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Re Magnetic Loop !!!
To continue the saga......
I fitted a variable capacitor to my 10 foot circumference 1mtr loop and am able to tune 80 to almost 15 meters...works well..but......... How to fit a motor drive !!!.....there seems to be nowhere to acquire cogs, pulleys or gear sets....no model shops around that carries components, that i`ve googled for anyway..(now when i were a lad! ).... ;-) Can you imagine how fast that electric motor turns the capacitor, even with reduced voltage???..you have to be quick i can tell you or you miss the sweet spot heh,heh.... Seriously, any ideas to slow the tuning rate will be most welcome . Thanks. Lee......G6ZSG..... |
On Sat, 08 Oct 2005 06:26:31 GMT, "Lee"
wrote: Seriously, any ideas to slow the tuning rate will be most welcome . Hi Lee, Use the old stand-by of the screwdriver antennas, a screwdriver motor. However, I expect you want two speed - then try a variable speed battery powered drill. For fine resolution and high speed both, look into the world of Stepper motors. You can even design for calibrated settings instead of hunting - or rough position indicators. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message ... On Sat, 08 Oct 2005 06:26:31 GMT, "Lee" wrote: Seriously, any ideas to slow the tuning rate will be most welcome . Hi Lee, Use the old stand-by of the screwdriver antennas, a screwdriver motor. Tried that, it has far to much torque even on low volts .... damaged the gearing on my spare cap :-/ However, I expect you want two speed - then try a variable speed battery powered drill. Good idea, worth looking into!! For fine resolution and high speed both, look into the world of Stepper motors. You can even design for calibrated settings instead of hunting - or rough position indicators. I`ll do a google for steppermotors .... thanks. Lee.....G6ZSG.... 73's Richard Clark, KB7QHC |
Lee wrote:
To continue the saga...... I fitted a variable capacitor to my 10 foot circumference 1mtr loop and am able to tune 80 to almost 15 meters...works well..but......... How to fit a motor drive !!!.....there seems to be nowhere to acquire cogs, pulleys or gear sets....no model shops around that carries components, that i`ve googled for anyway..(now when i were a lad! ).... ;-) Can you imagine how fast that electric motor turns the capacitor, even with reduced voltage???..you have to be quick i can tell you or you miss the sweet spot heh,heh.... Seriously, any ideas to slow the tuning rate will be most welcome . Lots of model-making and robotics suppliers in the UK have small plastic gearboxes, plastic shafts, couplers etc. Try some of the links from my "Components and Suppliers" page. As a short-cut, try to find a small battery-powered barbecue spit motor at one of the home and leisure stores (probably in the end-of-season bargain bin). They are still a bit fast, but will run at reduced voltage or with pulse-width control to slow them down. -- 73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On Sat, 08 Oct 2005 08:05:35 GMT, "Lee"
wrote: Use the old stand-by of the screwdriver antennas, a screwdriver motor. Tried that, it has far to much torque even on low volts .... damaged the gearing on my spare cap :-/ Hi Lee, One way to control that, at least speed-wise, is to turn the DC voltage into pulse-width modulated voltage. You are always applying the same voltage, but you turn it on for a short-to-long time, and off for a long-to-short time over any interval. You change the speed by changing these times. This is more properly called Dwell Time. This offers the prospects of maintaining a constant torque over a considerable range of speed. I'm not certain by what you mean about "too much torque," unless you have too much resistance for the gears to drive.... Stepper motors are also noted for high torque - it goes with the turf. 73's Richard Clark, KB7QHC |
On Sat, 08 Oct 2005 06:26:31 GMT, "Lee"
