DUMMY ARIAL FOR ALIGNMENT OF FRAME AERIAL RECIEVERS

Since the frame arial also forms part of a tuned circuit, the method of injecting a test signal must avoid influencing the response of the tuned circuit.

The following is taken from the first volume (1953) of the Radio and Television Servicing book.


Where receivers are normally operated from internal frame arials (e.g., portable and "personal" models) re-alignment must be carried out with these arials in place.

Shielded coil construction diagram [27K]

For R.F. alignment the signal should preferably be injected via an R.M.A. standard shielded coil spaced about 1 ft. from the frame arial. This coil is defined as follows: "This shall be a cylindrical coil, 5 cm in radius and 6 cm. deep, wound with 20 turns to an approximate inductance of 40 microhenrys. The whole coil shall be shielded by a wire cage arranged to avoid magnetic screening (i.e., there shall be no complete circuits whose planes are normal to the axis of the coil). The connecting leads shall also be screened."

Where a coil of this type is not available, a small loop arial (about 4 turns) of approximately the same area as the frame arial of the receiver should be connected to the output leads or socket of the signal generator and set up at a minimum distance of 2 ft. from the reciever. For preliminary adjustments, it may prove necessary to loosely couple the signal-generator output lead to the grid of the frequency changer valve by laying the lead near to this point.

The receiver should always be aligned with the batteries and loudspeaker in the same position relative to the frame aerial as would be the the case under normal operating conditions, otherwise the inductance of the frame aerial may be affected.


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