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|golfguru||Posted - 20 November 2012 20:37 |
>>>>>>>>> How tight was the coupling for the "approximate tracking" situation that you mention?
My comment was WRT homebrew's "rubber band" tracking arrangement. If the right diameter pulleys were chosen when turning the shafts on a 2 stage, capacitively coupled set, then close enough might be good enough. No worse than a "Tuggle" single tuned?
Edited by - golfguru on 11/20/2012 8:42:05 PM
|homebrew||Posted - 21 November 2012 7:42 |
J- "Then, of course, if you can rig the pulleys and strings to pre-emphasize that out, or if tuning is not sharp, maybe it still kinda works?"
Not extremely high Q circuits. Solid wire spaced cylinder coils and diamond weave usually. The initial set up is usually not set for max selectivity. And my design style may overcome some of the matching problems you mention. I use multi tapped coils initially and if the circuit proves promising I copy the coils with only the taps used the most.
In case you did not catch it, the rubber banded variables can be set at different initial capacitances if needed for better tracking together.
|Richard||Posted - 21 November 2012 7:57 |
"I figured the better cap belonged in the detector stage because its Q could be maintained higher than the antenna stage which was swamped by the antenna and ground loading."
Not necessarily, Garry. In my experience very light ground loading in a double tuned triple cap set enhances overall selectivity in the upper third of the band.
To achieve this requires separately tuned caps in the antenna tuner but the selectivity gains realized over ganged caps have proven beneficial in separating adjacent channels above 1200 khz.
With no changes to the detector circuit, selectivity gains are significantly reduced when phenolic insulated caps are substituted for ceramic insulated ones in the antenna tuner.
Of course it's best to have ceramic or even lower loss insulators in all circuits but it's far easier to reduce ground resistance loading in the antenna circuit than it is to reduce the combined effects of diode, transformer and headset resistance in the detector circuit.
Edited by - Richard on 11/21/2012 8:01:27 AM
|Garry Nichols||Posted - 21 November 2012 16:34 |
Thanks, Richard. I did not mention that the phenolic insulated cap in the antenna section was a dual wired up in a Tuggle tuner configuration. So I was part way towards what you mention, I think.
I'll have to try that circuit again some time with independent caps in the antenna section to see if I can squeeze more selectivity out above 1200.
To get increased high band selectivity can we say that the independent series cap will likely be tuned for less pF than the section of the dual used for the same function? Or am I once again trying to oversimplify things for my enjoyment?
It sounds like it might not matter whether the one phenolic cap that i have goes into the antenna section or the detector section. (I only have one decent ceramic insulated cap to use on this circuit.)
|Richard||Posted - 21 November 2012 19:35 |
"Can we say that the independent series cap will likely be tuned for less pF than the section of the dual used for the same function?"
Yes, definitely - far less series pf is needed at top of band. In fact, it sometimes helps to remove the ground connection entirely and rely on stray capacitance to ground. The ability to reduce series capacitance to ever lower values while maintaining a much higher capacitance in the parallel tank is the primary advantage of using individual caps in the antenna circuit.
Most dual section capacitors used in AM radios have dissimilar capacitance values in the sections. The lower value is used as the series cap in a Tuggle tuner. Typically, these dual caps have values between 365-500 pfd in the main section and a capacitance value ranging from 180 to 260 pfd. in the oscillator section, or thereabouts.
In a Tuggle circuit, as one tunes these dual caps near top of band the parallel cap may have a value of around 40-50 pfd with a series cap of 25-30 pfd. Compared to the cap ratios utilized at the low end of the band, this C1-C2 combination at the high end presents a somewhat different ratio that results in excessive ground loading. The result is increased bandwidth (reduced selectivity), when it's needed most.
By utilizing a separate series cap of lower maximum value one can incrementally reduce the series value well below the parallel cap value and compensate for detuning by slightly increasing the parallel cap value to remain on frequency. Using this method in a two handed round robin approach one can definitely improve selectivity.
Of course this assumes one is using very light inter-stage coupling to begin with to preserve selectivity. As you might expect, the only downside is somewhat reduced headset volume but that can be compensated for by slightly increasing coil coupling. In general, separate caps aid in separating adjacent or nearby channels.
"It sounds like it might not matter whether the one phenolic cap that i have goes into the antenna section or the detector section. (I only have one decent ceramic insulated cap to use on this circuit.)"
In that case I'd suggest using the lowest loss (best) variable cap in the parallel tank of the antenna circuit. If it's a dual section cap of equal values I'd add a low loss styrene fixed value cap of approximately the same capacitance in series (ant. or ground) to reduce the overall amount of A/G loading to the tank. I prefer a series cap on the ground side but Ben tells me it makes no difference. I've tried it in each side with inconclusive results as to which is best.
