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Old 31st May 2017, 12:06 pm   #41
Al (astral highway)
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Default Re: RF signal path in this circuit...

Hello again, Argus!

Thank you for this really informative and helpful post. It emphatically confirms my conclusions!! It's great to understand the methamatical underpinnings of what's going on here in a good or poor magnetic loop design.

You've made two very inventive and good looking loops there , beauties! How did you wind those turns inside the PVC tubes?

I will adapt turns for centre frequency of 1MHz. Litz is very expensive here but the investment may be worthwhile in terms of performance ...

I wonder how I can wind them neatly around the outer surface of the snooker triangles without unsightly additions ?
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Old 31st May 2017, 2:41 pm   #42
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Litz is very expensive here but the investment may be worthwhile in terms of performance ...
I don't think performance is much of an issue here simply up the input power a bit (what is a 100mW twixt friends).

Quote:
I wonder how I can wind them neatly around the outer surface of the snooker triangles without unsightly additions ?
Small grooves in the outer edge?
 
Old 31st May 2017, 4:36 pm   #43
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Default Re: RF signal path in this circuit...

Small grooves in the outer edge?[/QUOTE]

Here you go...

Can you explain a bit more please?
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Old 31st May 2017, 4:43 pm   #44
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Default Re: RF signal path in this circuit...

Thanks,


I have one of those so I might use it as a "Pantry" TX rather than build one.

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Old 31st May 2017, 10:21 pm   #45
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Default Re: RF signal path in this circuit...

Quote:
Originally Posted by astral highway View Post
How did you wind those turns inside the PVC tubes?
The pvc tube is just the usual electrical conduit stuff from the hardware store around 20mm diameter with the pre-formed corner sections, making it very easy to create a rectangle with rounded corners. So while it was laid out on the floor, prior to putting it together with the recommended glue, I threaded some enamelled copper draw wires into the assembly. Then, after the glue dried, I used those to pull around the turns of litz wire. The base part is also made from a standard conduit junction box and it provides the space to mount the base connector (a UHF plug) and two toroidal cores for the impedance matching transformer. (AES Antique Electronic supply in the USA sells very good and economical Litz wire and you can buy it per foot)
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Old 1st Jun 2017, 8:16 am   #46
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Small grooves in the outer edge?

Here you go...

Can you explain a bit more please?
Some grooves in the cylinders you are using at the corners, these being the outer edge. Or replace them with spikey hair rollers to space the wires.
 
Old 1st Jun 2017, 5:59 pm   #47
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Default Re: RF signal path in this circuit...

Argues - Thanks again , that's very helpful!

Restoration73 - leaky feeder may be appropriate, but what do I need for this ? Is it a special kind of Coax ? I couldn't find anything by Googling around ...
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Old 2nd Jun 2017, 3:09 pm   #48
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Default Re: RF signal path in this circuit...

I've followed this and the 'bandwidth' thread for a while now and I think things are slowly going off the rails a bit. I read the original article here:

http://www.techlib.com/electronics/amxmit.htm


The modulator + loop antenna at the end of that page looks OK to me. It's been designed to rebroadcast 'talk' stations rather than music and uses a simple diff pair modulator feeding a simple loop antenna. Everything you need to know is in the article and if you build it to the circuit and then adjust the circuit as per the instructions it should be fine as a pantry transmitter as long as you accept that the bandwidth has been optimised for speech and not music. The author claims it works across a large house and that sounds about right for this design. Maybe a 10 metre usable range? If you want to increase the bandwidth for music then it will need some circuit tweaks as well as tweaks to the antenna if you want to maintain a decent range.

It's probably going to produce some distortion in the basic modulator, probably 5-10% but this will still be OK for casual listening to talk stations and music will be OK as long as you don't expect Hifi quality and you also mod the system to get more bandwidth.

