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Old 19th Nov 2017, 6:32 pm   #21
Al (astral highway)
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Default Re: Another Pantry MW Thing

Hi Hugh...
Quote:
Originally Posted by Argus25 View Post

The output passes to a 750uH output coupling coil of 80 turns of 0.2mm diameter enameled wire wound on a 48mm length of 90mm diameter PVC stormwater pipe (I know that this coil would appeal to Astral Highway)

Yes, spot on! A classy feature indeed. Interesting to see how people go about antenna matching, and it's better-looking than a hidden bunch of low uH chokes which, on inspection to the untrained eye, sometimes look just like resistors.

It kind of draws attention to the importance and precision of this matching component. Thanks for sharing the circuit!

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Old 23rd Nov 2017, 1:35 pm   #22
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Default Re: Another Pantry MW Thing

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Originally Posted by ThePillenwerfer View Post
I've converted the layout diagram into PCB artwork but HAVEN'T TESTED IT so approach it with caution.
Scrap that - I found an error in the level indicator, and I've added a 4-way DIP switch for the output match. Three of the four ways are used to short out the first three inductors in the chain. I've also added additional holes for different sizes of output trimmer and crystal.

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I'm also toying with the idea of adding a transistor and an LED for a signal strength indicator - you'd tune the output trimmer for brightest LED. There's just enough room (I think) near to the power input area of the board. I'll experiment with a simple indicator circuit later today and see what I can come up with.

Last edited by mictester; 23rd Nov 2017 at 1:59 pm. Reason: More Info
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Old 23rd Nov 2017, 3:49 pm   #23
Karen O
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Default Re: Another Pantry MW Thing

Those output inductors aim to resonate with the aerial capacitance so as to cancel it. This leaves only the inductor DC resistance and the radiation resistance. The latter is tiny for a domestic aerial so very little signal develops across it. I recall someone once writing 'this loading coil heater is inefficient - some of the power escapes as RF!'

In saying that, you only need microwatts to service the average house.

Last edited by Karen O; 23rd Nov 2017 at 3:53 pm. Reason: Added comment that only low power really needed
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Old 23rd Nov 2017, 4:06 pm   #24
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Default Re: Another Pantry MW Thing

I've always thought a resonant loop would be the most sensible approach to getting good local signal strength from minimal RF power with a pantry-transmitter. Though I could forsee issues with a decently-high-Q loop causing some sideband-truncation of the higher audio frequencies.
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Old 23rd Nov 2017, 6:13 pm   #25
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Default Re: Another Pantry MW Thing

Quote:
Originally Posted by Karen O View Post
Those output inductors aim to resonate with the aerial capacitance so as to cancel it. This leaves only the inductor DC resistance and the radiation resistance. The latter is tiny for a domestic aerial so very little signal develops across it.
I'd say the opposite - radiation resistance for a short domestic aerial is very high, nearly infinite! There is some capacitance, to which is added the output trimmer.

So signal voltage at the aerial terminal is pretty big, from the small swing at the input to the pi network.
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Old 23rd Nov 2017, 8:09 pm   #26
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Default Re: Another Pantry MW Thing

I'd agree with KarenO.

For any antenna you want the radiation resistance as high as possible, it is the fictitious part of the resistance that radiates the EM waves and, for any rms antenna current, the radiation resistance is directly proportional to the radiated power.

Unfortunately when the geometry of the antenna is much smaller than the wavelength of the carrier, the radiation resistance is very low.

In a loop, it is easier to establish higher rms currents in the conductors of the loop as it is a closed resonant circuit. To get a substantial rms current into a 2m length wire or whip it requires a high rms voltage at its base. However the radiation resistance at a low operating frequency like 1.5Mhz for a 1m (square) loop is much lower than for a 1m whip.

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Old 23rd Nov 2017, 10:08 pm   #27
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Default Re: Another Pantry MW Thing

Ahah, Hugo,

And for those others new to this, there is a huge discussion about this on this thread,

in which you were one of the significant contributors!
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Old 23rd Nov 2017, 11:17 pm   #28
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Default Re: Another Pantry MW Thing

Al, yes and there was quite a bit on this thread too:

https://vintage-radio.net/forum/showthread.php?p=970986

On that one I put up the equations for the radiation resistance of whips & loops.

The interesting thing is that if you re-format those equations for the ratio of the radiation resistance for a loop:whip, you end up with

4pi(squared)/wavelength (squared) when the loop is 1m x 1m and the whip is about 1m tall

What this means is that at a wavelength of about 6.3m (47MHz) the radiation resistance of a 1 turn loop with a side of 1m is about the same as a whip 1m high.

At 200m (1.5MHz) wavelength the 1 turn loop only has about 0.1% of that of the whip, suggesting it would be hopeless to use a loop.

But there are mitigating factors, for example a 5 turn loop is 25 times better than a 1 turn loop and as I noted its easier to get higher currents at resonance in the loop (and remembering the radiated power is the product of the radiation resistance and the square of the rms current) and most of the effects of pantry transmitters take place in the near field as the wavelength is so long.

In my experiments on this issue, I gained substantially better performance out of a 5 turn resonant loop than the whip of comparable geometry, as G6Tanuki suggested. So I stick with loops.

