Subtract the total heater voltage from the supply voltage to obtain the voltage to be dropped by the dropper. Divide the result by the current (in Amps) to get the dropper resistor value (in Ohms).

Example: We have five valves in a superhet radio and we need to work out the value for a dropper resistor. The valves are UCH42 UF41 UBC41 UL41 UY41 and, with the heaters connected in series, the total voltage required is 116 Volts and the current is o.1 Amps (100mA)

The mains voltage is 230 Volts
Total heater voltage is 116 Volts
Voltage to be dropped is 114 Volts
Heater current is .1 Amps

Total heater voltage = 116
Supply voltage = 230
Voltage to be dropped = 114
Total current = 100mA
Dropper resistor = 1140 Ohms

We will need to know the power the dropper resistor will have to handle so, using ohm's law... P=I x V where P is power in Watts - I is current in Amps - V is voltage in Volts.
0.1 Amps multiplied by 114 Volts = 11.4 Watts.

The wavelength in metres = 300 divided by the frequency in kHz.

Example: The frequency is 1323 kHz - a medium wave radio station.
300 divided by 1323 = 226 metres.