Tom Coates wrote:
There's an old design that puts an electrical half wave of coax between the
two connectors on the balanced side.
What drawbacks does this old design have compared with more modern ones?

As with stubs, it is frequency dependent and therefore is
limited to a single band. One advantage of a toroid-based
transmission line transformer is that it is broad-banded
often covering a 10:1 frequency range. If you confine it's
use to 20m, the "old design" should work well enough.
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73, Cecil http://www.qsl.net/w5dxp