The links to the loop calculator programs aren't much use for analysing this type of loop antenna. It's too small in terms of diameter vs wavelength so the online calculator won't give meaningful results. Besides, it's best to view this little loop as a near field 'energy storage' loop rather than a regular/larger loop antenna that is designed to radiate energy away off into the far field.

The circuit has been fairly well slated here but I'm going to stick up for it and say that it is a fairly elegant design and I think the author understands how it works and has described how to adjust it quite well. However, I think he has pasted the wrong scope plot into the article as that plot appears to be the loop voltage rather than the voltage at the collector. The basic design guidelines all make sense to me including the use of the (nominal) 10k resistor to bring down the resistive part of the loop to approach the ideal value for the output of the modulator circuit.

One other thing, I'm a bit confused by your description of your loop circuit. I get the impression you have used two 5nF caps in the loop. The tank needs to have 10nF if you want resonance with 2.5uH at 1MHz and so I think it should be like the image below in your case with 2 x 20nF caps arranged as below. Is yours like this? Either way, I would suggest you choose a frequency up near the top of the band for best performance and the author hints at this too. Hope this doesn't all sound too critical but the general criticism of the original circuit on this thread is a bit unfounded I think
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Old 2nd Jun 2017, 10:09 pm   #49
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Originally Posted by G0HZU_JMR View Post
The circuit has been fairly well slated here but I'm going to stick up for it and say that it is a fairly elegant design and I think the author understands how it works and has described how to adjust it quite well. However, I think he has pasted the wrong scope plot into the article as that plot appears to be the loop voltage rather than the voltage at the collector.
Hi Jeremy, thank you for taking the time to post these detailed observations.
I love the way this forum can bring so many differing viewpoints to overlap, even among experts (I mean, professional electronics engineers in this case, as well as amateur license holders, and sometimes both at once.). It's all good, from my perspective; I learn a great deal along the way. One thing I've learnt on this forum is that 'elegant' with reference to a design is always strong praise indeed!!

Quote:
One other thing, I'm a bit confused by your description of your loop circuit. I get the impression you have used two 5nF caps in the loop.
Hah! I can see where you got that - just a rushed post on the hoof, possibly on a bus at the time, and distracted.

I'm more careful when I'm actually putting things together and always have a dedicated project notebook to hand at home, and the calculator never far away. My actual build was on resonance at (my) just 15KHz below my design frequency of 1MHz, and tweakable with a trimmer, but thanks for taking time with the diagram.

I take your and the author's point about going for the top of the band and I did mention this earlier, but I just happen to have a crystal in the middle of the band, and thought it worth a try. I know it complicates efficiency...

Quote:
Hope this doesn't all sound too critical but the general criticism of the original circuit on this thread is a bit unfounded I think
It just comes across as a very reasoned opinion, so I wouldn't worry about that at all.

For all that, I had actually scrapped the one-turn design already by the time you posted, and wound a 20 turn version, this time with 1.5mm wire, on the same 'snooker-triangle six' former. It is self-resonant at around 2.3 MHz, so a small parallel capacitor and a trimmer will sort it out nicely for resonance at 1MHz. I haven't coupled it to the transmitter yet - I will first research the 'how' of using a single coupling loop a bit more deeply.

I did find that the effective range of the original was disappointing and the bandwidth far too low for music. You explain that this is in line with the author's comments. I'm not expecting hi-fi (especially on my 81 year old Marconi!) but yes, I'd like the 3dB points to make music sound palatable.

With my single turn, 3mm wire lash-up, my experience was very poor indeed. The explanation, of course, is that I was not using the 15mm dia copper plumbing pipe in the author's design , but only 3mm dia wire, and so I was never going to achieve even close to the results of the author. The voltage at the loop was way off his design, and the circulating current also far, far lower, because of my materials choice. I was hoping that I made this part clear and didn't criticise the author for this part of my experience and design choice.

