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#1




Constant bandwidth TRF circuit
In article > , "Henry
Kolesnik" > wrote: > Got any idea how it maintains constant BW as BW is a function of Q, a > relative constant and frequency which varies? Also I don't understand your > notation "12uh centertapped" (3uh persection). > tnx I have moved this response from "alt.binaries.pictures.radio" to "rec.antiques.radio+phono" so that it will not be quickly deleted by the server. The following is my take on how these circuits work, if you don't like the explanation consider that you got exactly what you paid for, as I thought this explanation up all by myself, I did not find it in the RDH4, nor is it handed down to me from the ancients. I believe there are two ideas incorporated in this circuit. The first is the idea of a tunable tank circuit whose Q, and hence bandwidth is proportional to frequency, and the second idea is coupling two such circuits, such that the coupling coefficient is inversely proportional to frequency, to take advantage of the better shape factor that double tuned circuits provide. If this could be done in practice we would have a bandpass tuning circuit that would maintain constant bandwidth and selectivity across the entire broadcast band. Theoretically if we had perfect Ls and Cs with infinite Q, and if we eliminated all shunt losses like diode detectors, antenna source resistance, and coils with frequency dependent losses, we could build the required tank circuits. A variable capacitor tuned tank circuit using a coil of infinite Q, with the loaded Q controlled by a small series resistance in the tank circuit will have the desired Q that is proportional to frequency. At this point we could build a traditional TRF type receiver using these constant bandwidth tank circuits alternated in the traditional way with RF amplifier stages, making sure that we don't load the tank circuits with any significant shunt resistance like a diode detector, or an RF amplifier tube with a high input conductance. For the detector we would use something like an anode bend detector, or reflex detector to minimize the grid conductance. Of course in a practical radio such a circuit is impossible, and can only be approximated, but we try to do the best we can, accepting some broadening of the bandwidth at the upper end of the band due to the inevitable shunt losses. Since the response curve of each tank circuit is rounded, and when we cascade several single tuned tank circuits the rounding and response roll off increases, we realize that it would be a nice idea if we could couple the tank circuits in pairs as is commonly done with the IF transformers in superhetrodyne receivers to provide a better shape factor. For this to work we need the coupling coefficient of the two coils to vary inversely with frequency so that the product of "k" and "Q" remains constant vs. frequency. Normal mutual inductance coupling as is typically used in IF transformers won't work here because with mutual inductance coupling the coupling coefficient remains constant with frequency. In a variable capacitor tuned circuit what we need is a coupling reactance that is independent of frequency, which will then cause the coupling coefficient to vary inversely with frequency. There is not a real component that has a fixed reactance vs. frequency, but we can simulate one to quite a good degree of accuracy across the MW broadcast band by using an ordinary capacitor in series with a negative inductor. The negative inductor acts like a capacitor whose reactance increases with frequency, and when the decreasing reactance of an ordinary capacitor is added to this decreasing reactance, the result is a relatively constant coupling reactance across the MW broadcast band, thus providing the desired decrease in "k" or coupling coefficient vs. frequency. It should be noted that the reactance of both a capacitor and a negative inductor have the same sign, which is negative. Now the only problem is where to find the mythical "negative inductor"? In the context of coupled circuits the effect of a negative inductor is easily simulated by using a center tapped inductor where the two halves of the inductor are closely coupled with k = 1, and connecting the two tuned circuits to opposite ends of the tapped inductor, the capacitor then goes in series with the tap, and we have the desired result. Now in the real world we find that we can't really build our perfect series loaded tank circuits, and some shunt losses intrude, causing the tank Q to not increase as much as we would like at the high frequencies, which results in a somewhat wider bandwidth at the top of the dial. I suspect that the designers of these sets made an effort to compensate somewhat for this effect, by choosing Qs that made the bandwidth slightly narrower than optimal at the low end of the band, and then tweaking the values of the coupling reactances, the capacitance and negative inductance, so that the circuit becomes slightly under coupled at the high end of the band, tending to narrow the bandwidth, although making the response more rounded, and causing the circuit to be slightly over coupled at the low end of the band widening the compromise bandwidth a little at the expense of a slightly humpbacked response curve. That's just my take on how these sets were designed, and obviously there are a lot of moving parts which probably were adjusted in different ways by different designers with different tastes in design. I await Patrick's take on how these so called "band pass" double tuned TRF circuits actually work. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ 