wrote: To continue the saga...... I fitted a variable capacitor to my 10 foot circumference 1mtr loop and am able to tune 80 to almost 15 meters...works well..but......... How to fit a motor drive !!!.....there seems to be nowhere to acquire cogs, pulleys or gear sets....no model shops around that carries components, that i`ve googled for anyway..(now when i were a lad! ).... ;-) Can you imagine how fast that electric motor turns the capacitor, even with reduced voltage???..you have to be quick i can tell you or you miss the sweet spot heh,heh.... Seriously, any ideas to slow the tuning rate will be most welcome . Assuming it is a permag motor, reduce the motor voltage... add a current limited driver to limit the torque, an electronic shear pin if you like... except it doesn't shear. I am assuming that you are trying to tune the reactance out, to achieve an acceptable VSWR on the line to the ATU/transceiver. Have you considered automating the capacitor drive. You could build a phase detector (between V and I at the antenna input) as a sensor to control the motor drive. Owen -- |
Here in the USA ALL Electronics had 12volt 6RPM DC motors. I drive my large
4600v variable with it using a DC pulse width motor speed control. The motor control was around $22 as a kit in the USA. I would actually like about 3RPM since the motor control slow speed needed for fine tuning is in a small range. I could gear it but a mechanical mess. A simple DPDT switch is used to reverse direction. Check your electronic parts dealers to see if they have motor with built-in gear boxes. Best way to do it. Dick N3HKN PS: My loop is 1 meter on a side square and is used on 20 meters in the attic. I have set it outside where the noise level is less but on really cold days the motor will not turn. Used a simple plastic container from a local discount store. I'll stick to the attic since I have a GAP vertical as well. Yes, they are comparable given the dipole like pattern of the loop. "Owen Duffy" wrote in message ... On Sat, 08 Oct 2005 06:26:31 GMT, "Lee" wrote: To continue the saga...... I fitted a variable capacitor to my 10 foot circumference 1mtr loop and am able to tune 80 to almost 15 meters...works well..but......... How to fit a motor drive !!!.....there seems to be nowhere to acquire cogs, pulleys or gear sets....no model shops around that carries components, that i`ve googled for anyway..(now when i were a lad! ).... ;-) Can you imagine how fast that electric motor turns the capacitor, even with reduced voltage???..you have to be quick i can tell you or you miss the sweet spot heh,heh.... Seriously, any ideas to slow the tuning rate will be most welcome . Assuming it is a permag motor, reduce the motor voltage... add a current limited driver to limit the torque, an electronic shear pin if you like... except it doesn't shear. I am assuming that you are trying to tune the reactance out, to achieve an acceptable VSWR on the line to the ATU/transceiver. Have you considered automating the capacitor drive. You could build a phase detector (between V and I at the antenna input) as a sensor to control the motor drive. Owen -- |
"Richard Clark" wrote in message ... On Sat, 08 Oct 2005 08:05:35 GMT, "Lee" wrote: Use the old stand-by of the screwdriver antennas, a screwdriver motor. Tried that, it has far to much torque even on low volts .... damaged the gearing on my spare cap :-/ Hi Lee, One way to control that, at least speed-wise, is to turn the DC voltage into pulse-width modulated voltage. You are always applying the same voltage, but you turn it on for a short-to-long time, and off for a long-to-short time over any interval. You change the speed by changing these times. This is more properly called Dwell Time. Yes, i`ve been looking into that and it seems the best way to go, a local supplier has a panel mount version in stock £15.00 ($16.00??)......so!!. This offers the prospects of maintaining a constant torque over a considerable range of speed. I'm not certain by what you mean about "too much torque," unless you have too much resistance for the gears to drive.... Not enough resistance, the capacitor comes to the end of its travel but the torque wants to continue..... hence the damage to my spare capacitor!!!. Regards. Lee...G6ZSG...... 73's Richard Clark, KB7QHC |
