Another approach to 13.556 MHz

scwis's picture
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 Coil Design I used the Coil Calculators here: and 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.

Nice little board for experimenting! Just remember that part15 SW requires strict carrier control. A frequency tolerance of +/- 0.01% at a temp of -20 to+50C and at a supply voltage variance of 85% to 115% of rated voltage at 20C. I dont know what the specs are but I suspect an LC VCO will not meet this requirement. Do you have any specs for that unit?

+/- 0.01%? Well...

Fortunately I'll be using a reglated and filtered power supply and I'm sure hoping not to get those temperature variances in my work room :-)

One of the few pieces of Test Equipment I DO have is a fairly good Frequency Counter, so we'll see if I can keep it between 13.5574 and 13.5546 at STP :-)

Experimental broadcasting for a better tomorrow!

Experimental broadcasting for a better tomorrow!


Squinting at the circuit picture it looks like a 2-stage transmitter. I'm guessing oscillator and final, but maybe not.

What is the method of modulation?

Carl Blare

Coil Calculations

It looks as if, from the picture, the coil is wound on a ferrite rod and if so then the inductance equations used in the calculations will overestimate the number of turns since the rod will increase the inductance. The equation in the first link is for an air core coil. I think you will find the number of turns to be fewer than calculated.

You may need to do this by experiment keeping the ratio as specified. Good project, and good luck.


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