A radio field signal strength indicator is very usefull for determine radiation patterns of transmitters antennas. An ancillary antenna must be put close to main transmitter antenna. The received signal is applied to L1,L2 and C2, this allows the field indicator circuit to adjust to the same frequency as the transmitter. With the coils values from the radio signal indicator wiring schematic, the frequency band is 6 to 60MHz. A pair of headphones can be connected to hear the transmission. The total impendance of the headphones has to be at least 2.2Kohm.