"Owen Duffy" wrote in message ... On Sat, 08 Oct 2005 06:26:31 GMT, "Lee" wrote: To continue the saga...... I fitted a variable capacitor to my 10 foot circumference 1mtr loop and am able to tune 80 to almost 15 meters...works well..but......... How to fit a motor drive !!!.....there seems to be nowhere to acquire cogs, pulleys or gear sets....no model shops around that carries components, that i`ve googled for anyway..(now when i were a lad! ).... ;-) Can you imagine how fast that electric motor turns the capacitor, even with reduced voltage???..you have to be quick i can tell you or you miss the sweet spot heh,heh.... Seriously, any ideas to slow the tuning rate will be most welcome . Assuming it is a permag motor, reduce the motor voltage... add a current limited driver to limit the torque, an electronic shear pin if you like... except it doesn't shear. I am assuming that you are trying to tune the reactance out, to achieve an acceptable VSWR on the line to the ATU/transceiver. Have you considered automating the capacitor drive. You could build a phase detector (between V and I at the antenna input) as a sensor to control the motor drive. Doesn`t work with a magloop!!!.....thanks anyway. Lee.....G6ZSG..... Owen -- |
"Adelphia" wrote in message ... Here in the USA ALL Electronics had 12volt 6RPM DC motors. I drive my large 4600v variable with it using a DC pulse width motor speed control. The motor control was around $22 as a kit in the USA. I would actually like about 3RPM since the motor control slow speed needed for fine tuning is in a small range. I could gear it but a mechanical mess. A simple DPDT switch is used to reverse direction. Check your electronic parts dealers to see if they have motor with built-in gear boxes. Best way to do it. Yes, thanks Lee....G6ZSG...... Dick N3HKN PS: My loop is 1 meter on a side square and is used on 20 meters in the attic. I have set it outside where the noise level is less but on really cold days the motor will not turn. Used a simple plastic container from a local discount store. I'll stick to the attic since I have a GAP vertical as well. Yes, they are comparable given the dipole like pattern of the loop. |
"Lee" wrote in message ... "Richard Clark" wrote in message ... On Sat, 08 Oct 2005 08:05:35 GMT, "Lee" wrote: Use the old stand-by of the screwdriver antennas, a screwdriver motor. Tried that, it has far to much torque even on low volts .... damaged the gearing on my spare cap :-/ Hi Lee, One way to control that, at least speed-wise, is to turn the DC voltage into pulse-width modulated voltage. You are always applying the same voltage, but you turn it on for a short-to-long time, and off for a long-to-short time over any interval. You change the speed by changing these times. This is more properly called Dwell Time. Yes, i`ve been looking into that and it seems the best way to go, a local supplier has a panel mount version in stock £15.00 ($16.00??)......so!!. Typo ($26.00??)...... This offers the prospects of maintaining a constant torque over a considerable range of speed. I'm not certain by what you mean about "too much torque," unless you have too much resistance for the gears to drive.... Not enough resistance, the capacitor comes to the end of its travel but the torque wants to continue..... hence the damage to my spare capacitor!!!. Regards. Lee...G6ZSG...... 73's Richard Clark, KB7QHC |
Lee wrote:
Seriously, any ideas to slow the tuning rate will be most welcome . What's wrong with a gear-head motor? I have some in my junk box. -- 73, Cecil http://www.qsl.net/w5dxp |
Lee wrote:
Seriously, any ideas to slow the tuning rate will be most welcome . The latest QST has a 6 RPM 12v gearhead motor from All Electronics advertised on page 161 for $13.75. Runs 3 RPM at 6v. -- 73, Cecil http://www.qsl.net/w5dxp |
Hi Lee
