[Radio Wrangler]

I suspect the author was playing games with dielectric constant and trying to improve the impedance at higher frequencies.

If you think about the resistor at different levels down its length, the effective impedance changes. At the bottom it is a short to ground, at the top it is 50 Ohms to ground and is connected to whatever is driving it looks like.

A cunning designer of wide band loads... ones that will do VHF as well as HF, puts a contoured-diameter tube around the resistor and this contributes the right amount of C to ground to turn the big cylindrical resistor into a lossy transmission line whose Z suits each level along it.

I seem to remember an example in an American publication. Oil filled (They used either Regent or Aeroshell turbine oil) the grounded tube was a simple cone outwards and a simple cone back inwards and it made a useful difference at 2m and 70cm.

Sand provides a bit of thermal mass, but it's a good heat insulator.

David