Roller Inductors

[W8JI]

Tom, by "progressively shorting switch", do you mean a type of bar that would slide across the coils shorting all the unused coils as one?

Yes. The switch has to pick up and hold at least some critical taps to the common contact. It does not have to hold all of them, just enough so the resonances in unused coil areas are kept out of the bands being used.

I think I'm following your reasoning here - the problem with a roller inductor is there is always a section of coils both in front of and behind the contact. The section in front is the inductance used to "tune" the circuit while the portion behind the contact is not used, but still affects the efficency of the tuner? This is true even if the unused end is grounded or connected to the roller contact.

Absolutely. This is a big killer of roller inductor Q and even can cause tuning problems and arcing over very large distances on higher bands.

Think about what you are doing with a roller coil. On the higher bands like 15 meters you might have two turns active with 20 turns floating between the common and the end. That means you have a 2:10 voltage step up to the middle of that section even if you float the far end. You can have very high voltages in the roller, and that leads to a loss of Q even if the roller does not arc or have a perfect resonance.

This has historically been a big problem with roller inductors that is pretty well known in serious engineering circles when a roller has to operate over a wide frequency range. Collins solved it with a ribbon wire roller that rolls the unused ribbon over on a solid metal core.

Most rollers for a given physical size with similar construction run less than half the Q of an equivalent tapped coil with a switch at the upper frequency end for this reason. This is why I never use roller inductors in large multiband amplifiers, and I sometimes avoid them in tuners. There is no real range advantage, and the circuits are generally not loaded Q critical anyway, so why use one??

The ONLY tuner that generally needs a roller is an L network, because it requires exact L and C for lowest SWR. With a T or Pi, the inductor only has to be reasonably close because there are two other adjustments to trim. You don't need three.

The typical tapped coil that moves the contact from tap to tap, but leaves the unused taps open would have the same problem - is this true?

Yes. This is why HF PA's use a progressively shorting switch or at least will short the tank in multiple points on unused lower frequency taps. The coil left hanging is a huge problem.

IF the unused taps where all shorted together this takes care of the problem - is what you describe in the "short and hold" statement? I'll bet that is the "progressively shorting switch"? It's also a LOT faster to go from band to band with a tapped coil/switch than a roller inductor. Cranking all those turns sure can take a lot of time, especially from 160 meters to 20 meters.

That's why I generally use a switch on things I build. The switch has to be rated for open contact to ground or any other terminals at more than the capacitors peak voltage breakdown because a switch arc can never be tolerated.

The roller inductor can be cheaper to use because it saves wiring time and might even be cheaper in material cost, but it is also less easy to use and for a given size and construction will have much less Q on higher bands.

There are pros and cons to almost anything we do, so you have to look at it all.

Adding a roller will NOT increase the matching range over a switch provided the switched system has the same minimum to maximum range. As a matter of fact it is easier to get a wider range with a switch because the stray reactances can be better controlled. The problem is all the wiring and getting enough positions.

If you run the numbers on a T network and actually look at the range you can see why many times a switch is a much better choice.

MFJ started building its own rollers because the Cardwell rollers and the Cardwell knock-offs from OEP had so many problems, but of course they started having all their own new problems too mostly from material choices


Think about how poor it is to wind any high reactance inductor on a solid dielectric form of any type. The dielectric increases the stray C and that brings the unwanted resonances lower in frequency. This is why the dielectric is a major de-Q'ing factor in compact rollers.

It is far more difficult than people realize to build a good roller, or to get one that has enough inductance for 160 and still works above 20 meters well.

It is all a complex consideration of what we want to do and what we can afford to do, but as a rule the roller actually trades off performance over a wide impedance range for simplicity in the design.

73 Tom

[KL7AJ]

Yes. The switch has to pick up and hold at least some critical taps to the common contact. It does not have to hold all of them, just enough so the resonances in unused coil areas are kept out of the bands being used.



Absolutely. This is a big killer of roller inductor Q and even can cause tuning problems and arcing over very large distances on higher bands.

Think about what you are doing with a roller coil. On the higher bands like 15 meters you might have two turns active with 20 turns floating between the common and the end. That means you have a 2:10 voltage step up to the middle of that section even if you float the far end. You can have very high voltages in the roller, and that leads to a loss of Q even if the roller does not arc or have a perfect resonance.