Two thoughts Re stepper motors. You can often find them in old junked scanners. A nice cheap source. If you are feeling really cheap the interface can intially be a simple TTL counter cct and some pass transistors. I once used one off my PC parallel port for an EzAl antenna system. Also look at the possibility of using a piston capacitor and driving that with a threaded bar connected to your motor. This may still be a little fast though. You might also get away with a long lever and threaded bar arrangement if turning a standard rotary capacitor. Kind of limits you to something less than 180 degrees tho. A car windscreen wiper motor is another possibility also through a threaded bar. (Overkill on the torque tho) Cheers Bob Vk2YQA Lee wrote: "Richard Clark" wrote in message ... On Sat, 08 Oct 2005 06:26:31 GMT, "Lee" wrote: Seriously, any ideas to slow the tuning rate will be most welcome . |
On Sat, 08 Oct 2005 12:50:30 GMT, "Lee"
wrote: torque wants to continue..... hence the damage to my spare capacitor!!!. Hi Lee, Then the proper design incorporates a travel limit switch. 73's Richard Clark, KB7QHC |
For a stepper motor check
http://www.herbach.com/Merchant2/mer..._Cod e=H2-553 and for the controller check http://www.alliedelec.com/Search/Pro...p?SKU=316-0061 "Bob Bob" wrote in message ... Hi Lee Two thoughts Re stepper motors. You can often find them in old junked scanners. A nice cheap source. If you are feeling really cheap the interface can intially be a simple TTL counter cct and some pass transistors. I once used one off my PC parallel port for an EzAl antenna system. Also look at the possibility of using a piston capacitor and driving that with a threaded bar connected to your motor. This may still be a little fast though. You might also get away with a long lever and threaded bar arrangement if turning a standard rotary capacitor. Kind of limits you to something less than 180 degrees tho. A car windscreen wiper motor is another possibility also through a threaded bar. (Overkill on the torque tho) Cheers Bob Vk2YQA Lee wrote: "Richard Clark" wrote in message ... On Sat, 08 Oct 2005 06:26:31 GMT, "Lee" wrote: Seriously, any ideas to slow the tuning rate will be most welcome . |
On Sat, 08 Oct 2005 12:58:53 -0000, Lee
wrote: Hi Lee, Some years ago i used a 1 RPM reversible dc motor from Edmond Scientific supply. you may want to look see if they still carry them if not an Email to them may produce the Manufacter's name. It work very well with my home brew 30 to 15 mtr Mag loop. -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/ |
On Sat, 08 Oct 2005 12:53:11 GMT, "Lee"
wrote: Doesn`t work with a magloop!!!.....thanks anyway. Why is that? Owen -- |
"Owen Duffy" wrote in message ... On Sat, 08 Oct 2005 12:53:11 GMT, "Lee" wrote: Doesn`t work with a magloop!!!.....thanks anyway. Why is that? Owen ========================== Because there is only one phase - that which corresponds to the motor driving voltage. ---- Reg. |
On Sat, 8 Oct 2005 21:36:59 +0000 (UTC), "Reg Edwards"
wrote: "Owen Duffy" wrote in message .. . On Sat, 08 Oct 2005 12:53:11 GMT, "Lee" wrote: Doesn`t work with a magloop!!!.....thanks anyway. Why is that? Owen ========================== Because there is only one phase - that which corresponds to the motor driving voltage. Call me thick, I still don't understand why you can't sample the RF V and I and feed them to a phase detector to provide an indication of whether to adjust the capacitor up or down to achieve resonance (V in phase with I). Owen -- |
Owen Duffy wrote: On Sat, 8 Oct 2005 21:36:59 +0000 (UTC), "Reg Edwards" wrote: "Owen Duffy" wrote in message .. . On Sat, 08 Oct 2005 12:53:11 GMT, "Lee" wrote: Doesn`t work with a magloop!!!.....thanks anyway. Why is that? Owen ========================== Because there is only one phase - that which corresponds to the motor driving voltage. Call me thick, I still don't understand why you can't sample the RF V and I and feed them to a phase detector to provide an indication of whether to adjust the capacitor up or down to achieve resonance (V in phase with I). Owen -- Hi Owen, Of course you can sample the RF-V and I and use it to adjust the capacitor via the motor control circuits. The motor driving voltage is DC, and I don't understand the inferences in this example. Automatic antenna tuners work on this principle, and work well. Gary N4AST |
Doesn`t work with a magloop!!!.....thanks anyway.
