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#31
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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 |
#32
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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 |
#33
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![]() 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 |
#34
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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 |
#35
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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 -- |
#36
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![]() "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 |
#37
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![]() "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 |
#38
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![]() "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 .................................................. .......... |
#39
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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 |
#40
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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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