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#2




John Byrns wrote: > In article > , "Henry > Kolesnik" > wrote: > > > Got any idea how it maintains constant BW as BW is a function of Q, a > > relative constant and frequency which varies? Also I don't understand your > > notation "12uh centertapped" (3uh persection). > > tnx > > I have moved this response from "alt.binaries.pictures.radio" to > "rec.antiques.radio+phono" so that it will not be quickly deleted by the > server. > > The following is my take on how these circuits work, if you don't like the > explanation consider that you got exactly what you paid for, as I thought > this explanation up all by myself, I did not find it in the RDH4, nor is > it handed down to me from the ancients. > > I believe there are two ideas incorporated in this circuit. The first is > the idea of a tunable tank circuit whose Q, and hence bandwidth is > proportional to frequency, and the second idea is coupling two such > circuits, such that the coupling coefficient is inversely proportional to > frequency, to take advantage of the better shape factor that double tuned > circuits provide. If this could be done in practice we would have a > bandpass tuning circuit that would maintain constant bandwidth and > selectivity across the entire broadcast band. > > Theoretically if we had perfect Ls and Cs with infinite Q, and if we > eliminated all shunt losses like diode detectors, antenna source > resistance, and coils with frequency dependent losses, we could build the > required tank circuits. A variable capacitor tuned tank circuit using a > coil of infinite Q, with the loaded Q controlled by a small series > resistance in the tank circuit will have the desired Q that is > proportional to frequency. At this point we could build a traditional TRF > type receiver using these constant bandwidth tank circuits alternated in > the traditional way with RF amplifier stages, making sure that we don't > load the tank circuits with any significant shunt resistance like a diode > detector, or an RF amplifier tube with a high input conductance. For the > detector we would use something like an anode bend detector, or reflex > detector to minimize the grid conductance. Of course in a practical radio > such a circuit is impossible, and can only be approximated, but we try to > do the best we can, accepting some broadening of the bandwidth at the > upper end of the band due to the inevitable shunt losses. > > Since the response curve of each tank circuit is rounded, and when we > cascade several single tuned tank circuits the rounding and response roll > off increases, we realize that it would be a nice idea if we could couple > the tank circuits in pairs as is commonly done with the IF transformers in > superhetrodyne receivers to provide a better shape factor. For this to > work we need the coupling coefficient of the two coils to vary inversely > with frequency so that the product of "k" and "Q" remains constant vs. > frequency. Normal mutual inductance coupling as is typically used in IF > transformers won't work here because with mutual inductance coupling the > coupling coefficient remains constant with frequency. In a variable > capacitor tuned circuit what we need is a coupling reactance that is > independent of frequency, which will then cause the coupling coefficient > to vary inversely with frequency. There is not a real component that has > a fixed reactance vs. frequency, but we can simulate one to quite a good > degree of accuracy across the MW broadcast band by using an ordinary > capacitor in series with a negative inductor. The negative inductor acts > like a capacitor whose reactance increases with frequency, and when the > decreasing reactance of an ordinary capacitor is added to this decreasing > reactance, the result is a relatively constant coupling reactance across > the MW broadcast band, thus providing the desired decrease in "k" or > coupling coefficient vs. frequency. It should be noted that the reactance > of both a capacitor and a negative inductor have the same sign, which is > negative. Now the only problem is where to find the mythical "negative > inductor"? In the context of coupled circuits the effect of a negative > inductor is easily simulated by using a center tapped inductor where the > two halves of the inductor are closely coupled with k = 1, and connecting > the two tuned circuits to opposite ends of the tapped inductor, the > capacitor then goes in series with the tap, and we have the desired > result. > > Now in the real world we find that we can't really build our perfect > series loaded tank circuits, and some shunt losses intrude, causing the > tank Q to not increase as much as we would like at the high frequencies, > which results in a somewhat wider bandwidth at the top of the dial. I > suspect that the designers of these sets made an effort to compensate > somewhat for this effect, by choosing Qs that made the bandwidth slightly > narrower