Following K6JCAs design approach, I will be using a low pass L-C network topology. I will also be using relays to put capacitors in parallel to build the desired capacitor value and inductors in series for the same purpose. These are the necessary tasks:
- Select the value of the capacitors and inductors in the design
- Select capacitors that meet the design requirements
- Design the inductors per the requirements
Capacitor and Inductor Values
By running the simulator, I got the maximum values of capacitors and inductors for each band to meet the design requirements. The values of importance to us are the values at the low end of the band (the higher value - and I work a lot of CW). Since I am not designing for the 160 meter band, I will skip those values and start with 2,728 pF as the maximum capacitance and 6,821 nH as the maximum inductance that needs to be synthesized.
| Band | C (pF) | L (nH) |
|---|---|---|
| 160m low | 5,305 | 13,263 |
| 160m high | 4,775 | 11,937 |
| 80m low | 2,728 | 6,821 |
| 80m high | 2,387 | 5,968 |
| 40m low | 1,364 | 3,410 |
| 40m high | 1,308 | 3,270 |
| 20m low | 682 | 1,705 |
| 20m high | 665 | 1,664 |
| 17m low | 529 | 1,321 |
| 17m high | 526 | 1,314 |
| 15m low | 455 | 1,137 |
| 15m high | 445 | 1,113 |
| 12m low | 384 | 959 |
| 12m high | 382 | 955 |
| 10m low | 341 | 853 |
| 10m high | 322 | 804 |
| 6m low | 191 | 477 |
| 6m high | 177 | 442 |
To build a "power of 2" set of values, I will divide the maximum value eight times and then sum the resulting 8 values to see how close I can get to the target value.
| C (pf) | L (nH) |
|---|---|
| 2,728 | 6,821 |
| 1,364 | 3,411 |
| 682 | 1,705 |
| 341 | 853 |
| 171 | 426 |
| 85 | 213 |
| 43 | 107 |
| 21 | 53 |
| 11 | 27 |
| Sum | Sum |
| 2,718 | 6,795 |
The sum of 6,795 is close enough to 6,821 and the sum of 2,718 close enough to 2,728 to be a good starting point. Since I am not going to design for the 160 meter band, I will drop the data for this band from here on.
Maximum Voltages and Currents
Let's look at the maximum voltages and currents (from the simulator) that the inductors and capacitors must handle:
| 200 W | 500 W | 800 W | |
|---|---|---|---|
| Vpk L | 475 | 750 | 950 |
| Vrms L | 336 | 530 | 672 |
| Irms L | 6.2 | 9.9 | 12.5 |
| Vpk C | 442 | 697 | 883 |
| Vrms C | 313 | 493 | 624 |
| Irms C | 6.9 | 10.8 | 13.7 |
The 800 volts inductor peak voltages occur on voice peaks, but when a relay is open, I have to select relays that don't break down at these levels. The same is true of relays in the capacitor legs of the circuit.
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