Chapter 14 - Front ends, Make your own coils

Either it be a Receiver circuit or a general hobby circuit there cannot be a single design that might satisfy every hobbyist equally. Not only individual requirement that varies but also building capabilities to complete the project, not to mention the environment. Two prominent factors that lead to failure are the hesitation to take in expert advise and our lose attitude towards component's quality. These factors are enough to spoil too much money and time. The most serious damage this attitude does is to own enthusiasm to continue with this hobby.

In Fig 12/3 (Chapter 12) a general purpose circuit of a model BC Receiver circuit using the IF and AF stages generally used is given. This circuit is equally usable for any IF frequency below 9 Mhz. Only T1, T2, T3 and those capacitors in their primary that are to change. Certainly the value of coupling capacitors also may require an updation. When T1, T2, T3 are assembled for a definite frequency all the disciplines which we observed while assembling a 9 Mhz Transformer are expected. If it is 455Khz IF that is proposed to be used in this circuit, instead of T1, T2 and T3 a set of 455 Khz IFTs (having capacitors in the primary) available from the market is enough in Yellow, Green White order respectively. Here, no capacitors are needed at their primaries. R9 and R15 in the circuit are presets. If it is ceramic filter that is suggested, space is provided on the board at the area for C3 and C 19. Only after confirming the IF stage performance without any filter that a filter shall be introduced there. May I repeat that more filters means more the strength is reduced. If you are connecting the signal to C 16 through a ceramic filter, you may not need another filter on the board. 

What that were not shown in the above mentioned diagram were Front end local oscillator and mixer stages. Only if a communication receiver could avoid all sorts of unwanted signals and is capable to take in even the weakest of signals that a Receiver is useful in its fill width. This means that every good Receiver needs a good filter at the front end. Already the details of model front end was given in the last chapter. Here two front end circuits proposed for 20 (C-14/1A) and 40 (C-14/1B) meters are shown in C-14/1.
 

Notes: 
T1 - Primary 4 Turns, Sec. 16 Turns (Tap at 3rd turn - from ground)
T2 - Primary 16 Turns, Sec. 5 Turns
      (T1 and T2 are wound on Philips Can type Antenna coil using 40 SWG wire
T3 - Primary 4 Turns, Sec. 36 Turns (Tap at 3rd turn from ground)
L1 - 36 Turns. 
T3 and L1 are wound on 8mm slug tuned core using 36 SWG wire. 

Designing a tuned circuit  for a particular frequency for oneself is impossible without enough preliminary knowledge on assembling a coil of a definite inductance. In most circuits, capacitors and inductors of definite reactances may be required for the tuned circuit there. Inductance and Reactance that coils and capacitors develop at spot frequencies are given in charts C-14/2A, C-14/2B. 

C-14/2A	100 Khz	1Mhz	10Mhz	100Mhz
1 PF	1.6M	160K	16K	1.6K
10 PF	160K	16K	1.6K	160
50 PF	32K	3.2K	320	32
250 PF	6.4K	640	64	6.4
1000 PF	1.6K	160	16	1.6
2000 PF	800	80	8	0.8
0.01 μF	160	16	1.6	0.16
0.05 μF	32	3.2	0.32	0.032
0.1 μF	1.6	0.16	0.016	-

C-14/2B	100 Khz	1Mhz	10Mhz	100Mhz
1μH	0.63	6.3	63	630
5 μH	3.1	31	310	3.1K
10 μH	6.3	63	630	6.3K
50 μH	31	31	3.1K	31K
100 μH	63	630	6.3 K	63 K
250 μH	160	1.6K	16K	160K
1mH	630	6.3K	63K	630K
2.5mH	1.6K	16K	160K	1.6M
10mH	6.3K	63K	630K	6.3M
25mH	16K	160K	1.6M	16M
100mH	63K	630K	6.3M	63M
Reactance of Inductors on Spot Frequencies

Along with capacitance and inductance, even though applied resistance in the circuit also will be considered while calculating the impedance of any circuit, in HF ranges resistance is limited to the length of the coil wires - it is negligible. 

Fig. C-14/3 shows the current change that happens in a series resonant frequency circuit where the resonant frequency is shifted plus or minus 20 percent due to changes in the Q factors by 10, 20, 50 and 100. Here, even if the 'Q' is different Resonant Frequency current is the same. But in circuits with 'Q' at 100 the current peaks sharply. That is, if the Q is high bandwidth will be low. The 'Q' (Quality factor) of a circuit is the figure that you get while dividing the reactance (Xl or Xc) developed to a resonant frequency with the resistance of the circuit. That is, Q = x/r. For example, suppose the inductive reactance and capacitive reactances of a coil and capacitor respectively, used in a circuit is 150 ohms each and the resistance in the circuit is 5 ohms, 'Q' will be 150/5 = 30. Even if the purpose could be different, a tuned circuit designer first considers the 'Q' of a circuit while planning a new tune circuit design.  The value of components in all 7 MHz tuned circuits need not be the same. They change according to the differences in the 'Q' of that particular circuit. If we know the 'Q' required for a particular circuit it is easy to find the reactance of the circuit. Further, we can easily find out the capacitance or inductance of the components required for the suggested reactance values in that circuit. If capacitors are available at the intended values coils are to be assembled by oneself. A lot of formulas are available to find out the coil assembling specifications of our inductance mark.

To find out the number of turns, use the formula n = √L(9a+10b). n = number of turns, L = inductance in micro henry, a = coil radius in inches, b = coil length in inches). Here,  he required inductance should be known and we suggest the diameter and winding length of the coil. w = √L/D(l/D+.44) is another formula used to assemble air core coils. Here, w=number of turns, L = inductance in micro Henry, l = winding length in centi meter and D = coil diameter in centi meter. Formula w=10 x √L/1.6 x 1.69 is enough to assemble coils on 'Jawahar' type .8cm iron core formers.

None of these formulas claim 100% accuracy. All these given formulas are usable only at single layer coils. Multitudes of variety coils and transformers are available. Variety formulas are required for each. To be used in general complex situations, use the formula W= √XL. This is enough for practical purposes. Here, W= number of turns, Xl = inductance in micro Henry. 'X' refers to an imaginary figure. 'X' at different types of cores are also different. Suppose that the inductance of a 1cm IFT with 24 turns at specified wire gauge is 18 micro henry, we find that 'X'= 31.85. Further, if we want to assemble a 5 micro henry coil using this 1cm IFT, follow W= √31.85 X 5 = 12 1/2 turns. Using this method, the inductance of slugs, pot cores, and balloon cores can be found out. First find out the inductance of a sample coil using the proposed core, that's all. 

When local coils are assembled we usually ignore the losses and damages that is because of assembled coils with inappropriate inductance/reactance values. This in turn hits the performance of the circuit  as a whole. The shape, size and material of the core used are important factors. A definite type of core itself may be required for a particular purpose. 

Chapter 13                                                 Chapter 15