This has historically been a big problem with roller inductors that is pretty well known in serious engineering circles when a roller has to operate over a wide frequency range. Collins solved it with a ribbon wire roller that rolls the unused ribbon over on a solid metal core.

Most rollers for a given physical size with similar construction run less than half the Q of an equivalent tapped coil with a switch at the upper frequency end for this reason. This is why I never use roller inductors in large multiband amplifiers, and I sometimes avoid them in tuners. There is no real range advantage, and the circuits are generally not loaded Q critical anyway, so why use one??

The ONLY tuner that generally needs a roller is an L network, because it requires exact L and C for lowest SWR. With a T or Pi, the inductor only has to be reasonably close because there are two other adjustments to trim. You don't need three.



Yes. This is why HF PA's use a progressively shorting switch or at least will short the tank in multiple points on unused lower frequency taps. The coil left hanging is a huge problem.



That's why I generally use a switch on things I build. The switch has to be rated for open contact to ground or any other terminals at more than the capacitors peak voltage breakdown because a switch arc can never be tolerated.

The roller inductor can be cheaper to use because it saves wiring time and might even be cheaper in material cost, but it is also less easy to use and for a given size and construction will have much less Q on higher bands.

There are pros and cons to almost anything we do, so you have to look at it all.

Adding a roller will NOT increase the matching range over a switch provided the switched system has the same minimum to maximum range. As a matter of fact it is easier to get a wider range with a switch because the stray reactances can be better controlled. The problem is all the wiring and getting enough positions.

If you run the numbers on a T network and actually look at the range you can see why many times a switch is a much better choice.

MFJ started building its own rollers because the Cardwell rollers and the Cardwell knock-offs from OEP had so many problems, but of course they started having all their own new problems too mostly from material choices


Think about how poor it is to wind any high reactance inductor on a solid dielectric form of any type. The dielectric increases the stray C and that brings the unwanted resonances lower in frequency. This is why the dielectric is a major de-Q'ing factor in compact rollers.

It is far more difficult than people realize to build a good roller, or to get one that has enough inductance for 160 and still works above 20 meters well.

It is all a complex consideration of what we want to do and what we can afford to do, but as a rule the roller actually trades off performance over a wide impedance range for simplicity in the design.

73 Tom

"Collins solved it with a ribbon wire roller that rolls the unused ribbon over on a solid metal core."

I had one of these gems once, Tom. Built a really great tuner around it, and gave it to a new Novice who really didn't know what he had.....such is life!

As we learned at HIPAS, shorting turns of a tank coil isn't without its own set of problems. Our eight 125 KW transmitters had the "progressive shorting bar" you described...servo driven, no less. However the circulating current in the shorted sections was so high under certain conditions we had to actually water cool the tank coils or we'd melt the contacts into oblvion...and these were made of 2" tubing! (These were from the original Platteville ionospheric heater system. built in the mid 60s) The plumbing on this system was a nightmare, as you can imagine.

As it turned out, we really didn't need all that frequency agility...we just had to run it on two frequencies...so we solved the problem by switching in one of two tank coils....almost back to the plug-in coil concept of the dark ages!

Eric

[W8HDU]

Is there any reason why someone could not roll their own out of solid copper grounding wire from the hardware store and a heavy duty PVC form?

Nope. Done it several times, for ham, and commercial AM radio.

[KL7AJ]

Nope. Done it several times, for ham, and commercial AM radio.

If you can POSSIBLY use some sort of standoff...like lucite rods or such, so that the coll is NOT right up against a plastic cylinder, you'll have a better inductor. You want this thing to approach an AIR CORE as closely as possible...and while the lucite rods aren't perfect, they can make a big difference. With six rods evenly spaced around the 'core' you can still have a very rigid inductor with a lot of air space.

eric

[W8HDU]

The materials I use is scrap cutting board material trimmed into .625" x .375" x 12" strips. The strips are then put in a mill and a notch is made at the correct intervals for the windings. The triangle frame is supported by strips of metal which are usually from old computer cases.

I'll use #10 solid copper, or .25" or .50" copper water line for the coil. Taps are usually the cable clamps like you get at Graybar.

