasma TVs or noisy battery chargers


I've not used the MiniVNA but I'd recommend you do a few tests with it to see what range of impedances it can reliably measure in your band of interest. The specs imply it is quite limited. Hopefully it is much better than the specs when used at MW band frequencies.
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Old 23rd Jun 2017, 7:26 pm   #29
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Default Re: Broadband transmission line transformer design ?

I first saw that circuit, and I wasn't impressed.

If asked to guess who had been involved in it, I would never have guessed Charles Wenzel. I remember articles by him in RF design magazine in the 1980s. His odd-order frequency multiplier is a true gem (the inductor floating in a bridge rectifier). I remember him winning their design competition a couple of times and getting an HP8590A as a prize. I've seen some of his low phase noise crystal oscillators and been impressed (that's an area I've been professionally active in).

But now I know who was responsible, I still think it looks like a bit of a quick lash-up.

If I was doing one I'd have spent more components on it and isolated the functions more and taken measures to get low distortion. When I need a pantry transmitter I put a bit of wire on the output of a spare marconi sig gen sitting on the shelf. If I wanted to go to the trouble of building a piece of dedicated gear, I'd want to do a very careful job just on the principle that if I'm going to do something, I want to do something which feels right to me.

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Old 23rd Jun 2017, 9:09 pm   #30
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Default Re: Broadband transmission line transformer design ?

If a circuit is criticised then it is no defence to say 'it was by respected designer X, so don't criticise it'. That is the way of fashion and the arts, not science and technology. We don't know whether X was having a bad day, or just threw it together in a hurry for a non-critical application, or added clever features which go over our heads. Note that we are not criticizing X, but a circuit designed by X - and we did not know it was by X anyway until recently. It could be argued that a poor circuit by X should attract more criticism than a poor circuit by someone else, on the grounds that it is bad manners to criticise the inadequate efforts of inexperienced people but experts are fair game.

I am still puzzled by the collector-base capacitors in the LTP. The last one appears to do nothing useful, while the first one (if I recall correctly) appears to magnify Miller effect and so add some FM to the AM.
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Old 23rd Jun 2017, 9:47 pm   #31
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Default Re: Broadband transmission line transformer design ?

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If a circuit is criticised then it is no defence to say 'it was by respected designer X, so don't criticise it'.
I made that point immediately after revealing who designed it. Just because it was designed by Charles doesn't mean it is perfect. But I did take the time to analyse it and simulate it and build it and I think it's fine and I got mine to work across my whole house and into the garden. Maybe you missed this because it is on another thread.
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Old 23rd Jun 2017, 10:16 pm   #32
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Default Re: Broadband transmission line transformer design ?

Charles did offer an option in the writeup to improve the distortion. He suggested (and so did Argus25) that you can improve the distortion by increasing or removing the 22R emitter bypass resistor and increasing the audio level.

I'd then expect the distortion in the basic modulator to be lower than it probably needs to be when the signal is received on a typical AM receiver. The original loop antenna will dominate/spoil the distortion and response here at high (music?) AF frequencies anyway. The version with the loop antenna was design centred for talk stations so this is less of an issue.

I'd expect the AM distortion figures for a typical bench sig gen to be no better than this modulator. The sig gen will have the luxury of some level control on the external modulation input but the distortion specs for many sig gens are average at best. Good enough for a pantry transmitter but maybe not as good as this modulator if driven correctly up to (say) 90% modulation?
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Old 23rd Jun 2017, 10:53 pm   #33
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Default Re: Broadband transmission line transformer design ?

With the emitter resistor removed and the modulation set to 5kHz at 90% depth in the modulator here's the spectrum of the received and demodulated output from the Racal 9008M AM detector. The Racal 9008M said it was 90% modulation, it looks like 90% modulation on the scope and the Tek RSA3408A AM demod function says just over 90% mod depth.

I get a similar result for harmonic levels on the RSA3408A if I look at the AF spectrum from the Racal 9008M demodulated output. Note that this is with the modulator run into a 1200R dummy load at full voltage swing of about 35V pkpk at the collector.

