Tank circuit in a confined space

[W1RKW]

I've run into a dilemna with building an input tuning circuit for the 813 rig.  I didn't plan this correctly when selecting my chassis box.  I wanted to put the input tuning network underneath the chassis but the coil dimensions along with the air variables shape and dimensions and other components mounted underneath won't give me the configuration I was hoping for.  It seems my only solution at this point is to attach another chassis box on the rear of the main chassis.  The box I have has just the right about space for all the tuning components and will give me the configuration I'd like.  My only concern is with the confined space that the input tuning network tank will be in (sides of the metal box), will it work correctly if the tank coil is in close proximity to the sides of the chassis?

[KL7OF]

Grid tuning coils are affected by close proximity to the chassis.  If you are building a rack mount unit, consider mounting it so that it can hang down below the chassis.  It will be out in the free air...If there is no interference with the components on the deck below, who cares if it is hanging down.....This works especially well if you are using plug in coils...Makes them much easier to change.The air cap doesn't seem to care if it is near the chassis...

[WA1GFZ]

An inductor should be at least 1/2 the diameter away from chassis minium or Q will suffer. Not a big deal on the input side but big deal for output. Say you stand it vertical make sure the low Z end is near the chassis but not too close. Caps near chassis will be fine but increase the minimum C and could be a problem on 10 meters.
Layout the whole circuit before you drill. study it for days in fear.

[W1RKW]

 I've come up with a solution to my problem.  I'm going to put the input tuning network on a different chassis since I have to build a screen and bias supply and that will give good separation between input and output tanks.  I will fit nicely there and I'll just wire it's output to the PA.  That should do the trick and  then I can use the free space underneath the PA chassis for mods and repairs if need be.

[Steve - WB3HUZ]

One other thought is to use toroidal coils. You can mount them almost anywhere.

[WU2D]

I agree on the toroidal idea. Fantastic Q's in the hundreds are possible with low permeability Powdered Iron cores at HF. The field concentration will be excellent.

Go to the toy store an buy a kid ring toss game which has a sucession of poly rings. Put the poly rings in the microwave with a glass of water. Cook until the water is boiling. Check the plastic for any signs of heating. Another idea is a doggie doughnut. This air core toroid should work in the confined space. It will not have a good a performance in terms of field concentration as the powdered iron, but it will be compact and will outperform a solenoid coil.

Mike WU2D

[WD8BIL]

Amidon T68 cores work great to 100 watts.
low HF use T68-2 (red) above 20 meters use T68-6 (yellow)

[WA1GFZ]

Don't mount them directly over a chassis put a lottle spacer under them. Rf will couple from wire to chassis and increase stray C. I use T68-2 in my homebrew watt meter.

[KF1Z]

Amidon T68 cores work great to 100 watts.
low HF use T68-2 (red) above 20 meters use T68-6 (yellow)

I assume the 100 watt limitation is due to core saturation?

What could you use for 300 watts + ?

Larger diameter cores...... or maybe stack 2 or 3 ?

Or possiblly a stack of larger beads? ( like the 1" diameter 1.125" tall )
or do they know the Q down maybe.........

Different type altogether......?


Don't try?

[WB3JOK]

For my homebrew amp plate tank, I'm trying three stacked T200-2 cores. It gets a bit warm at 225 watts carrier output on 75m. Heating may change the tuning over time since the permeability does have a temperature coefficient.

Tom W8JI tells me that you can't use a progressively shorting switch on a toroid core (as is commonly done with air-core tank coils)... and if you let the end float, very high voltages can be generated in the unused turns by autotransformer action. I think it'll work ok with just two bands (80/40) but I wouldn't try it with 160-10m except very low power...

-Charles

[WD8BIL]

What could you use for 300 watts + ?

Looks Like the T130-2 or T157-2 would do that nicely.
I've use their T300 for antenna matching transformers up to 1KW with no problem.