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19th Nov 2017, 6:32 pm | #21 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
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Re: Another Pantry MW Thing
Hi Hugh...
Quote:
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! Al
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23rd Nov 2017, 1:35 pm | #22 | |
Triode
Join Date: Dec 2007
Location: Sometimes Suffolk and other times Limburg, NL
Posts: 37
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Re: Another Pantry MW Thing
Quote:
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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23rd Nov 2017, 3:49 pm | #23 |
Rest in Peace
Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
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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 |
23rd Nov 2017, 4:06 pm | #24 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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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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23rd Nov 2017, 6:13 pm | #25 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,088
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Re: Another Pantry MW Thing
Quote:
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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23rd Nov 2017, 8:09 pm | #26 |
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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. Last edited by Argus25; 23rd Nov 2017 at 8:14 pm. |
23rd Nov 2017, 10:08 pm | #27 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
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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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23rd Nov 2017, 11:17 pm | #28 |
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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 |
24th Nov 2017, 12:45 am | #29 |
Rest in Peace
Join Date: Jul 2011
Location: Bridgnorth, Shropshire, UK.
Posts: 787
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Re: Another Pantry MW Thing
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? |
24th Nov 2017, 1:30 am | #30 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,088
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Re: Another Pantry MW Thing
Quote:
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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24th Nov 2017, 10:27 am | #31 |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,580
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Re: Another Pantry MW Thing
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. |
24th Nov 2017, 12:55 pm | #32 |
Triode
Join Date: Dec 2007
Location: Sometimes Suffolk and other times Limburg, NL
Posts: 37
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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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24th Nov 2017, 3:25 pm | #33 |
Heptode
Join Date: Sep 2009
Location: Tonbridge, Kent, UK.
Posts: 687
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Re: Another Pantry MW Thing
Yes please!
Gordon |
24th Nov 2017, 4:42 pm | #34 | |||
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Re: Another Pantry MW Thing
Quote:
Quote:
Quote:
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24th Nov 2017, 5:22 pm | #35 |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,580
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Re: Another Pantry MW Thing
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24th Nov 2017, 5:22 pm | #36 |
Rest in Peace
Join Date: Jul 2011
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Posts: 787
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Re: Another Pantry MW Thing
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 |
24th Nov 2017, 10:43 pm | #37 | |
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
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Re: Another Pantry MW Thing
Quote:
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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Regards, Jeremy G0HZU |
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25th Nov 2017, 12:17 am | #38 |
Triode
Join Date: Dec 2007
Location: Sometimes Suffolk and other times Limburg, NL
Posts: 37
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Re: Another Pantry MW Thing
As requested, here's the Veroboard layout I've used.
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25th Nov 2017, 12:27 am | #39 |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,580
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Re: Another Pantry MW Thing
Excellent mictester, thanks for that. I am going to have a go at building your circuit over the next few weeks.
Regards Symon. |
25th Nov 2017, 1:51 pm | #40 |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,580
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Re: Another Pantry MW Thing
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. Regards Symon |