Ham Band Preselector

An amplifier or preselector is often used to improve the reception of amateur signals. By employing a selection of series and parallel capacitors, it is possible to arrange that each band shall completely fill the tuning scale. However, this does not contribute anything in terms of efficiency, and can be tiresome to adjust. For these reasons, tuning here is with the single capacitor VC1, with parallel fixed capacitor C2. Values are chosen to accommodate the largest band, so there is coverage to spare on the smaller bands. This is of little or no disadvantage in practice, since tuning of the preselector is merely to peak up the wanted signals in each band.

The amateur bands are as follows: 1.8-2.0MHz (160 metres), 3.5-3.8MHz (80 metres), 7.0-7.1MHz (40 metres), 14.0-14.35MHz (20 metres), 21.0-21.45MHz (15 metres), 28.0-29.7MHz (10 metres). Outside Great Britain some of these frequencies are extended.



In Figure 11, a 2-pole 6-way switch selects the required coils, for 160, 80, 40m and other bands. Signal input is to gate 1. The potentiometer VR1 provides a gain control, to avoid over-loading of the receiver with strong signals. Drain and source circuits are arranged as in Figure 7, with capacitor coupling to the receiver. If the drain circuit were also tuned, twelve coils would become necessary, with ganged tuning.

Winding of the six aerial coils is not too critical, as it is only necessary to set the cores so that amateur signals throughout each band can be peaked by VC1. All coils are wound on formers approximately 9/32in or 7 mm in diameter, and 1in long.

• 10m. Seven turns 26swg side by side. Aerial coupling four turns.
• 15m. Nine turns 26swg side by side. Aerial coupling five turns
• 20m. Fifteen turns 32swg side by side. Aerial coupling eight turns.
• 40m. Twenty eight turns 32swg side by side. Aerial coupling fifteen turns.
• 80m. Sixty turns 34swg side by side with two cores. Aerial coupling thirty five turns.
• 160m. 120 turns in compact pile occupying ¼in width, 34swg. Aerial coupling 60 turns in pile.

Details of coil winding will be found earlier. It is important that windings are not covered with adhesive, cement, or other substances. Winding should begin very near the top of the formers, to give clearance from a metal chassis if inductors are mounted on it. At least one winding diameter should be left between any metal and the coil (say ½in minimum).

Layout should place the switch near VC1 and G1, and should permit very short connections for 10m, 15m and 20m coils in particular.

In view of the number of turns required for 160m, it may be noted that a receiver type medium wave band coil can often be used here. Some coils of this type will reach 160m with the core screwed fairly well out. If not, some turns need to be removed. A Litz wound coil here will give some improvement in efficiency.

Medium Frequency Amplifier - improve long distance reception วงจรขยายภาครับย่าน MF ช่วยเพิ่มความไวให้ดีขึ้น

This circuit, Figure 7, is primarily intended for use over the 1.7MHz to 30MHz range, and will be found to provide considerable gain. RF amplifiers of this kind are generally used to improve long distance short wave reception, to increase volume, and to reduce second channel interference on the higher frequencies.



To avoid winding coils and permit easy band changing, Denco (Clacton) miniature plug in coils may be used. These are they “Blue” (Aerial) ranges, valve type. The most useful coils will be Range 3, 1.67-5.3MHz, or 580 to 194 metres; Range 4, 5-15MHz or 60 to 20 metres; and Range 5, 10.5-31.5MHz, or 28 to 9.5 metres. Exact coverage depends on the setting of the adjustable cores, and will also be modified if VC1 is of different value. The coils are inserted in a B9A type holder. If only single range is wanted, the coil can be mounted by its threaded end, and leads are then soldered directly to the pins.

VR1 is an adjustable aerial input control, as overloading may easily arise with strong signals. R1 and R2 provide the voltage for gate 2, and R3 is for source bias.

The drain circuit is arranged for capacitor coupling by C4 to the aerial socket of the receiver. This lead should not be unnecessarily long, as this may cause losses, as well as picking up signals which cause second channel interference. If the lead is screened, it must be no longer than necessary. A 2.6mH short wave sectionalised radio frequency choke will be satisfactory for the frequencies mentioned.

Construction is best in a metal case, which can have a hinged
lid if plug-in coils are to be fitted. (An alternative, for several bands,
is to use switching as shown for Figure 11.) No ganging difficulties
can arise with VC1, which is adjusted for best volume.



Second channel interference is caused by signals which are 2xIF frequency from the wanted signals. With a 470kHz intermediate frequency, these offending signals will be 940kHz from the
wanted transmission. As a result, interference from this cause is unlikely at low frequencies, but very probable at high frequencies. Such second channel interference is considerably reduced, or completely avoided, by using a tuned RF stage of this kind, actual results in this direction depending on the receiver IF, and frequencies tuned.

A 9v supply is adequate, and current may be drawn from the receiver if convenient. Only about 2mA to 3mA or so will be wanted. The MEM618, 40602, and 40673 will be found satisfactory here.