Why is that? ========================== Because there is only one phase - that which corresponds to the motor driving voltage. ---- Reg. ====================================== Automatic Antenna Tuners and Magloops. All ATUs work on the principle of adjusting tuning components such that the load presented to the transmitter has a reflection coefficient of RC = 0+j0 ohms. When mismatched the RC has both magnitude and phase. So there must be two L and/or C variables. That's why there are always at least two knobs on the front of a manual tuner. Or, when automatic, two electric motors. Or a set of relays which accomplish the same tasks. In the case of a T-network the variable components are usually a pair of reversible motor-driven capacitors. There is always a reflexion coefficient bridge which is at balance when the transmitter load is simultaneously R = 50 and jX = 0. Or impedance Magnitude = 50 and Phase = zero. The RC bridge circuit is similar to the so-called SWR bridge except that there are TWO voltage outputs - proportional to magnitude and phase of the RC. Output voltages are amplified to drive the reversible capacitor motors or operate the set of relays. The motors go backwards and forwards and react with each other similar to what a human operator does. When Z Magnitude = 50 and Phase Angle = zero, or sufficiently near, motors stop and the green LED comes on. Now, in the case of a magloop there is only one variable - the tuning capacitor. The other component involved with the tune-up process is FIXED. It is the diameter of the small coupling loop inside the main loop which is fixed. The small coupling loop, in conjunction with the main loop, behaves as the single-turn primary winding of an impedance matching transformer. The main loop, when at resonance, has a very low purely resistive impedance, the sum of the radiation and loss resistances. The effective turns-ratio of the transformer is approximately proportional to the ratio of the areas enclosed by the two loops. It so happens that when the diameter of the coupling loop is approx 1/5th of the main loop diameter, the impedance looking into the coupling loop is 50 ohms. If a 75-ohm feedline is used then the coupling loop needs to be a little larger. A nice property of a magloop is that an impedance match is approximately independent of frequency. When changing bands it is not necessary to change the diameter of the coupling loop even if could easily be adjusted. It can now be seen why the conventional ATU cannot be used with magloops. What is needed is something which searches for minimum loop impedance, or zero phase angle, as the tuning capacitor is varied. It then stops. Any ideas? ---- Reg, G4FGQ |
On Sun, 9 Oct 2005 01:31:54 +0000 (UTC), "Reg Edwards"
wrote: magloops. What is needed is something which searches for minimum loop impedance, or zero phase angle, as the tuning capacitor is varied. It then stops. Any ideas? ---- Reg, G4FGQ Isn't that what I described, a control system that samples RF V and I at the antenna end of the transmission line, and uses that to control the direction of the capacitor drive motor to send it to the point where the phase difference is zero? Yes, I am aware that is only adjusting the reactance of the load presented, but yes, I was aware the R component doesn't vary over a very wide range over the loop's operating frequency range, and that if X was tamed, the line losses would be acceptable. Owen -- |
"Owen Duffy" wrote in message ... On Sat, 08 Oct 2005 06:26:31 GMT, "Lee" wrote: To continue the saga...... snip Assuming it is a permag motor, reduce the motor voltage... add a current limited driver to limit the torque, an electronic shear pin if you like... except it doesn't shear. Yes, good idea......i`ll look into that. I am assuming that you are trying to tune the reactance out, to achieve an acceptable VSWR on the line to the ATU/transceiver An ATU does not work too well with a magloop as the magloop has its own tuning capacitor (ATU) built in all that happens is that the antenna and feed are tuned as one....unless i misunderstand your comment!!!. .. Have you considered automating the capacitor drive. You could build a phase detector (between V and I at the antenna input) as a sensor to control the motor drive. Yes, but that is a bit too involved when pulse width modulation will do the job, albeit, manually.....good thought tho`...... Thanks for the input .... Lee.........G6ZSG..... Owen -- |
"Cecil Moore" wrote in message t... Lee wrote: Seriously, any ideas to slow the tuning rate will be most welcome . What's wrong with a gear-head motor? I have some in my junk box. Wish it was in my junkbox!!! ;-) Lee....G6ZSG...... -- 73, Cecil http://www.qsl.net/w5dxp |
"Owen Duffy" wrote in message ... On Sat, 8 Oct 2005 21:36:59 +0000 (UTC), "Reg Edwards" wrote: "Owen Duffy" wrote in message .. . On Sat, 08 Oct 2005 12:53:11 GMT, "Lee" wrote: Doesn`t work with a magloop!!!.....thanks anyway. Why is that? Owen ========================== Because there is only one phase - that which corresponds to the motor driving voltage. Call me thick, I still don't understand why you can't sample the RF V and I and feed them to a phase detector to provide an indication of whether to adjust the capacitor up or down to achieve resonance (V in phase with I). Owen -- No doubt you can Owen, but that is a little beyond my present technical learning curve unfortunately.....but i`ll get there eventually, i`m learning more as i post!!!. Thanks. Lee.....G6ZSG.... |