than optimal at the low end of the band, and then tweaking the > values of the coupling reactances, the capacitance and negative > inductance, so that the circuit becomes slightly under coupled at the high > end of the band, tending to narrow the bandwidth, although making the > response more rounded, and causing the circuit to be slightly over coupled > at the low end of the band widening the compromise bandwidth a little at > the expense of a slightly humpbacked response curve. > > That's just my take on how these sets were designed, and obviously there > are a lot of moving parts which probably were adjusted in different ways > by different designers with different tastes in design. > > I await Patrick's take on how these so called "band pass" double tuned TRF > circuits actually work. I find the above dissertation too difficult to fully digest on a sunday. I doubt I could fully explain how mutual coupling works in dual LC circuits where the mutual element is a reactive shared element, without an enormous dissertation with lots of formulas and equations, that not even I know about. But its enough for most folks to know that the basic config will give a wider than single tuned response, and then play with values and a plot the response to find out experimentally what is possible But double tuned circuits with mutual reactive coupling in the earthy ends of the Ls are not all that easy to get right, and anyone attempting them will run into difficulties, which is why we never see any in old tube radios made for the majority of consumers in bygone days. In the above paragraphs, there is too much talk of perfect LC tank circuits and "what ifs". The facts are that no matter what we do with LC, R still exists to affect what we do in the real world, so R has to be included in all perceptions of LC workings at all times. There is enough ideas presently on the web about mutually coupled pairs of tuned LCs anyone keen amoung you to go to your workshops and build something which may turn out a complete waste of 10 sundays, but then again if you emerge with something giving good selectivity, wide AF bw, and low N&D, then you will have achieved something. Its no good rabbiting on about rabbit eared response curves forever, the time for action is nigh, so away from the PC and to the workshop! For TRF, I'd suggest trying a "loosely coupled" antenna input coil, then 3 other identical LC coils so a 4 gang tuning cap is required, or a pair of old but identical twin gangs, with those large dia wheels attatched of the same dia, allowing the dial cord to be run around the two wheels. Locating the coils and Cs is critical, and determining amp gain betwen stages. Its easiest to use jfets with low gain and 12volt supplies until a thourough undertsanding of the practicalities ahve been attained. breadboard construction would be fine at first. All coils can be wound at home using 1.5" cardboard tubes and 0.4mm dia wire. The coils need screening, so don't be tempted to use old steel tins from the kitchen, that will damage the Q. Cans must be 1" away from windings, and done with Al or Cu sheet, but need only be quite thin material. This is a shirt load of work to trim for equal performance along the band. I recommend a superhet damped specially treated 455 kHz IFTs, or possibly 2MHz IFTs, or perhaps cascaded pairs of single tuned 2 MHz IF LC circuits. The simplest good performance tubed superhet AM tuner which offers quite fair performance is one using a moderately selective input coil with loose coupling to the antenna, a 6BE6 converter, two IFTs, and a 6BA6 IFamp with AVC to both. The detector using a 12AU7 plus germanium diode and tone control should be as I described in detail last night in another post. But 9 kHz AF BW is available from such a tuner, and in one I altered in a guys old Kenwood AM/FM receiver, I used a series LC for a broad 9 kHz notch filter which removed most monkey chatter as well as the carrier whistle for distance listening. The use of wide pass band ceramic filters is an explorable persuit. The other simple type of AM receiver is a chip based synchrodyne, using a simple double tuned input circuit for initial broad selectivity, although one single LC input could be tried until the rest of the circuit is got working, so the effect of greater input selectivity can be later measured if a dual LC input is tried. The oscillator runs at the wanted station's F, and is held synchronised with a PLL. Final selectivity is attained with the audio filter following the detector. One only needs one double gang tuning cap, and $10 worth of chips, and a +15v supply, plus minor parts. The hard part is understanding what you are doing, and applying what you have learnt, and ironing out the bugs, ie, getting things to work like they do in the text books, without any spurious behaviors. In conclusion my answer is to build, observe, and learn. Patrick Turner. > > > Regards, > > John Byrns > > Surf my web pages at, http://users.rcn.com/jbyrns/ 
#3




Patrick Turner wrote:
