13-30MHz Grounded Gate Stage

If a receiver has an intermediate frequency of 455 to 470kHz, second channel interference is always present at frequencies higher than about 15MHz, unless one or more tuned RF stages are included. This trouble arises because the second channel is so close to the wanted frequency. As example, if the receiver is tuned to 20MHz, or 20,000kHz, and the receiver IF is 470kHz, then the oscillator will be operating on 20,000 plus 470 = 20,470kHz. However, unwanted signals which are 470kHz higher in frequency than the oscillator will also be converted to 470kHz. As a result, transmissions around 20,940kHz or 20,94MHz will be passed through the receiver, in this instance, as the aerial circuit will not reject these, when tuned to 20MHz. This effect, with a superhet, can only be avoided by raising the intermediate frequency, or providing additional selectivity before the mixer or frequency changer. The grounded gate stage in Figure 9 introduces two additional tuned circuits before the mixer, so is of considerable aid in reducing interference from second channel signals. By keeping parallel capacitances to a low value, approximately 30MHz to 13MHz can be tuned in a single band, with good efficiency.



L2 and L3 are identical, and are wound with 24swg enamelled wire, using 7mm formers with adjustable cores. The cores must be suitable for use at 30MHz or higher frequencies. L2 and L3 are each eighteen turns, but L2 is tapped at three turns from the grounded end for C1 and R1. Begin winding from near the top ends of the formers, securing the wire with a spot of adhesive. Turns are side by side. L1 is wound immediately below the grounded end of L2, and has seven turns. L4 is wound at the grounded end of L3, and has five turns.

Layout should place L1/L2 at one side of the ganged tuning capacitor VC1/VC2, and L3/L4 at the other side. The output lead from L4 and ground line are connected to the aerial terminal and chassis or earth of the receiver. It may be possible to draw a positive supply from the receiver, 9v to 12v being required here.

No connection must ever be made to the chassis of an AC/DC type receiver which draws current directly from the mains, and may thus have a live chassis.

To obtain maximum band coverage without increasing VC1/2 in value, T1 and T2 must be set at low values. First unscrew T1 and T2 completely. Set the cores of L2 and L3 in approximately similar positions. A signal should then be tuned in around 28-30MHz, and T1 or T2 can then be adjusted for best volume. Subsequently find a stable signal around 14-16MHz, and set the cores for best volume. These adjustments should be repeated a few times, adjusting T1 and T2 at the HF end of the band (VC1/2 nearly fully open) and the coil core towards the LF end of the band (VC1/2 nearly closed).

It the receiver has a signal strength or tuning meter, this will aid critical alignment. Alternatively, choose weak signals or if possible switch off the automatic volume control circuits, so that AVC action does not mask adjustments.

Should a receiver with no RF stage be in use, check that the grounded gate stage is actually being tuned to the correct signal frequency, and not to the second channel, as second channel signals may come through strongly with such a receiver.

With receivers having an IF of 1.6MHz, the second channel will fall at 3.2MHz from wanted transmissions, so is less likely to be troublesome.