This circuit for the 60 kHz range was found in 73 magazine and published in 1995. The circuit uses a ferrite rod. Note the direction of the coil windings.
This simple intercom for home use was found in a 1979 publication. It can be easily mounted with...
This circuit was found in a Popular Electronics from the 90's. The CMOS integrated circuit used is...
This circuit was found in old Maxim documentation. Check the availability of the IC if you are...