(SNIP, SNIP, SNIP) >>Now in the real world we find that we can't really build our perfect >>series loaded tank circuits, and some shunt losses intrude, causing the >>tank Q to not increase as much as we would like at the high frequencies, >>which results in a somewhat wider bandwidth at the top of the dial. I >>suspect that the designers of these sets made an effort to compensate >>somewhat for this effect, by choosing Qs that made the bandwidth slightly >>narrower than optimal at the low end of the band True. (more SNIP) > > For TRF, I'd suggest trying a "loosely coupled" antenna input coil, > then 3 other identical LC coils so a 4 gang tuning cap is required, or a pair of > old > but identical twin gangs, with those large dia wheels attatched of the same dia, > allowing > the dial cord to be run around the two wheels. Be advised that the old 1920's multigang tuning caps are generally found nowadays in very leaky condition. A thorough cleaning and baking is quite often called for in order to have them behave respectfully. > All coils can be wound at home using 1.5" cardboard tubes and 0.4mm dia wire. > The coils need screening, so don't be tempted to use old steel tins from > the kitchen, that will damage the Q. Cans must be 1" away from windings, and > done with Al or Cu sheet, but need only be quite thin material. Cardboard tubes are desirable if one is looking for MINIMUM Q. If your humidity hovers above zero percent you can count on even lower Q...but it won't be predictable :) Nowadays a nice solid/consistent BCB inductor can be made with an FT8261 toroid core with +/ 50 turns of #26 enamelled wire approaching midband unloaded Q numbers of 300 or better. I've yet to find any coil from an old BCB TRF set that comes even close to this. Under 100 is not atypical. An added advantage using toroids is that screening is not normally required. Shoving a 1.5" solenoid coil into a box with only 1" of spacing is a good way to kill the Q of the ckt. But, I've described a method of getting 3kc selectivity at the low end of the band that will likely be 2025kc (measured, not a guess) at the high end in a 2stage set. The point is only to illustrate why this isn't as good an idea as superhetting. In the real world my experience says a 2 or 3 or 4 stage set works great on a 650kc station with another strong local present at 700kc. But it (the same scheme) will NOT work for your 1450kc station with a strong local at 1500kc. > This is a shirt load of work to trim for equal performance along the band. Hasn't yet been accomplished in 80+ years of radio... BM 
#4




Bill wrote: > Patrick Turner wrote: > > (SNIP, SNIP, SNIP) > > >>Now in the real world we find that we can't really build our perfect > >>series loaded tank circuits, and some shunt losses intrude, causing the > >>tank Q to not increase as much as we would like at the high frequencies, > >>which results in a somewhat wider bandwidth at the top of the dial. I > >>suspect that the designers of these sets made an effort to compensate > >>somewhat for this effect, by choosing Qs that made the bandwidth slightly > >>narrower than optimal at the low end of the band > > True. > > (more SNIP) > > > > For TRF, I'd suggest trying a "loosely coupled" antenna input coil, > > then 3 other identical LC coils so a 4 gang tuning cap is required, or a pair of > > old > > but identical twin gangs, with those large dia wheels attatched of the same dia, > > allowing > > the dial cord to be run around the two wheels. > > Be advised that the old 1920's multigang tuning caps are generally > found nowadays in very leaky condition. A thorough cleaning and baking > is quite often called for in order to have them behave respectfully. The last 1932 radio I serviced seemed to work as intended without coil baking, ( or even frying or grilling ) , :) > > > > All coils can be wound at home using 1.5" cardboard tubes and 0.4mm dia wire. > > The coils need screening, so don't be tempted to use old steel tins from > > the kitchen, that will damage the Q. Cans must be 1" away from windings, and > > done with Al or Cu sheet, but need only be quite thin material. > > Cardboard tubes are desirable if one is looking for MINIMUM Q. I was going to suggest old toilet paper roll inners, soaked in varnish before winding, and waxed after. One don't want an extremely high Q. > If your > humidity hovers above zero percent you can count on even lower Q...but > it won't be predictable :) Nowadays a nice solid/consistent BCB > inductor can be made with an FT8261 toroid core with +/ 50 turns of > #26 enamelled wire approaching midband unloaded Q numbers of 300 or > better. I've yet to find any coil from an old BCB TRF set that comes > even close to this. Under 100 is not atypical. Even with Q = 50, the BW at 550 kHz is 11 kHz, which is OK. It only allows 5.5 kHz of audio, so hence you'd need two LCs stagger tuned at the low end of the band. > > > An added advantage using toroids is that screening is not normally > required. Shoving a 1.5" solenoid coil into a box with only 1" of > spacing is a good way to kill the Q of the ckt. Again I have to pay homage to my ancestors and say that many an ancient old radio had sufficient Q with RF coils which were a 1" dia solenoid inside a 3" can. No worries. Using ferrite reduces the size of the coil, and hence the can. But DIYers wanting a spectacular radio should keep all the coils and vari caps nice and big alongside old 6U7 pentodes with top caps. > > > But, I've described a method of getting 3kc selectivity at the low end > of the band that will likely be 2025kc (measured, not a guess) at the > high end in a 2stage