To me this looks very good. Too good for the job in hand. But I never know who I'm dealing with on the internet. To some 'audio' people this amount of distortion would be gross.
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Old 23rd Jun 2017, 11:13 pm   #34
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Default Re: Broadband transmission line transformer design ?

Here's the AF distortion of the demodulated output from the Racal 9008M AM detector when the modulator is fed a two tone test that gives 35Vpkpk at the collector into a 1200R dummy load at 1.6MHz. So the PEP is the same as the single tone test earlier. The two tone AF source being fed to the modulator was the LF AF output from an Agilent vector signal generator. This is a versatile waveform/function generator output that gives very low distortion for the two AF tones so the AF source isn't the weak link here. The plot below shows the amount of AF distortion added by the modulation up to RF and the Racal 9008M detection/demodulation back to two audio tones.

Like the previous plots, this is with the 22R emitter resistor removed. The IMD products look very low to me. Easily good enough for pantry use.
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Old 24th Jun 2017, 9:12 am   #35
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Default Re: Broadband transmission line transformer design ?

I should clarify that I'm only testing the raw modulator section here for it's contribution to distortion so the antenna is replaced by a dummy load and I'm using a sig gen to drive the diff amp at a few hundred mV pkpk at 1.6MHz. I don't have a suitable 1.6MHz crystal to make the crystal LO. So the crystal oscillator may become the weak link once you mod the circuit by removing the 22R emitter resistor. But I'd argue that most people struggle to detect 5% THD on music anyway and the signal is going to be received on a domestic AM receiver that will contribute its own distortion via its IF section, AGC, AF section and its speaker. The other issue is that on randomly selected audio content it will be difficult to protect the system from brief modulation peaks that exceed 100% modulation unless you go down the ALC/compressor/clipper route.

So maybe this is academic anyway. However, I could try and get a 1.6MHz crystal to verify operation with the crystal oscillator but it won't be any time soon.
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Old 24th Jun 2017, 12:49 pm   #36
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Default Re: Broadband transmission line transformer design ?

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I am still puzzled by the collector-base capacitors in the LTP. The last one appears to do nothing useful, while the first one (if I recall correctly) appears to magnify Miller effect and so add some FM to the AM.
As I said on the other thread we can only guess. The 56pF cap looks to be part of a capacitive divider. It will present a tiny version of the (modulated/distorted) collector waveform back at one input to the diffamp.

The other half of the diffamp has a 22R resistor in the collector. When modulation is applied this can't swing over the full modulation cycle because the load is resistive. So it will produce a tiny rectified version of the modulated RF envelope here but this should have low distortion in the rectified envelope. The 27pF cap will feed some of this back to the diff amp input. For this to do 'something' I think you probably need to make it with the oscillator circuit interfaced to it.

Maybe it is able to limit distortion/bandwidth with high modulation levels. I didn't bother with these caps on my version here with the sig gen replacing the oscillator. They won't affect the range performance which is what I built my version to prove using the loop antenna. So I ignored these caps. They were used on more than one version of Charles' circuit so they may even be lazy carry over components from one schematic to the other.
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Old 24th Jun 2017, 4:50 pm   #37
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Default Re: Broadband transmission line transformer design ?

Quote:
The other half of the diffamp has a 22R resistor in the collector. When modulation is applied this can't swing over the full modulation cycle because the load is resistive. So it will produce a tiny rectified version of the modulated RF envelope here but this should have low distortion in the rectified envelope. The 27pF cap will feed some of this back to the diff amp input. For this to do 'something' I think you probably need to make it with the oscillator circuit interfaced to it.
I'm not clear how a resistor can produce a rectified signal. Surely the signal at the resistor is just a smaller inverted version of the output voltage from the other collector? Miller effect will magnify the capacitance, although not by much as the gain to there is low. The gain is modulated by the audio, so the input capacitance is modulated. This will probably add to the variation of the parasitic capacitance of the collector-base junction, hence produce a little FM by adding variable loading to the oscillator.

Quote:
So I ignored these caps. They were used on more than one version of Charles' circuit so they may even be lazy carry over components from one schematic to the other.
That is what some of us suspected. Only the man himself can tell us.

