[600MRG] What is the best way to tune a coil?

[Warren K2ORS]

Collins had probably the most efficient scheme - it consisted of two 
drums one was a low loss coil former, the other was a metal drum. The 
coil conductor was a silver plated ribbon. The two drums rotated - you 
basically wound as much coil as you needed onto the former, the 
remaining unused ribbon was left on the drum. Theoretically more 
efficient than either a variometer or tapped coil.


73 Warren

On 10/10/2017 3:44 PM, John Andrews wrote:
> There is another method to varying the inductance of a coil. When I 
> worked in broadcasting, we had a 4-tower directional array at the bottom 
> of the AM band. Many of the coils had eddy-current disks which could be 
> run in and out near one end of the coil. The disks were made of 
> aluminum, and were about the same diameter as the coil. This would only 
> work with a paramagnetic metal such as aluminum or copper, and aluminum 
> is cheaper. Threaded rod attached to the disk center was used to move 
> the it in and out. Moving the disk closer to the coil reduced the 
> inductance, due to the eddy current circulating in the disk.
> 
> It did not seem that significant losses were introduced. The end towers 
> ran about 13A a night, and nothing got unusually warm.
> 
> I have also seen coils with a disk (smaller than the coil diameter, 
> obviously) mounted inside the coil. In those cases, the disk was a ring 
> of aluminum with a non-conductive shaft attached. The shaft ran through 
> bearings on both sides, and by turning it, you could run the ring from 
> being parallel or normal to the plane of the windings. Never saw one in 
> actual operation though.
> 
> John, W1TAG
> 
> On 10/10/2017 3:20 PM, Ralph Hartwell wrote:
>>
>> Gaah!  I hate lists that do not automatically let you reply to ALL 
>> instead
>> of the original poster!  Paul just got an extra copy of my message 
>> that was
>> supposed to be sent to the list, not to him.  SRY Paul.
>>
>>
>>> I was going to take an old metal water bucket, put a few rocks in it and
>> tie it to a rope to lower it (or raise it) into the center of the coil.
>>
>> Unfortunately the iron in the bucket will have a very RF loss due to the
>> circulating current in the iron caused by the magnetic field of the coil.
>> The system will  dissipate a lot of the power heating the bucket.
>>
>> Now, if the bucket made out of pure silver, it might work reasonably 
>> well.
>> <G>
>>
>>
>> The only reasonably effective methods of varying the inductance are 
>> either
>> to use a variometer or multiple taps on the coil.  Using taps on the coil
>> will give you the least loss since you have only as much wire in the 
>> circuit
>> as is required for the desired inductance.  The variometer has somewhat
>> higher losses than a tapped  coil but it is much more convenient and 
>> lends
>> itself more easily to remote operation.
>>
>>
>> Best DX & 73,
>>
>> Ralph  W5JGV - WD2XSH/7
>>
>>
>>
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> 
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