set. The point is only to illustrate why this > isn't as good an idea as superhetting. I have been saying all along that TRFing is a pita because of the unfixed bw at different F. Its only good for locals well spaced apart. > > In the real world my experience says a 2 or 3 or 4 stage set works great > on a 650kc station with another strong local present at 700kc. But it > (the same scheme) will NOT work for your 1450kc station with a strong > local at 1500kc. Fortunately, no such situation occurs here in Oz for local strong metropolitan stations. But in regional areas, its all a bit different, and its where the superhet kills all TRFs. > > > > This is a shirt load of work to trim for equal performance along the band. > > Hasn't yet been accomplished in 80+ years of radio... The radio of mine has stagger tuning of two LC circuits simply cascaded, both before a resistance loaded cascoded triode amp, which feeds the mixer of a supehet. The first LC has a loose coupled untuned primary winding for the antenna, so the antenna capacitance/inductance does not much affect the secondary, which is tuned by a gang cap. Then the output from the top of this LC has a 39k 1 W carbon resistor to couple to the next LC, tuned by a second gang cap. The R cas the right amount of stray C and R to make a good couple. Both LC coils are lowish Q, and the attenuation just outside the 22 kHz pass band at anypoint of the BCB is steeper than having a single tuned LC, and provides some initial selectivity to stop a strong station cross modulating a weaker station in the mixer. I could have soldered on to make similar set up with 2 or 3 twin gang caps each with identical tuning caps. I think it wopld have worked OK. It'd have been harder to align. Patrick Turner. > > > BM 
#5




That was really interesting. Thanks for going to the trouble of writing it out.

#6




John
I'm from a school that taught me that Q was a physical characteristic of a coil and I still believe that. Q = {2 * pi * f)/R Selecting the optimum sizes of wire, number of turns, and diameter to maximize Q is well documented. However in this Miller TRF circuit I see no mechanism for changing any physical characterisic of the coils. There are many ways of reducing Q and I see none in the circuit. I assume your neat little one stage mock up works and it's interesting that L1/T2 and L2 are in seperate shielded cans but T1 is open. I'm curious as to how RF is coupled from L1/T2 to L2 by T1. I've struggled to try to simplify this simple circuit further but so far no cigar. Is it possible for you to tune this to a known stations and then measure the variable capacitor so that the Ls can be worked out from f = 1/(2 * pi * (LC)^0.5) tnx  73 Hank WD5JFR "John Byrns" > wrote in message ... > In article > , "Henry > Kolesnik" > wrote: > > > Got any idea how it maintains constant BW as BW is a function of Q, a > > relative constant and frequency which varies? Also I don't understand your > > notation "12uh centertapped" (3uh persection). > > tnx > > I have moved this response from "alt.binaries.pictures.radio" to > "rec.antiques.radio+phono" so that it will not be quickly deleted by the > server. > > The following is my take on how these circuits work, if you don't like the > explanation consider that you got exactly what you paid for, as I thought > this explanation up all by myself, I did not find it in the RDH4, nor is > it handed down to me from the ancients. > > I believe there are two ideas incorporated in this circuit. The first is > the idea of a tunable tank circuit whose Q, and hence bandwidth is > proportional to frequency, and the second idea is coupling two such > circuits, such that the coupling coefficient is inversely proportional to > frequency, to take advantage of the better shape factor that double tuned > circuits provide. If this could be done in practice we would have a > bandpass tuning circuit that would maintain constant bandwidth and > selectivity across the entire broadcast band. > > Theoretically if we had perfect Ls and Cs with infinite Q, and if we > eliminated all shunt losses like diode detectors, antenna source > resistance, and coils with frequency dependent losses, we could build the > required tank circuits. A variable capacitor tuned tank circuit using a > coil of infinite Q, with the loaded Q controlled by a small series > resistance in the tank circuit will have the desired Q that is > proportional to frequency. At this point we could build a traditional TRF > type receiver using these constant bandwidth tank circuits alternated in > the traditional way with RF amplifier stages, making sure that we don't > load the tank circuits with any significant shunt resistance like a diode > detector, or an RF amplifier tube with a high input conductance. For the > detector we would use something like an anode bend detector, or reflex > detector to minimize the grid conductance. Of course in a practical radio > such a circuit is impossible, and can only be approximated, but we try to > do the best we can, accepting some broadening of the bandwidth at the > upper end of the band due to the inevitable shunt losses. > > Since the response curve of each tank circuit is rounded, and when we > cascade several single tuned tank circuits the rounding and response roll > off increases, we realize that