The coils don't look pretty, but there is one station still running 5kw through one so it must still work.

The thing I have to find some information on is silver plating, and then finding the little wheels to make rollers. The roller crank and feed is easy, but I've been all over the web looking for brass wheels for the roller contact.

[KL7AJ]

The materials I use is scrap cutting board material trimmed into .625" x .375" x 12" strips. The strips are then put in a mill and a notch is made at the correct intervals for the windings. The triangle frame is supported by strips of metal which are usually from old computer cases. I'll use #10 solid copper, or .25" or .50" copper water line for the coil. Taps are usually the cable clamps like you get at Graybar. The coils don't look pretty, but there is one station still running 5kw through one so it must still work. The thing I have to find some information on is silver plating, and then finding the little wheels to make rollers. The roller crank and feed is easy, but I've been all over the web looking for brass wheels for the roller contact.

For silver plating, there's a product called Cool-Amp. It's powdered silver with a cold flux. You take a damp rag and rub the powder into the surface of copper till it looks all silvery and purty. We used that at HIPAS all the time to keep our coils efficient. (at least in theory!) Eric

[KI4WCA]

The materials I use is scrap cutting board material trimmed into .625" x .375" x 12" strips. The strips are then put in a mill and a notch is made at the correct intervals for the windings. The triangle frame is supported by strips of metal which are usually from old computer cases.

I'll use #10 solid copper, or .25" or .50" copper water line for the coil. Taps are usually the cable clamps like you get at Graybar.

The coils don't look pretty, but there is one station still running 5kw through one so it must still work.

The thing I have to find some information on is silver plating, and then finding the little wheels to make rollers. The roller crank and feed is easy, but I've been all over the web looking for brass wheels for the roller contact.

Cyanide. Sodium cyanide , a silver dollar and a battery will work great.
Best to take it to a professional plater!
I suspect obtaining cyanide is well nigh impossible now!

[AD5ND]

For surplus RF components try Max-Gain systems in Marietta, GA.

In the last several minutes I've learned more about roller and switched inductors then I thought possible.

[W8HDU]

Cyanide. Sodium cyanide , a silver dollar and a battery will work great. Best to take it to a professional plater!

I think not. I've gone through some training sessions on this stuff, so I don't think it's worth it.

I did find one recipe that I'm checking out. It calls for...

50 ml demineralised water (H2O)
2 gr. silver nitrate (AgNO3)
1 gr. ammonium chloride (NH4CL)
5 gr. calcium carbonate (CaCO3)
4 gr. natrium tiosulphate (Na2S2O3)

I have to talk to the chemistry chair at the local university about this. I'm an engineer and not chemist, so I don't know how dangerous this stuff is. Plating might be the solution to it all, as well as a way to get away from the liability.

I still need to find the little wheels that touch the coil windings. They have to be out there. I've just not included the sufficient Google terms to find the darn things.

[KI4WCA]

I think not. I've gone through some training sessions on this stuff, so I don't think it's worth it.

I did find one recipe that I'm checking out. It calls for...

50 ml demineralised water (H2O)
2 gr. silver nitrate (AgNO3)
1 gr. ammonium chloride (NH4CL)
5 gr. calcium carbonate (CaCO3)
4 gr. natrium tiosulphate (Na2S2O3)

I have to talk to the chemistry chair at the local university about this. I'm an engineer and not chemist, so I don't know how dangerous this stuff is. Plating might be the solution to it all, as well as a way to get away from the liability.

I still need to find the little wheels that touch the coil windings. They have to be out there. I've just not included the sufficient Google terms to find the darn things.

Silver nitrate is very, very dangerous.Doctors used to use a silver nitrate pen to burn off small warts.When the pen touches the flesh, smoke is emitted copiously.IE:silver nitrate is a severe burn hazard.Don't even think of getting it in your eyes.Full face sheild and protective gear MANDATORY.The cyanide is fine provided you do not add an acid to it....that liberates cyanide gas.(I assume you would not drink it or touch it in any way either)
Seriously, a jeweler or metal finishing place is better.

Silver is much better for vhf and above....otherwise bare copper is just fine.Plating generates hazardous waste....who wants to deal with that.
Let a pro deal with it.