Reg Edwards wrote:
It can now be seen why the conventional ATU cannot be used with magloops. What is needed is something which searches for minimum loop impedance, or zero phase angle, as the tuning capacitor is varied. It then stops. Any ideas? There was an article in RadCom some years ago describing an auto-tuning Top Band vertical, which used a simple inline phase detector and a little DC tuning motor. The phase detector is nothing complicated - it's very like a toroid-type SWR bridge rearranged components - but there was a lot of practical information about what's necessary for RF shielding and decoupling. The whole thing should work equally well for a magloop, since the electronics don't care what kind of antenna it actually is. The article was by Tim Forrester, G4WIM, and was reprinted in G4LQI's 'HF Antenna Collection' book... my copy of which is unfortunately in another country. -- 73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
There was an article in RadCom some years ago describing an auto-tuning Top Band vertical, which used a simple inline phase detector and a little DC tuning motor. ================================== Ian, A magloop is an altogether different kettle of fish to a top-band vertical. For a start, the Q of a magloop is in the order of 1000. For a top-band vertical it may be about 50. For most antennas it is about 10. What I would like to know is has anybody ever made an automatic tuner which works with a magloop. Or has manufactured one for sale? By the way, thanks for the Teslar papers although I am unable to run the programs. ---- Reg. |
Reg Edwards wrote:
There was an article in RadCom some years ago describing an auto-tuning Top Band vertical, which used a simple inline phase detector and a little DC tuning motor. ================================== Ian, A magloop is an altogether different kettle of fish to a top-band vertical. For a start, the Q of a magloop is in the order of 1000. For a top-band vertical it may be about 50. For most antennas it is about 10. Not fundamentally different - it only means the magloop tuning will be more sensitive. The servo will still try to drive the system to resonance at zero phase angle. The overall gain around the servo loop will be the product of the antenna Q, the mechanical gear ratios and the voltage gain in the electronics. The last of these can be adjusted with a single pot. If you have a higher antenna Q, you simply need less voltage gain. What I would like to know is has anybody ever made an automatic tuner which works with a magloop. Or has manufactured one for sale? Don't know, never looked. By the way, thanks for the Teslar papers although I am unable to run the programs. (I only downloaded the program for coupled inductors, but haven't studied it yet. It ran OK, after having extracted all of its files into a real directory; it won't run from inside the .zip 'directory'.) -- 73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Not fundamentally different - it only means the magloop tuning will
be more sensitive. The servo will still try to drive the system to resonance at zero phase angle. ================================= Ian, what slightly worries me is - (1) The resistive component of antenna input impedance, as measured at the input of the small coupling loop, when the main loop is even only slightly off-resonant, is altogether different from 50 ohms but is not included in the bridge balancing process. The diameter of the coupling loop is fixed. Yet magnitude and phase adjustments react upon each other as is experienced by a human operator with two variable controls. (2) The coupling between the two loops is very loose. We are trying to adjust the main loop exactly to resonance via a means which is very insensitive to its resonant condition. Direct voltage and current sampling connections to the main loop itself are impossible. (3) We can imagine a situation where the impedance phase-angle is zero at the measuremnt point, and the green LEDs light up, but which does not correspond to exact resonance in the main loop. And exact resonance matters with a magloop. (4) Because the system is trying to reduce a phase angle to zero in the presence of two unknowns, instability can result. We can imagine the system continuously hopping about trying to find the zero. As you can see, I have difficulty in describing what I think happens circuitwise. But I shall be convinced only when somebody produces something which WORKS reliably without human intervention. It may be possible but where is it? ---- Regards, Reg, G4FGQ |
Reg, G4FGQ wrote:
"It may be possible but where is it?" In the shortwave broadcast plant I worked in 50 years ago we had a 3.5 KW AM Raytheon "Autotune" transmitter we used to talk back to our program relay transmitting station in another country. We called it our "order-wire " transmitter. It or its twin were sometimes used for broadcasting but it was low in power for that job. This autotune transmitter had a rotary telephone dial on its panel for programming its mode, operating drequency, etc. You could instruct it to listen to instructions, then dial in A-3 for AM, followed by the frequency you wanted it to operate on, such as 15,925, hit the go button, then stand back and watch the knobs spin as it tuned itself up completely. including putting the desired power into a dummy load. A ready lamp informed you it was good to go on the air at the push of a button. It worked like a charm. Collins made autotune transmitters which are now military relics of WW-2. I never toyed with one of those. Best regards, Richard Harrison, KB5WZI |