The project is still live, and it won't be long before I can test the new magnetic loop and see how we get on. 10m range is just about i ok, but I do think the author implies a rather more reachy range in his comments. He says:
In contrast, 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!
I am curious about this as my home is a new-build flat with lots of structural steels and reception and propagation are extra complicated at these low frequencies.

I didn't know very much at all about the mathematics of magnetic loop antennas until I read Argus25's post. I found this very illuminating and various things that were a mystery beforehand suddenly became clear.

And even though perhaps the links on MM's page are clearly designed for larger antennas, the content and some of the modelling available via those applets is really handy.

Thank you again for a very informative, balanced, and helpful post, Jeremy.
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Old 2nd Jun 2017, 10:32 pm   #50
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I'd expect it to still work with a usable signal strength to 10metres even with fairly skinny 3mm wire but it might work a bit further if you could live with weaker reception. This assumes that the background noise/interference levels in your location are reasonably low and you use a receiver with a typical ferrite rod type antenna inside that picks up the magnetic near field properly. It also assumes that you have used caps with low ESR at 1-1.5MHz. The caps can easily be the weak link in this system and it pays to use parallel caps to get the net ESR down. If you used skinny wire and lossy caps and didn't adjust the modulator design to suit then you would get quite low voltage swing at the loop.

However, if you want to send/listen to music then I think you will need to make quite a few changes to get more bandwidth and ideally you should try for some AF compression to keep the modulation below 100% and minimise distortion on peaks. Also you may need to add some emphasis to the high AF frequencies before you feed the AF to the modulator. I would expect the system would sound quite flat otherwise in comparison to other AM music stations. Based on a quick peek at stations on the medium wave band with an old Tek analyser here, I think they typically hard limit the RF channel bandwidth to about 12kHz. But I suspect they add some emphasis at the top end of the 6kHz AF BW available to make it sound bright despite the 6kHz AF limitation. A lot also depends on how wide the receiver bandwidth is as well. If you use a receiver with wide BW and make a transmitter with wide BW then it could sound quite good compared to the fairly limited BW of a typical AM station on a typical AM receiver.
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Old 2nd Jun 2017, 10:44 pm   #51
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Default Re: RF signal path in this circuit...

Quote:
Originally Posted by G0HZU_JMR View Post
...However, if you want to send/listen to music then I think you will need to make quite a few changes to get more bandwidth and ideally you should try for some AF compression to keep the modulation below 100% and minimise distortion on peaks. Also you may need to add some emphasis to the high AF frequencies before you feed the AF to the modulator.
Thanks, Jeremy. That's very helpful indeed. I will try to keep modulation between 50-70% - would that work?

I'll also put in an active or passive filter in to add high AF emphasis, as you suggest, and see how we get on.

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Old 2nd Jun 2017, 10:57 pm   #52
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I love the way this forum can bring so many differing viewpoints to overlap, even among experts (I mean, professional electronics engineers in this case, as well as amateur license holders, and sometimes both at once.).
The chap who designed your circuit and wrote the article is Charles Wenzel the founder and president of Wenzel associates

http://www.wenzel.com/

I spotted his name at the bottom of the article and recognised it from RFDesign magazine. He's won the odd design award here and there including one for a novel frequency tripler a few years back. I still have simulations and prototypes of his tripler circuit here somewhere.

Of course this doesn't mean his pantry transmitter design is perfect or flawless but I went over the circuit and analysed it and it seems good to me as long as you accept the limited bandwidth and the fact that it isn't going to work strongly outside (maybe) a 10m radius. I'm also going to assume that his crystal oscillator circuit is OK because I didn't analyse that part of the circuit. But his company does have about 40 years' experience in the design of crystal oscillators if you click on the link

http://www.wenzel.com/about_wenzel1.htm

.
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Old 2nd Jun 2017, 11:03 pm   #53
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Default Re: RF signal path in this circuit...

Wenzel associates did (still do?) some very low phase noise crystal oscillators.

