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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello, I am thinking about the best method for homemade high power tapped rf coils, such as a resonator for a "plasma flame" tube oscillator.
Here we have 2 problems: 1. Proximity effect. This can quite easily be solved by spacing the turns apart. 2. Skin effect. The only solution I can think of is Litz wire. But how do you tap a Litz wire? Are there any other types of conductors that can be used to wind rf coils?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Litz is useful below 1 MHz, above that you're better off with 'magnet wire' 3/16 copper brake tubing is good. eBay 1.5mm od un-insulated silver plated copper wire is one of my favourites for rf coils. What frequency and power are you considering?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Something like this ...
Assuming a loaded 'Q' of say 5, 1 kVA output = 5 kVAR inductor and capacitor.
For a driver for your tube consider a CB 'burner' (amplifier) or something like this which I'm currently winning. (probably it has 'blown' output transistors so I ain't increasing my bid)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks for the link Sulaiman, however the postage on that coil is huge... I have a GU-43b tetrode which is good up to 100 MHz, which I want to use. I will put additional topload to increase the Q of the resonator, the tank circuit will be the coil itself with its self-capacitance and the topload as the capacitor.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Something similar, teravolt, but just the coil itself. He seems to have made the coil out of solid copper wire or tubing, however judging from the discoloration of copper in the middle he also had problems with the coil overheating.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Silver plated copper tube or strap is common in high-power 13.56MHz ISM power sources. Anything else will get very hot at these frequencies if carrying lots of current. Even still, forced air or water cooling might be necessary.
Rather than having an adustable tap or wiper on the inductor, I'd favour using fixed connections and vary the inductance by another method. Sliding a low perm core in and out of the coil is one way to change the inductance. Spliting the inductance into two series connected coils, and adjusting the mutual coupling between them is another method. Both of these techniques avoid crowding the RF current through a moveable single point wiper or roller contact.
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The construction of high power tapped rf coils