Last edited by Argus25; 23rd Nov 2017 at 11:43 pm. Reason: add info & fix typo
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Old 24th Nov 2017, 12:45 am   #29
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There is an identity that applies at single frequencies by which a low value capacitor in SERIES with a low value resistor can be converted to a small capacitor in PARALLEL with a large resistor. At a single frequency, both of these networks would present the same complex impedance. I half suspect that's what Smith charts are all about...?

So kalee is right in a way, and if you were to place the inductors in shunt position as oppose to series, then the aerial would look like a high value resistor (probably in 100s of k). Trouble is, you still get the same losses and your RF would mostly go into heating the coil.

You can tell I am a failed analogue engineer, can't you?
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Old 24th Nov 2017, 1:30 am   #30
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Quote:
You can tell I am a failed analogue engineer, can't you?
Hmmm... no I can't... I'd say you had a good grasp!

Yes I see where we are, you'd originally conceived as a small radiation resistance in series with a capacitance (and yes the notional voltage across this resistance would be very small).

I'd conceived as a very high radiation resistance in parallel with a capacitance - the notional voltage across the network would be fairly high, but because of the massive radiation resistance, radiated power would still be small. At any one frequency a bit of complex number arithmetic converts from one to another. I'd actually reckon this would be the more useful representation, because you can at least probe the voltage across this resistance (stick your 'scope probe on the aerial terminal) whereas the series case you can't access the series junction point.

Erm... can you tell I'm a failed wireless transmission engineer?
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Old 24th Nov 2017, 10:27 am   #31
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Thanks mictester for posting this very interesting project. I hope to have a go at building your circuit when I can find the time.

Regards
Symon.
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Old 24th Nov 2017, 12:55 pm   #32
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Default Re: Another Pantry MW Thing

You're welcome Symon. I've also got a Vero strip board layout that I've built twice now. I'll put that up on here for those who don't like etching boards. The Vero version works as well as the PCB version, but takes a little more time to assemble.
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Old 24th Nov 2017, 3:25 pm   #33
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Yes please!

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Old 24th Nov 2017, 4:42 pm   #34
G8HQP Dave
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Default Re: Another Pantry MW Thing

Quote:
Originally Posted by Karen O View Post
There is an identity that applies at single frequencies by which a low value capacitor in SERIES with a low value resistor can be converted to a small capacitor in PARALLEL with a large resistor. At a single frequency, both of these networks would present the same complex impedance.
Yes.

Quote:
I half suspect that's what Smith charts are all about...?
No.

Quote:
So kalee is right in a way
Only in the way of overlooking the actual definition of radiation resistance, which is that it is a series resistance. Of course it can be transformed into parallel; that is what happens when someone uses a 'hairpin' match on a Yagi.
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Old 24th Nov 2017, 5:22 pm   #35
Philips210
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Quote:
Originally Posted by mictester View Post
I've also got a Vero strip board layout that I've built twice now. I'll put that up on here for those who don't like etching boards.
Hi mictester.

That would be much appreciated, thanks.

Regards
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Old 24th Nov 2017, 5:22 pm   #36
Karen O
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Yes Dave, it is standard to express a network as A+jB where A is the ohmic resistance and B is the reactive (capacitive/inductive) component, i.e. as a series combination. Short aerials are almost all capacitive with a tiny bit of ohmic (the 'radiation resistance').

Actually, it's fairly obvious that an aerial has an ohmic component - capacitors and inductors only store energy. Only resistors can absorb energy.

Last edited by Karen O; 24th Nov 2017 at 5:23 pm. Reason: Make clear who I'm replying to
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Old 24th Nov 2017, 10:43 pm   #37
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Quote:
Originally Posted by Argus25 View Post
At 200m (1.5MHz) wavelength the 1 turn loop only has about 0.1% of that of the whip, suggesting it would be hopeless to use a loop.
But there are mitigating factors, for example a 5 turn loop is 25 times better than a 1 turn loop and as I noted its easier to get higher currents at resonance in the loop (and remembering the radiated power is the product of the radiation resistance and the square of the rms current) and most of the effects of pantry transmitters take place in the near field as the wavelength is so long.
Can you give details about the 5 turn loop antenna. eg the wire diameter and the expected ohmic loss in the 5 turns and also what bandwidth you get from this antenna. Then compare this to the 1 turn loop made from 1.3cm diameter copper pipe as per the Wenzel loop in that old thread. The only experience I have of this stuff is from a few quick tests of the single turn loop and it definitely wasn't 'hopeless' for pantry use. The bandwidth was a little narrow for music but it was design centred for talk stations. I thought the whole design for the modulator and how it interfaced to the loop was quite elegant.

Can you give me the design details for your 5 turn antenna and the matching and I'll have a go at building one over xmas? I still have the Wenzel loop here to compare it against.
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Old 25th Nov 2017, 12:17 am   #38
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Default Re: Another Pantry MW Thing

As requested, here's the Veroboard layout I've used.
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Old 25th Nov 2017, 12:27 am   #39
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Excellent mictester, thanks for that. I am going to have a go at building your circuit over the next few weeks.

Regards
Symon.
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Old 25th Nov 2017, 1:51 pm   #40
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Hi mictester.

Looking at the circuit diagram, I am a little confused about the frequency of the crystal. The crystal is quoted as 4.608MHz but at the end of the notes, it reads as 5.120MHz. I assume the correct frequency is 4.608MHz.

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