it would be a nice idea if we could couple > the tank circuits in pairs as is commonly done with the IF transformers in > superhetrodyne receivers to provide a better shape factor. For this to > work we need the coupling coefficient of the two coils to vary inversely > with frequency so that the product of "k" and "Q" remains constant vs. > frequency. Normal mutual inductance coupling as is typically used in IF > transformers won't work here because with mutual inductance coupling the > coupling coefficient remains constant with frequency. In a variable > capacitor tuned circuit what we need is a coupling reactance that is > independent of frequency, which will then cause the coupling coefficient > to vary inversely with frequency. There is not a real component that has > a fixed reactance vs. frequency, but we can simulate one to quite a good > degree of accuracy across the MW broadcast band by using an ordinary > capacitor in series with a negative inductor. The negative inductor acts > like a capacitor whose reactance increases with frequency, and when the > decreasing reactance of an ordinary capacitor is added to this decreasing > reactance, the result is a relatively constant coupling reactance across > the MW broadcast band, thus providing the desired decrease in "k" or > coupling coefficient vs. frequency. It should be noted that the reactance > of both a capacitor and a negative inductor have the same sign, which is > negative. Now the only problem is where to find the mythical "negative > inductor"? In the context of coupled circuits the effect of a negative > inductor is easily simulated by using a center tapped inductor where the > two halves of the inductor are closely coupled with k = 1, and connecting > the two tuned circuits to opposite ends of the tapped inductor, the > capacitor then goes in series with the tap, and we have the desired > result. > > Now in the real world we find that we can't really build our perfect > series loaded tank circuits, and some shunt losses intrude, causing the > tank Q to not increase as much as we would like at the high frequencies, > which results in a somewhat wider bandwidth at the top of the dial. I > suspect that the designers of these sets made an effort to compensate > somewhat for this effect, by choosing Qs that made the bandwidth slightly > narrower than optimal at the low end of the band, and then tweaking the > values of the coupling reactances, the capacitance and negative > inductance, so that the circuit becomes slightly under coupled at the high > end of the band, tending to narrow the bandwidth, although making the > response more rounded, and causing the circuit to be slightly over coupled > at the low end of the band widening the compromise bandwidth a little at > the expense of a slightly humpbacked response curve. > > That's just my take on how these sets were designed, and obviously there > are a lot of moving parts which probably were adjusted in different ways > by different designers with different tastes in design. > > I await Patrick's take on how these so called "band pass" double tuned TRF > circuits actually work. > > > Regards, > > John Byrns > > > Surf my web pages at, http://users.rcn.com/jbyrns/ 
#7




In article >, "Henry
Kolesnik" > wrote: > John > > I'm from a school that taught me that Q was a physical characteristic of a > coil and I still believe that. There are two values of "Q" associated with a coil, ignoring that the "Q" may change with frequency. The first is the "Q" of the coil by itself, which is what you describe above, but there is also the "loaded" or "circuit" "Q", which includes the effects of external circuit elements like the resistors that Patrick has mentioned. > Q = {2 * pi * f)/R Selecting the optimum sizes of wire, number of turns, > and diameter to maximize Q is well documented. However in this Miller TRF > circuit I see no mechanism for changing any physical characterisic of the > coils. There are many ways of reducing Q and I see none in the circuit. I would have to look but I believe you are correct about the Miller circuit. I don't know what the "Q" of the Miller coils was, perhaps they were not all that great, so they didn't have to add any series resistors to reduce the "Q" at the low end of the band. I don't know, these designs are not perfect, they are just one way to go. > I assume your neat little one stage mock up works and it's interesting that > L1/T2 and L2 are in seperate shielded cans but T1 is open. I'm curious as > to how RF is coupled from L1/T2 to L2 by T1. I've struggled to try to > simplify this simple circuit further but so far no cigar. Is it possible > for you to tune this to a known stations and then measure the variable > capacitor so that the Ls can be worked out from f = 1/(2 * pi * (LC)^0.5) It's not my "neat little one stage mock up", it's Robert Casey's, but L1 and L2 are coupled in the same manner as in the W.E., Miller, and the other similar tuners, by means of a common impedance in the ground end of L1 and L2, this common impedance is T1 & C2 in Robert's design. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ 
#8




In article >,
(John Byrns) wrote: Please delete rec.radio.shortwave from the news group header. Thanks.  Telamon Ventura, California 
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