If the impedance of a delta loop is about 100-120 ohms,just curious why a 2:1 balun instead of a 4:1 balun isn't recommended for use?

Making a proper 2 : 1 impedance ratio balun isn't as simple as it sounds, especially if

it is designed to operate over a wide frequency range. Typically a balun will have an

integer turns ratio between two sides, and the impedance ratio is the square of the

turns ratio. You can make a voltage balun using a tapped transformer, but that has

limitations, and getting a good current balun in other multiples is certainly not easy.

A 1 : 2 turns ratio gives 50 : 200 ohms, and is comparatively simple to make. Also,

for good wideband balance, you'd like the coax shield to connect to the center tap

of the secondary winding, so the secondary should have an even number of turns.

A 2 : 3 turns ratio would match 112 ohms, but wouldn't allow you to ground the

center tap of the secondary easily. A 3 : 4 ratio would match 89 ohms, but that

would traditionally require a septafiliar winding, and the coupling among the wires

in the bundle becomes more questionable as you get beyond 3.

That's why, for broadband use, the 1 : 1 and 4 : 1 baluns are the most practical

units. Other ratios are possible, but performance tends to be less reliable.

Besides, for monoband use, the quarter wave transformer is much easier. You can

wind it up into a choke if desired.