Edited by - Richard on 11/22/2012 12:11:08 PM
|Garry Nichols||Posted - 23 November 2012 13:19 |
Thank you for the interesting discussion, Richard. When I was using an MRL-2A (dual cap) I found that about 15 feet of counterpoise ground wire run around the baseboard gave a boost to selectivity compared to a "real" ground. I did all of my contesting with the set rigged up that way. I preferred to "listen hard" rather than have a jumble of stations.
One issue Ben mentioned with regard to placement of a series cap was that if it is in the ground lead there will be the opportunity for increased losses due to capacitive coupling to ground through random objects and the listener. Leaving the set hard wired to ground and putting it in the antenna lead eliminates this problem.
But I always used the series cap in the ground lead because it was easy to just leave my Tuggle front end the way it was. And this placement did not seem to hurt Mike Tuggle's scores!
I think that an alternative to a variable cap for the series connection could be an array of fixed caps. Since Ben has mentioned that the more ordinary variables are the lossiest at low pF values, some good quality fixed caps and either a jumper or a good quality switch might work out better.
I have purchased NPO ceramic caps down to 5.6 pF. I think that they are easily available at half that value, or two 5.6 pF caps could be put in series to cut the value in two.
I'm not sure what increments I would use, however. Roughly doubling the value each time (5.6, 10, 22, 47 . . . .)?
If one did not have too many increments, a multi-scale dial could be calibrated for such a set up. This would allow quick retuning as various series caps were tried.
|Richard||Posted - 23 November 2012 20:13 |
Yep, a variety of fixed caps work well too.
"I always used the series cap in the ground lead because it was easy to just leave my Tuggle front end the way it was."
You could also leave your Tuggle front end as is and swap the A/G inputs to change from series ground to series antenna.
Also, Ben is no doubt correct about stray capacitance but in practice I never could discern a noticeable difference. Measurable? Perhaps, I don't know. As usual, YMMV. :)
Edited by - Richard on 11/23/2012 8:15:04 PM
|Garry Nichols||Posted - 24 November 2012 6:45 |
Any thoughts on fixed cap increments?
Do you think 5.6 pF is a good starting point?
|Richard||Posted - 24 November 2012 8:13 |
"Any thoughts on fixed cap increments?"
I've never measured the series values used in the top third of the band so don't have any suggestions. My approach to finding the best value to use was arrived at using a variable cap and my ears. Too many variables to pin down.
A/G characteristics and frequency tuned would affect the degree of needed coupling. Experimenting with your setup may be the best way to determine values.
Edited by - Richard on 11/25/2012 4:07:00 PM
|neazoi||Posted - 25 November 2012 11:5 |
So, What fo you think about my setup?
Would it that be better to make everything inside a big enclosure or make separate enclosures for the detector/ and each of the two transformers or maybe even the traps?
Also, what about coupling the coils? I think I am going to make basket weave coils out of standard enameled wire because they are easier to make than the litz spider ones.
|golfguru||Posted - 25 November 2012 14:32 |
I think with "coaxial" you will need greater distances between coils, than if you use "rotation" to vary the coupling.
"Sliding sideways" might work but I don't know if that brings phase changing into the mix.
Others might comment.
Edited by - golfguru on 11/25/2012 2:35:20 PM
|gzimmer||Posted - 25 November 2012 17:48 |
You can null coils by putting them on right-angle axis (but three coils need three perpendicular axis)
Or you can use the old trick used in old TRF receivers having all coils on the same plane but slanted at 45 degrees.
At 45 deg there's a magic spot where the fields cancels and there's no coupling.
Edited by - gzimmer on 11/25/2012 5:54:22 PM
|golfguru||Posted - 25 November 2012 18:14 |
TRF angles Quote:
""These receiver designs also often had the coils angled at 54.7 degrees between the axis of each coil and a line connecting the centres of the coils to reduce undesirable inter-stage coupling, as in this FADA.""
|Garry Nichols||Posted - 25 November 2012 18:18 |
No kidding?! I've seen those old TRF sets and never knew about that!
As I recall, it's as if the coils were all lined up side by side or end to end (with appropriate spacing) and then each rotated by 45 deg.
Does the spacing between them have anything to do with the spot at which the coupling is reduced?
|golfguru||Posted - 25 November 2012 19:5 |
Just checked a Fada 175-A that I have - coil angles are in line and ~54 degrees (not 45) to the horizontal.
Here is the link to the quote in my previous post:
Hazeltine Neutrodyne - see page 8
Edited by - golfguru on 11/25/2012 7:40:19 PM
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