That 'bridge rectifier' frequency tripler he did was elegant. I think he got an HP 3580 spectrum analyser as the prize for it.

I'd never have guessed it was his work, but I still don't like it.

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Old 3rd Jun 2017, 12:28 am   #54
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I still can't see what there is not to like... It looks a really elegant circuit to me. I suppose that some people may build it using lossy capacitors and some may have trouble finding/tuning for resonance as the loop has to be trimmed spot on for the target frequency. So in that respect I guess it may prove disappointing for some builders in terms of the component requirements and the precise tuning required?

However, I think Charles has probably tweaked this circuit a few times because the numbers he gives in his analysis are a bit out wrt his circuit. But not by much. eg the diff current will be 12mA and not 10mA. Some of the other values he gives have similar offsets so I think he has optimised the diff current and other aspects of the design at some point and not updated the text.

Probably the most suspicious part in the circuit (for me) is the 22uH choke. I'm not sure this is a good choice here as the Rparallel of a typical/lossy 22uH choke at 1.5MHz will be significant here in terms of how much it will contribute to the load of the modulator output. He does suggest the use of a 100uH choke and this should be a lot better. Maybe he was lucky with his choice of 22uH choke but 100uH should be a safer bet.

I calculated that the optimum load presented to the modulator at the capacitive tap should be a bit less than 1400 ohms at resonance. Maybe 1200R to allow some margin?

So because of the (mid point) capacitive tap, the total shunt Rp in the loop should be four times this 1200 x 4 = 4800 ohm. At 1.6MHz I think this will deliver an RF BW of about 9kHz if the loop is a single turn at 2.5uH. This will be just about OK for speech.

In his article text, Charles suggests the shunt Rp loss resistance in the loop should be 'near 4000 ohm' and this seems to agree. I can post up what I think are all the design equations for the whole circuit if anyone wants to see them?

This would allow someone to optimise the circuit for a given antenna loop such that optimal swing is achieved from the modulator and this would give optimal range performance with reduced distortion?
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Old 3rd Jun 2017, 2:51 pm   #55
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Default Re: RF signal path in this circuit...

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Originally Posted by G0HZU_JMR View Post

Probably the most suspicious part in the circuit (for me) is the 22uH choke. I'm not sure this is a good choice here as the Rparallel of a typical/lossy 22uH choke at 1.5MHz will be significant here in terms of how much it will contribute to the load of the modulator output. He does suggest the use of a 100uH choke and this should be a lot better. Maybe he was lucky with his choice of 22uH choke but 100uH should be a safer bet.

I calculated that the optimum load presented to the modulator at the capacitive tap should be a bit less than 1400 ohms at resonance. Maybe 1200R to allow some margin?

.

Hi Jeremy or anyone else so inclined ...

Please can you make explicit the effect of a higher inductance on the bias of the output transistor , following what you've already posted there? I found a 22uH choke self-resonant at about 3MHz and subbed a 220uH instead , as the author implied that the design value wasn't that critical . But i have no way of modelling the effect of a higher inductance on the whole circuit ...

Thanks!
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Old 3rd Jun 2017, 6:20 pm   #56
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Default Re: RF signal path in this circuit...

I read through his circuit description but could find no mention of the role of the collector-base capacitor on the output stage. Possibly some sort of neutralisation?
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Old 4th Jun 2017, 9:58 am   #57
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As you say, there isn't a description given for the role of the c-b caps. So we are left to guess...

I think that the 56p cap is part of a capacitive divider with the 100nF cap and this will present a small signal back into the differential amplifier. The ratio of the divider looks huge at first but there is a huge waveform at the collector and the linear range of the diff amp input is only about +/- 100mV. So a few mV arriving at the diff amp input via this divider could be relatively significant.
The signal fed back to the base via the 56pF cap will contain the wanted swing but also unwanted distortion in that swing (as an error signal wrt the 'clean' signal fed to the other base?) so maybe this c-b feedback in the diffamp is some form of distortion cancellation. But I'm only guessing here. I'm not sure if this would even be effective?