Reg Edwards wrote:
Not fundamentally different - it only means the magloop tuning will be more sensitive. The servo will still try to drive the system to resonance at zero phase angle. ================================= Ian, what slightly worries me is - (1) The resistive component of antenna input impedance, as measured at the input of the small coupling loop, when the main loop is even only slightly off-resonant, is altogether different from 50 ohms but is not included in the bridge balancing process. The diameter of the coupling loop is fixed. Yet magnitude and phase adjustments react upon each other as is experienced by a human operator with two variable controls. (2) The coupling between the two loops is very loose. We are trying to adjust the main loop exactly to resonance via a means which is very insensitive to its resonant condition. Direct voltage and current sampling connections to the main loop itself are impossible. (3) We can imagine a situation where the impedance phase-angle is zero at the measuremnt point, and the green LEDs light up, but which does not correspond to exact resonance in the main loop. And exact resonance matters with a magloop. (4) Because the system is trying to reduce a phase angle to zero in the presence of two unknowns, instability can result. We can imagine the system continuously hopping about trying to find the zero. As you can see, I have difficulty in describing what I think happens circuitwise. But I shall be convinced only when somebody produces something which WORKS reliably without human intervention. It may be possible but where is it? I see your point... but what do you actually tune for, when you do it manually? -- 73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Ian White wrote I see your point... but what do you actually tune for, when you do it manually? =============================== Frequency is selected by the receiver, not the transmitter. The transmitter is OFF when I do it manually and I tune for maximum noise in the receiver. How do YOU do it? smiley If I can't do it when the transmitter is ON then neither can an automatic ATU. It would have to be more clever than I am. ---- Regards, Reg, G4FGQ |
Reg Edwards wrote:
Ian White wrote I see your point... but what do you actually tune for, when you do it manually? =============================== Frequency is selected by the receiver, not the transmitter. The transmitter is OFF when I do it manually and I tune for maximum noise in the receiver. How do YOU do it? smiley I freely admit, I've never touched the things... just trying to be helpful :-) But what do you think "maximum noise" means? You hope it's going to mean maximum field strength when you come to transmit, but what does that actually mean in terms of loop tuning conditions? If I can't do it when the transmitter is ON then neither can an automatic ATU. It would have to be more clever than I am. If we're not clever enough to build an automatic ATU for a magloop, it's a sign that there's something about magloops we still need to know... not abandon the idea. First of all, somebody needs to build a phase detector for an existing manually tuned loop, and see what results it gives. -- 73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On Mon, 10 Oct 2005 08:18:35 +0100, Ian White G/GM3SEK
wrote: Reg Edwards wrote: Ian White wrote I see your point... but what do you actually tune for, when you do it manually? =============================== Frequency is selected by the receiver, not the transmitter. The transmitter is OFF when I do it manually and I tune for maximum noise in the receiver. How do YOU do it? smiley I freely admit, I've never touched the things... just trying to be helpful :-) But what do you think "maximum noise" means? You hope it's going to mean maximum field strength when you come to transmit, but what does that actually mean in terms of loop tuning conditions? If I can't do it when the transmitter is ON then neither can an automatic ATU. It would have to be more clever than I am. If we're not clever enough to build an automatic ATU for a magloop, it's a sign that there's something about magloops we still need to know... not abandon the idea. First of all, somebody needs to build a phase detector for an existing manually tuned loop, and see what results it gives. Ian, Reg, I don't for a moment think automation of a loop tuner is trivial, but I don't think it is impractical with modern processor control techniques. For a practical solution, it will probably need some king of position sense from which to derive end limits and velocity for implementing a second order control loop. It is probably a lot like autotuning a PA tank (and that was done in closed loop linear systems decades ago), except that the loop Q is probably much higher and the tuning range wider. The same problems will be encountered in finding the true resonance where phase changes very quickly from a large positive value to a large negative value or vice versa depending on the direction of tuning. It would be interesting Reg, just the install an SWR meter (this is not a windup!) at your magloop, shut your eyes and tune it up on rx as you describe, then see whether is is close to minimum SWR )which will probably be at or very near zero reactance) on tx. To some extent, the tuner algorithms will be simpler than for a two to n variable tuner (many autotuners vary more than two components, they may switch in an autotransformer, or change from L to PI or PI-L configuration). I am more interested to hear from someone who says it can't be done, what they tried that didn't work. A remote autotuner could be just the thing that makes a magloop a very attractive, small, frequency versatile antenna (well, for those who don't have the antenna in the shack or very long arms). Owen -- |