This speculation reminds me somewhat of what happens in many English literature classes. The teacher says "At this point the author is saying such-and-such." A famous Indian author said a few years ago that English teachers talk rubbish: he knew that what they said about his books was wild speculation, so he assumed that much of what they said about other books was equally untrue. The teachers' reply suggested that they thought they knew what was in the mind of an author perhaps better than the author himself did!
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Old 24th Jun 2017, 5:21 pm   #38
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I'm not clear how a resistor can produce a rectified signal. Surely the signal at the resistor is just a smaller inverted version of the output voltage from the other collector?
Obviously the resistor isn't a rectifier as such.. I didn't mean to imply that. When the differential pair has the current source modulated by an AF sine wave then the current at the collector of the current source (i.e. the common current source transistor in the emitter) will look like an AF sine wave at about 24mA pkpk at full modulation. Eg a sinewave at 1kHz for example.

But in order to satisfy the diff amp requirements (remember it is being hit on one side by a small RF waveform so it is acting a bit like a switch if driven hard by the RF) then this 24mA AF sinewave will split into each transistor in the diffamp and it has to do it in a way that obeys the rules for a diffamp. i.e. it also has to accommodate the RF waveform and still keep a total current budget set by the current source. So the current waveform through each C-E of the diff amp pair will look very different compared to that 'common' sinewave at AF. It will actually look like one half of the modulated RF waveform (appearing to the casual observer as if it had been rectified)

So because the 22R collector transistor on the left is 'just' a resistor it will produce a voltage waveform that also looks like 'half' of the complete envelope because it is being fed a current waveform that is only half the envelope.

The reason you get a full envelope on the other collector is because of energy storage/release in its collector circuit. This can't happen with just a 22R resistor in the other side
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Old 24th Jun 2017, 5:57 pm   #39
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Default Re: Broadband transmission line transformer design ?

Sorry for the hasty/scruffy drawing below but maybe this will help?
The common (AF modulation) current at the collector is shown by the thick blue sine wave. But each transistor in the diffamp will have a current waveform like the green trace. Obviously each will be out of phase if zoomed/examined at RF but they will look like the green (RF) current waveform. This (6 mA mean) current waveform will pass through each of the C-E of the diff transistors to whatever load is in the collector.
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Old 24th Jun 2017, 6:39 pm   #40
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Default Re: Broadband transmission line transformer design ?

If the carrier signal at the base of the diff pair is large, then the diff pair starts to approximate a current switch. The current in this case is set by the DC bias of the tail transistor, plus or minus an amount due to the modulating signal.

The current either gets dumped into the resonated loop, where the square-ish RF current causes a sine-ish signal in the loop (The resonated loop filters off the harmonics of the square-ish RF) or it just gets dumped and lost via the left side collector.

The AF gain of the tail transistor is boosted over its DC gain (well overall transconductance is a better phrase than gain) by the series R-C pair across its emitter resistor. Looking at that resistor value, I think the tail transistor is being worked rather hard. It'll work, but it could have been better. As for the miller-positioned capacitors, I have no knowledge of why they would be there. I suspected someone bodging in capacitors willy-nilly in pursuit of some spurious oscillation. Given that there was a reputable designer behind it this area becomes more of a mystery.

All designers have some silly circuits to their name. In my case I was in a bunch of fairly well known people (G3ROO, G3RJV, G3PDL, G0BPS, and Peter Halpern of Ham Radio Deluxe fame, so I don't carry the entire blame) at a vicarage christmas party. For a bit of fun we decided to bodge together the most basic transceiver imaginable. A direct conversion receiver (made easier with a nobbled computer speaker) a coupe of diodes and an oscillator. The single balanced mixer was unbalanced by a short through a morse key so the leaked carrier was our TX. the transformer was the funny part, it was wound on two stuffed ferrets shaped into a ring. It worked a couple of stations in Holland. It didn't go in Sprat.

Vicars get some strange things left over after jumble sales.... The two ferrety things were sewn together and had a total of only six legs. For some reason it gave me a strong image of Giles' cartoon grandma wearing it. It's a nicer image than that of one stopping a bit too quickly in a burrow.

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