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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 27th Oct 2020, 11:25 pm   #1
G4YVM David
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Default Coil question

I wind a coil on a plastic former and add wire till it reaches, say, 100uH. All fine. If I insert an iron rod or something the uH goes up. Infact, winding coils on iron or ferrite formers is well understood (maybe not by me!).

Out of curiosity today I HALF wound a coil then wound the remainder back on itself, effectively winding it on a copper core and lo and behold the inductance went UP, a lot in fact. To achieve the same end I used less copper wire and a shorter former. Remember this was just on the bench.

Question...why dont we wind all coils back on themselves? There must be a good reason, but I havent found it yet...


On a similar vein, in many vintage radios I see coils wound tight spaced, then a big gap and another coil wound. Obviously winding many coils on one former...yet they are all in series! How does this work and why is it done?


I have also seen many different forms of windings - the most exotic I thing being basket wound coils...but then my head explodes and I offer great respect to engineers of old.

David
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Old 28th Oct 2020, 12:07 am   #2
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Arrow Re: Coil question

General comments - based on my understandings.

First, the sense of any overwind matters: does it aid the inductive coupling or decrease it?
Second, the capacitance between each turn for a single layer coil limits Q and thus bandwidth. Same applies to multi-layer coils.
Third, the capacitance between two windings on same former is relevant: mutual coupling must also be considered.
Fourth, when measuring inductance, the capacitance of the measuring device is relevant as is the inductance of the connecting wires, coil to instrument. (There is also the question of 'How accurate is my measuring device?' of course).
Finally, inserting a core with into any coil often changes the flux density, depending on the material of that core. Ferrous materials increase that density; brass, OTOH, decreases it. The greater the flux density, the more magnetic lines of force cut the coil, thus increasing its inductance.

In summary, it's mutual inductance and inherent capacitance that must be considered in coil design, not just number of turns, length of coil and spacing between coil turns. As we all have experience of, amongst the three physical factors in analogue design - resistance, capacitive reactance and inductive reactance - its the latter that causes the most uncertainties.

Al.
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Old 28th Oct 2020, 8:03 am   #3
Craig Sawyers
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Default Re: Coil question

That is why medium and long wave coils were wave wound - to reduce winding capacitance. You'll see that on rod ariels and IF transformers. Lots of information on what is needed and at what frequencies in the Radiotron Designer's Handbook, 4th Ed.

The other technique, used from the year dot in the first generation of radio receivers is basket winding. Same idea - reduce the winding capacitance and increase the self-resonant frequency.

Craig
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Old 28th Oct 2020, 8:47 am   #4
G4YVM David
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Default Re: Coil question

Ah...i KNEW id get answers here. Thanks

D
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Old 28th Oct 2020, 9:33 am   #5
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Default Re: Coil question

You'll also see vintage chokes where the winding is broken up into a number of separated sections, all connected in series, each section wave-wound. The aim is to reduce internal stray capacitance and to limit how much inductance that capacitance effectively connects across. Instead of one stonking big self-resonance at an inconveniently low frequency, this pushes the resonance up in frequency. Really wily designers would make the sections dissimilar so the resonance broke up into several on higher frequencies, but all slightly different.

The amateur radio world needed broadband chokes to feed DC power to the anodes of their transmitting valves. These were a right devil to design and tales abound of smoke, flashes and bangs when someone tuned to a resonance. So chokes got broken up into sections, artfully chosen to dodge the bands they would be used on.

Bang up to date and at GHz, there are chokes made wound on a conical mandrel which spreads out self-resonance. You can get tiny surface mount ones. So the old principles live on and still get exploited today.

David
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Old 28th Oct 2020, 3:35 pm   #6
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Default Re: Coil question

Quote:
Originally Posted by Radio Wrangler View Post

The amateur radio world needed broadband chokes to feed DC power to the anodes of their transmitting valves. These were a right devil to design and tales abound of smoke, flashes and bangs when someone tuned to a resonance. So chokes got broken up into sections, artfully chosen to dodge the bands they would be used on.

David
Hi

Like this National one perhaps?

Kind regards
Dave
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Old 28th Oct 2020, 6:41 pm   #7
Jon_G4MDC
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Default Re: Coil question

Did any common rig (or even linear amp) bandswitch down the sections of the anode choke?
It seems that you could arrange it to do that. I can't recall having seen it..

Maybe due to added capacitances it causes more problems than it solves?

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Old 28th Oct 2020, 7:29 pm   #8
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Default Re: Coil question

Quote:
Originally Posted by Dave757 View Post
Quote:
Originally Posted by Radio Wrangler View Post

The amateur radio world needed broadband chokes to feed DC power to the anodes of their transmitting valves. These were a right devil to design and tales abound of smoke, flashes and bangs when someone tuned to a resonance. So chokes got broken up into sections, artfully chosen to dodge the bands they would be used on.

David
Hi

Like this National one perhaps?

Kind regards
Dave
Thats a beaut!

Gotta doff yer cap to some of these fellas.

D
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Old 29th Oct 2020, 4:10 pm   #9
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Default Re: Coil question

Quote:
Originally Posted by Jon_G4MDC View Post
Did any common rig (or even linear amp) bandswitch down the sections of the anode choke?
It seems that you could arrange it to do that. I can't recall having seen it..
I haven't seen that done, but in a piece of gear I designed, I had to switch from a classic Pi-network tank to a series-tuned arrangement part-way down the frequency-coverage because the PA-valves had too high a capacitance to 'play' into a pi-tank at 40MHz. It involved several frightfully-expensive vacuum-relays and I always had a moment-of-angst when it tuned-though the switchover-point while delivering full rated power.

Much amateur-radio gear failed to learn from some US WWII Military gear which was to put the PA anode-choke on the _output_ of the pi-stage: this has the great advantage that when properly tuned the Pi-tank acts as a LPF and isolates unwanted choke-resonances from the PA. It also generally moves the choke from a high impedance to a low impedance point in the circuit, again reducing losses.
[Only downside is that the entire Pi-tank infrastructure sits at HT+ so you need good insulation - and wider air-gaps in the tuning capacitors since these are now subjected to the combined RF-voltage-plus-HT on peaks]
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Old 29th Oct 2020, 8:40 pm   #10
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Default Re: Coil question

The little nanovna would be a great (and cheap!) tool to use to explore the various resonance modes in inductors wound for the HF bands. To get the best results the nanovna would have to be used with a PC as this allows some powerful options.

The only issue would be which version of the nanaovna to buy. I think many people are opting for the genuine V2 with the larger display. I'm not sure what they cost or how long the delivery time is though.

One other demanding design task (for coils) is the design of a high power HPF. If the filter needs to operate over a wide bandwidth then the design of the coils can be quite challenging. A typical design at work might start at 25MHz and operate to 1GHz or so. This requires careful design of the coils to achieve adequate Q and also to avoid resonance modes falling into band at the top of the band. Things start to get challenging above about a 10W power level for this frequency range.
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