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

[John Andrews]

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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