the voltage reflection coefficient if it was perfect VSWR into 100 ohms is;
Tau (reflection coef.) =(100-50)/(100+50)= 50/150 = 0.33
VSWR is (1+Tau)/(1-Tau) = 1.33/0.66, so about 2:1, which is about -10 dB return loss or 90% efficiency, or 0.5 dB loss in efficiency.
That's not too bad. Most wide frequency systems use this VSWR level as a standard specification without burning things up or losing too much.
If you buy some 75 ohm cable, make it 1/4 wavelenth (electrical length at your operating frequency), and insert it between the 50 ohm and 100 ohm device, you'll get a better result in VSWR. Probably 1.3:1 (depends on frequency and connector quality).
kch
PS: don't forget the "a" in impedance. Remember you're slowing down ants.