Rf Coils

RF coils are very much like antenna coils in many respects and can be expected to have similar types of trouble and require similar methods of checking. There is this pne important difference between antenna and RF circuits however: the gain of the anttnna coil is independent of the AVC voltage on anv tube, since the gain is measured from the antenna to the first grid. Thus no tube is included in the measured circuit and the gain of the circuit will not be changed by the bias on the first tube (unless the tube loses all of its bias and constitutes a load on the tuned circuit).

In the case of the RF coil the measurement is made from the grid of the RF tube to the grid of the next tube. The measured circuit therefore is influenced by the AVC voltage on the tube included in the measured circuit. For highest gain a weak signal should be employed so that the AVC voltage is a minimum and the sensitivity is a maximum. Under these conditions the average gain of an RF stage in a two-gang TRF receiver may be as high as 75. In receivers of the same type but with more stages the gain-per-stage is lower, dropping down as low as 25 in receivers with four gancrs. Multi-band superheterodyne receivers may have an RF staee-^ain even lower, and may be as low as ten. This low gain is chosen purposely so that the sensitivity on the broadcast band will be about equal to or less than the sensitivity on the high-frequency bands. The reason for this choice is that there is a great deal more thermal noise generated in the Broadcast-band antenna coil than in the Short-Wave-band antenna coils and the receiver may therefore be made more sensitive on the Short-Wave bands than on the Broadcast band before internal set noises limit the useful sensitivity.

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