Another approach to 13.556 MHz
Posted on April 22, 2007
I just ordered this Budget Shortwave Transmitter kit from Kenneke Communications
It’s an unbuffered Hartley oscillator, LC/VCO with pretty minimal modulation, sold as pretty much any band you like as long as you are willing to wind your own coil. The included coil and iron powder bar are for the AM BCB.
The assembly instructions include mods for 7 MHz and I’m going to try to stretch it 13.556 and add an output filter as I’m guessing the harmonics will be intense. This circuit might actually be underpowered for Part 15 in 3.556, but we’ll see.
The kit is sold by the Greek outfit Smartkits and re-marketed here by Kenneke among others.
Coil for 13.556
For 13.556 MHz operation I get about 17 uH using the center value of 8 pF for C and 13.556 for F, as shown below:
L = K/F(squared) * C
K = 25330.3
L = 25330.3/(13.556 * 13.556) * 8
L = 25330.3/(183.765136) * 8
L = 25330.3/1470.121088
L = 17.23
I used the Coil Calculators here:
to arrive at a coil 0.5″ (12.7mm) in diameter and 1.5″ (38.1mm) long, of about 68 turns close wound.
To arrive at the 25% & 75% of total coil turns for the tapped coil, as specified in the instructions, it looks like 17 turns for the 25% section and 51 turns for the 75% section.
It also looks like the coil can be wound as two separate coils of 17 and 51 turns respectively and connected as shown in the instructions, because the PCB provides two solder terminals for the off-center tap.