here link to devices that recives signal from the transmitter belt same as watch does, or this device is in watch, on page 3 it shows the signal is transfered electro magnetic, i think, and on page 5 the reception frequency is 5.5 khz, i think anyway. also on page 3 the is some info on putting device near tv will cause interfernce is the anyway stopping this interference if you are near tv.
https://www.electro-tech-online.com/custompdfs/2013/02/RMCM01.pdf
here some info got from book on polar products
The Polar Transmitters send short signals for each heartbeat to the
receiver. These signals are low-frequency 5 kHz (kilohertz) electromagnetic
fields at very low power, less than 2nW (nanowatts). The magnetic field
strength at the transmitter is 5 uT (microtesla) and 1 nT (nanotesla) at a
1 m distance.
Polar Non-Coded and T31 Transmitters :
Transmission frequency: 5kHz
Means of transmission: On/Off -modulated electromagnetic field with a
5-7ms burst for each heart beat
Polar Coded, T31 C, WearLink 31 C and T61 Transmitters:
Transmission frequency: 5kHz
Means of transmission: On/Off -modulated electromagnetic field with a
train of three 5ms bursts for each heart beat. Time interval coding.
The strength of the electromagnetic field at the transmitter is 5 uT
(microtesla) and 1 nT (nanotesla) at 1 m distance.
many thanks