profile | register | search | main site
|Forums | Crystal Radio Think Tank ** Advanced Forum | Double slider question.||Post Reply||Send Topic To a Friend|
|Richard||Posted - 6 March 2013 9:11 |
If you build one I'd certainly be interested in seeing how you did it. I've considered varying both capacitance and inductance to achieve the best combination which would also allow spanning a wider range of frequencies - but it just seemed too complicated.
However, combining vernier drive variable capacitors with Bob's comment about push button inductor tuning might be just the solution I was looking for. My old six button car radio mechanical reluctance tuning system could form the basis of a LW-MW-SW set, without resorting to coil taps. Now that would be an interesting project.
Edited by - Richard on 3/6/2013 9:13:37 AM
|homebrew||Posted - 6 March 2013 10:45 |
The best bet for a multiband set is plug in coils. The old MRL booklets and Xtal Set Society books have a few good multiband sets most of which use plug in coils.
|Garry Nichols||Posted - 6 March 2013 10:59 |
I guess the L/D ratio of the coil and rod would effect tuning rate. That is, if the coil is slimmer and longer wouldn't it tune more slowly? (I'm thinking of my circa 1960 Remco Radio, which used a long thin coil with matching ferrite rod.)
I guess long and slim would lower Q, wouldn't it?
Maybe two slug tuned coils in series, with L/Ds made to tune the high and low ends with more amiable spreads.
Edited by - Garry Nichols on 3/6/2013 11:01:01 AM
An afterthunk: How about varying the winding pitch from one end of the coil to the other to spread out the high end a bit? Closewound at the bottom with space wound as we go up, going over one wire diameter at some point.
Edited by - Garry Nichols on 3/6/2013 1:50:52 PM
|Richard||Posted - 6 March 2013 14:12 |
"How about varying the winding pitch from one end of the coil to the other to spread out the high end a bit? Closewound at the bottom with space wound as we go up, going over one wire diameter at some point."
I suspect that would aid in linearizing core motion. Another project for the 'to try' list.
|golfguru||Posted - 6 March 2013 17:39 |
>>>>>>>>> I've considered varying both capacitance and inductance to achieve the best combination which would also allow spanning a wider range of frequencies - but it just seemed too complicated.
Can be done using a variometer (homemade if necessary) and a double-end shafted capacitor.
Some of the earlier "single knob tuning" radios had ganged, double ended caps.
Edited by - golfguru on 3/6/2013 5:44:15 PM
|golfguru||Posted - 6 March 2013 18:19 |
>>>> I guess the L/D ratio of the coil and rod would effect tuning rate.
Yes, but probably still OK - horses for courses. You would build it 'cos you can, not for electrifying selectivity.
Two knobs then?
You then need a longer rod and more "knob turns" to tune it.
Right at the top end (depending on the total number of turns) you might be down to tuning the last turn for the final few channels.
Edited by - golfguru on 3/6/2013 6:22:43 PM
|gzimmer||Posted - 6 March 2013 21:11 |
> I've considered varying both capacitance and inductance to achieve the best combination which would also allow spanning a wider range of frequencies - but it just seemed too complicated.
Have you seen the antenna tuner on the old Barlow-Wadley XCR-30 receiver?
It used a simple but ingenious arrangement of a tuning cap with a ferrite tuned coil on a dial string. Used a couple of micro-switches to switch coils.
With this arrangement it could cover 0.5 to 30MHz in one go. It worked very well.
And google on "Barlow-Wadley XCR-30", there's a YouTube video which shows it working.
Correction: Looking at the circuit, it seems there is no variable cap under the drum. I guess my memory was faulty. Be fun to try though.
Edited by - gzimmer on 3/6/2013 9:30:13 PM
|golfguru||Posted - 6 March 2013 21:34 |
|Garry Nichols||Posted - 7 March 2013 7:15 |
Very interesting tuning system! Three different pitches on the coil, but it appears denser wound sections are switched out as you tune higher. Do you know if there is a variable cap that tunes in unison with the permeability tuning?
Does this coil tune in unison with the mHz dial capacitor on the set?
I wonder if switching in/out coil sections would be necessary for the narrower range of the BCB? If so, they could be manually switched at specific points on the "dial" in order to simplify construction. Maybe only two coil sections would be needed.
If I did not have other projects lined up I would play around with a varied pitch coil for the BCB.
That video appears to be for a newer version of the B-W. No permeability tuning?
Neat idea on tuning mHz with one dial and kHz with another. Not quite sure how that is done . . . .
Unless there is some hidden permeability tuning to simultaneously adjust L across that broad spectrum as the variable caps are also tuned, as I suggested above.
|homebrew||Posted - 7 March 2013 8:30 |
Progressive winding with tighter windings at the low potential side works. Used in IF coils in the fifties.
Also tapping at 10% inductance points instead of every 10 windings improves high end tuning.
I have a couple LW/BCB/SW ferrites and they switch in separate coils for the bands. All on the LW, two for BCB, and one for SW.
The best multiband sets used separate coils on their own core. I suspect that the cores are designed for the band selected.
Schematic about half way down the page:
|Richard||Posted - 7 March 2013 8:53 |
Thanks so much for the pictorial schematic and video link. A fascinating receiver with automatic band switching no less! A very clever design. I want one! :)
|golfguru||Posted - 7 March 2013 12:32 |
>>>>>> That video appears to be for a newer version of the B-W. No permeability tuning?
Video at Time = 2:25.....2:40 clearly shows the operation of the wave change switches and the ferrite rod, popping in and out of the vertical antenna coil in the corner.
Edited by - golfguru on 3/7/2013 12:33:05 PM
|gzimmer||Posted - 7 March 2013 19:8 |
> Do you know if there is a variable cap that tunes in unison with the permeability tuning?
I thought there was, but apparently not.
> Does this coil tune in unison with the mHz dial capacitor on the set?
It's not physically connected to it. Is a separate knob to peak the antenna tuning.
> Neat idea on tuning mHz with one dial and kHz with another. Not quite sure how that is done . . . .
That's the magic of the Wadley Loop. Was used in a number of sets, see
RTV&H magazine even published a home brew Wadley Loop http://members.ozemail.com.au/~radio/wadley.html
google on "Wadley Loop"
|golfguru||Posted - 8 March 2013 3:31 |
>>>>>>> Can be done using a variometer (homemade if necessary) and a double-end shafted capacitor.
1920's ACME version here:
|homebrew||Posted - 8 March 2013 6:25 |
The Acme variable xformer is a variable MU with two series pancake coils in each side of the variometer. The design gives a non linear wide range of inductance values.
aligned - 4L + 4MU
180 out - 4L - 4MU
MU usually approaches L in value with close coupling.
Typical values would be 250 UH aligned and 50 UH at 180 degree rotatiion.
Edited by - homebrew on 3/8/2013 7:02:49 AM
Click Here To Close Thread, Administrators & Moderators Only.
Show All Forums | Post Reply