The initial advantage of the Frigidaire Radiantube units, like the TK units, was in the heating patterns with the 5 or 7 heat click position switches in use before infinite heat switches. The systems required two heating elements so that input to each could be switched on and off and voltages could be changed to achieve the five or seven heats. Most units gave heating patterns that varied the heated portions of the elements. Some made sense and performed well, others not so much. The fat Radiantube units allowed both heating elements to be imbedded on one piece of steel to give an even heating pattern. Those are the Radiantube units with three wires. The Radiantube units for use with infinite heat controls have only two wires and could just as easily have been the slim profile coils of more modern units. The exception to this were some Frigidaire HeatMinder units which used two elements inside, operating both to heat initially and then cycling only one to maintain temperature. Thermostatic units, whether gas or electric, have their own particular demons. Once Hotpoint/GE went to interwound Calrod units where both tubes spiraled next to each other from start to end, the heating was pretty even at each position. The exception to this was the 6 inch extra high speed unit with a separate outer winding and inner winding. While it heated across the entire element at HI, 3rd and Wm, it offered inner coil heating only on 2nd and Lo for small diameter utensils. The five heats on a two coil Calrod element were achieved by having both coils operate in parallel on 230 on Hi, one coil on 230 on 2nd or medium high, both coils in parallel on 115 for 3rd or medium, one coil on 115 for Lo and both coils in series on 115 for Wm. Allowing for the outer coil's slightly longer length giving it a slightly higher wattage, the heating wattage descends from 2600 on Hi to 1200 on 2nd to 650 on 3rd to 300 on Lo to 160 on Wm for an 8" element. For a similarly wound 1250 watt 6" element the wattage descends from 1250 to 560 to 310 to 140 to 75. The 75% drop in wattage between HI and 3rd (and between 2nd and Lo) shows the effect of cutting the voltage supplied to each coil from 230 to 115. The 75% drop in wattage between 3rd and Wm shows the effect of putting the elements in series on 115, meaning that the current is fed to only one, but passes through the length of both, sort of diluting the power. It is pretty ingenious and simple enough which is why it came before infinite heat controls. Hotpoint called the five heats "Recipe Heats" because they eliminated guesswork. Of course they lacked flexibility which is why most models of GE and Hotpoint ranges offered higher and lower wattage elements. Six inch elements could vary between 1600 and 1250 watts. Eight inch elements could vary between 2600 and 2050 watts. Often the higher wattage elements were at the front of the cooktop.
A big disadvanage to the Radiantube surface units was that they could warp badly because of the support system. I have seen old Frigidaire ranges with the elements warped up so badly they look almost like a snake fixing to strike. Unlike Calrod and Corox units, the Frigidaire and TK inits were not attached to their support frames so some older elements, while flat when cold could change position when heated. Sometimes the support spider under them sags because it has lost it temper due to overheating, allowing the center of the 8 inch element especially, to sag down in the middle when heated. In other cases, part of the element can lift up when heated.
Because the Radiantube surface units are slower to respond, the cook has to be more attentive in the initial stage of cooking operation when pans are being brought up to cooking temperature. You soon learn when to switch to a lower heat and that you do not leave anything unattended over high heat when cooking with electricity.
