I know this is an old posting, but considering the difficulty in finding suitable material for the dielectric for a trombone someone might be interested.
Use 12 x 3/4 in repair links for the outer part of the capacitor. You can transition to the 3/4 in part of the loop easily since the repair links are exactly the same as regular couplings. Then for the dielectric 1/2 in pex will slide into the coupling. Relatively easily. Then 1/2 inch copper pipe will slide perfectly into the pex. PEX has a dielectric constant of about 2.25.
A two legged trombone made this way will give you plenty of capacitance for a 8.5 foot loop that will cover 14 to 29 mhz. A faraday loop is far superior to a gamma match.
By the way PEX is cross linked polyethylene and is superior to using sheet Teflon in this instance.
Also some of the capacitor calculators just calculate the value for an air capacitor. Go ahead and use it, but multiply the value by the dielectric constant. In this case 2.25, but then you'll have to divide by 2 since the trombone is in parallel.
Or use this: http://www.66pacific.com/calculators/capacitance_calc.aspx