Whatever the true role of these c-b caps, I think we can all agree that they aren't essential components for the basic operation of this type of modulator
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Old 4th Jun 2017, 10:11 am   #58
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Please can you make explicit the effect of a higher inductance on the bias of the output transistor
I don't think it will affect biasing but a randomly selected 22uH across a variety of manufacturers may have a low(ish) Q of 20 at 1.5MHz. A good one might have a Q of 60.

If the Q was just 20 at 1.5MHz then the choke will look like an inductor with a 4150 ohm resistor in parallel at 1.5MHz. This resistance is getting significant wrt the target load resistance of about 1200 ohms at the modulator output.

In other words, this choke would automatically add damping to the system and maybe 'too much' damping (if the loop is already over damped due to its own losses in the 8nF caps) and it would prevent the modulator from reaching full voltage swing at the output. I think it's better to use low loss parts for the 8nF caps and also the loop and this choke and then adjust the 10k damping resistor value to get the target voltage swing. A 100uH choke will typically have a much higher Rp across a range of makes.

Of course, the 22uF choke may just give the correct damping by itself if you were lucky. The choke will also have some self capacitance but I've ignored that for this analysis
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Old 4th Jun 2017, 11:14 am   #59
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Hi again Astral Highway.

I can see that you are very keen on the notion of a good pantry transmitter and there is nothing quite like transmitting your own electromagnetic waves.

I agree with the remarks on other posts about the need to process the audio prior to the modulator.

I worked for some time on a good audio compressor and soft clipper.

I use my pantry Xmitter to transmit to my Hacker Sovereign and I have had many a good afternoon listening to music sourced from an iPod.

But it took a lot of work to get it right. One problem is the wide range of audio levels from mp3 source files. At the radio station it is all well controlled and the levels both within and between songs appear uniform, almost as though a human being is controlling the listening level.

To replicate this I started with the NE571 compander IC. Much to my surprise, even though it was taking the square root of the time averaged input signal the compression still wasn't adequate. It actually requires two in cascade, to take the 4th root, to come near the uniform listening level of a typical radio station. One IC has the required function for mono as it was intended for stereo.

I have attached a circuit of the compressor.

Also I have attached the circuit of the driver of an AM transmitter I made.

Even after compression of the audio, the dynamic range of music is still large. It is important therefore that there is soft clipping and a clipping indicator LED so that you can have the highest practical modulation with only the occasional peak clipped and the carrier never modulated out.

The best linear amplitude modulator device that is readily easy to obtain is the MC1496 IC.

The output RF of the circuit attached passes in my case to a broadband amplifier PCB with a 50R output that feeds my unique 50R loop antennas, but you could use it to feed a single transistor RF output amplifier too.

I'm not suggesting you should build this type of thing necessarily, but it shows the lengths I had to go to get a result I was happy with.The scope shot shows the effect of the soft clipping circuit.

(I hope the schematics are readable, I dug them out of my archives)
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Old 4th Jun 2017, 12:39 pm   #60
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Default Re: RF signal path in this circuit...

Yes, that looks like some work has gone into that design. I've not tried making a pantry transmitter but I think many people will start off with something basic. Once the novelty wears off and the reality kicks in that music isn't as rewarding to listen to as it could/should be then various upgrades will probably be attempted. Either that or they abandon regular use of it for music stations.

To be fair to Charles he does make it clear that his circuit is design centred for talk stations. I think it's an elegant solution in this respect.

I looked at his other AM modulator designs on his page and the first one uses a 220pF cap across the c-b of the modulator output rather than 56pF. But this design will have a lower voltage swing at the collector so I'd expect to see a bigger cap here if this is indeed meant to be part of a capacitive divider. But then again, the 56pF could be a lazy carry over from the second circuit? But I'm just guessing.
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