"Ian White G/GM3SEK" wrote in If we're not clever enough to build an automatic ATU for a magloop, it's a sign that there's something about magloops we still need to know... ==================================== We are able to analyse and predict the behaviour of magloops to any required degree of precision. What is missing is how both a magnitude-searching and phase-searching circuit of an automatic tuner works when denied access to the magnitude-searching component. When manually adjusting a tuner it is obvious to the operator that the controls INTERACT with each other. Both variable controls equally affect both magnitude and phase. That much can be gleaned from inspection of the circuitry. For example, in the case of a T-tuner with two variable capacitors, the operator cannot concentrate on one variable exclusively to the other. He continually has to swap from one to the other and obtain a balance by progressively closer approximations whilst keeping his eyes on the co-called SWR meter. An automatic tuner manages to complete the operation by varying both controls simultaneously. But it is obvious from observation of what the drive motors are doing, and the time taken to do it, that the circuit is behaving just like a human operator. Occasionally the motors even have to reverse and try again. When denied access to either one of the two variable controls, the automatic tuner doesn't know what to do next and would become lost. If the desired impedance magnitude is known to be 50 ohms and is somehow inserted in the circuit, this is of little assistance to how the circuit behaves because when the main loop is off-resonance the actual resistive component is miles away from 50 ohms yet the automatic tuner is obliged to do something about it. But without the ability to vary the diameter of the coupling loop, as I say, it is lost. So we need something different from and more sophisticated than the conventional automatic tuner with its relatively simple magnitude and phase-searching abilities. I'll believe it when I see one which works. Regarding your question about manual tuning up for maximum noise (or signal) in the receiver, at the frequency set by the receiver, reciprocity rules and fortunately, with modern transceivers, one can bawl into the microphone and answer a CQ call with confidence that it can be heard. But Ian, you already know all this. I have the time and I just like gabbing about it. I trust you are comfortably settling down in your new country. I have spent happy years, in bits, working in Scotland. It is a most civilised place. ---- Yours, Reg, G4FGQ |
"Owen Duffy" wrote Yes, I am aware that is only adjusting the reactance of the load presented, but yes, I was aware the R component doesn't vary over a very wide range over the loop's operating frequency range, and that if X was tamed, the line losses would be acceptable. ==================================== Line loss on transmit is relatively unimportant. What matters is accuracy of tuning up an extremely high Q loop on both transmit and receive. ---- Reg, G4FGQ |
"Owen Duffy" wrote I am more interested to hear from someone who says it can't be done, what they tried that didn't work. =================================== Owen, I cannot but agree with everything you say and am inclined to say it can't be done automatically with anywhere near to the required degree of accuracy. The obvious automatic way of tuning the loop is to use a remotely controlled signal generator at the other end of the back yard and tune up for maximum signal on receive. This should not be very difficult to arrange but it is too uneconomic to be practical. So I'll stick to manual tuning. But I hasten to add that I hope this conversation does not deter anybody from experimenting with magloops because magloops are, by far, the most power-efficient of all the small, space-saving, neighbor-friendly antennas. Try one on the 160, 80 and 40 meter bands. At higher frequencies there's usually enough space to erect a dipole or an inverted-L which will perform at least as well. (Ian, you could perhaps do a useful article on the subject.) For the basic design, download program MAGLOOP4 from the website below. .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. .......... |
Reg Edwards wrote:
If I can't do it when the transmitter is ON then neither can an automatic ATU. It would have to be more clever than I am. It would be relatively easy to use the SWR meter driving current from an MFJ-259 to control the ATU motor. -- 73, Cecil http://www.qsl.net/w5dxp |
Reg Edwards wrote:
So we need something different from and more sophisticated than the conventional automatic tuner with its relatively simple magnitude and phase-searching abilities. If you remembered what frequency the mag loop had been tuned to last time, would that alleviate the need to know phase? The reason I ask is that is how I auto-tune my screwdriver without knowing phase. -- 73, Cecil http://www.qsl.net/w5dxp |
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