Total posts : 45366
I’ve been interested in high Q coils for several years. Following are some attributes of the highest Q coils (for MW) I’ve made note of from various studies:
1. Solenoid coils with length/dia between 1:1 and 2:1 have the highest Q of any other geometry, including spiderweb coils
2. For solid (magnet) wire, spacing the wire one wire thickness results in the highest Q. To my knowledge, this been studied only in integer wire spacing, not fractional wire spacing. Suffice it to say, spaced magnet wire results in higher Q than non-spaced. Spacing wire makes much less difference with Litz, as distributed capacitance is already minimized in Litz wire. So Litz wire can be close wound with little loss.
3. Coil forms of HDPE (high density polyethylene) , PP (polypropylene), or polystyrene have lowest loss. PVC is high loss. If it gets warm in a microwave oven, it’s high loss.
4. For crystal radio use, optimum coil diameter is 3.5″-4.5″. Anything above 6″ in diameter starts acting like a loop antenna, receiving local radio signals. I don’t know if this is detrimental to loading coil use.
5. Maximum Q using magnet wire with the above procedures can be in the 400-500 range. Changing to Litz yields Q of 800-1000. Using special combinations of ferrite bars and Litz, Q can be 1500 and above.
6. When measuring Q with Ben Tongue’s method, a high quality scope probe is necessary. On my old Heathkit scope, I didn’t measure Q in the 1000 range until I built Dick Kleijer’s high impedance low capacitance scope amplifier. It doesn’t take much to load a coil where Q drops significantly. When measuring Q in the 1000 range, no objects, metal or otherwise, should be near the measured coil. Even 8″ away can drop measured Q by 100 or more. A link to Dick’s page is the third below.
There is a current Spiderweb coil thread on another forum, showing recently achieved high Q. See first link below:
There is another type of air spaced coil that is intend to be high Q, similar to Mike Tuggle’s coils. These are called “rook” coils, because the coil form looks like the top af a rook chess piece. The second link below is to an almost defunct website devoted to rook coils. My rook coil measurements are 100-200 points below my best solenoid coils.