Tam/WB2TT wrote:

If you are going to buy new coax, might as well get something decent. I used
RG11 FOAM for a 1/4 wave transformer at 3.8 MHz, and it worked per theory.

I think you were lucky. I've found that the velocity factor and
characteristic impedance of foam dielectric coax, even quality coax, to
vary a lot more than solid dielectric cable. Apparently they don't have
very good control over the foam density.

Roy Lewallen, W7EL