Milliwatt AM Transmitter For 13.560

By Tha Dood

A small transmitter intended for use in the 13 megahertz ISM (Industrial, Science and Medical) band under Part-15, which allows several milliwatts of power output. That sounds small, but the signal has been reported to reach a few miles locally, and hundreds of miles when shortwave propagation is good.

Top side of the transmitter, built into a mobile CB case


Bottom of the transmitter, open construction allows easy reach for modification and repair


Nothing in the circuit gets warm, Tha Dood says: "Maybe that wallwart gets a little luke-warm. I haven't noticed heat from that 2N3904 final either."

He also says, grounding the 13.560MHz crystal seems to take away any incidental phase modulation. "On FM, it's the quietest AM TX that I've tested, thus far, commercial, or consumer. On the TS-2000X on FM mode, I have to crank the volume all the way just to notice anything."

"Only one problem, is on audio bandwidth, it's not frequency limited. Thus, why I used the audio output of a Realistic AM only 1975 radio. Audio bandwidth is limited with that. Very simple, but effective. "

Back of transmitter, with antenna socket, grounding block, audio input and AC power cord


"What's the RF output of this TX? I calculated it from 500uW to MAX out at 18mW. At MAX RF, it doesn't MOD well at all, so that 200mA wallwart certainly limits things. If you put this into an HF power / VSWR meter, on it's lowest scale, the meter pointer may move only the width of the pointer itself. How's that for low? I have to use my MFJ antenna analyser to match an antenna to it. Consider this, at even 2mW and a perfectly matched 13.560MHz dipole, this TX will cover miles locally, and via Part #15."

"With that CB meter in it, when it indicated a "10" on that scale, that's about a 2mW carrier."

Schematic: Power, oscillator, driver and final amp, filtering



Parts list
R1 33k C6 330pf
R2 33k C7 1nf
R3 2.2k C8 5pf
R4 15k C8-2, C9 1uf
R5 47k VC1 3-30 pf trimmer
R6 560 ohms VC2 5-70pf
R7 820 ohms VC3, VC6 50-200pf
R8 50k trimpot VC4, VC5 330 pf variable
R9 3.3k Q1, Q2, Q3 2N3904 NPN
VR1 10k trimpot (pwr. adjust) X1 13.560 Mhz. Crystal
R10 1K L1 19-25uh
R11 1k L2 3.44uh
R12 50k trimpot L3, L5 .5-.9uh
    L4 1.1uh
C1 2200uf, 35 volt Regulator LM7810
C1A 220uf, 35 volt M1 Meter 1 ma assumed
C2 100pf F1 250ma slo-blo
C3 330pf D1 Germanium diode
C4 82pf LED1, 2 White, red 12 volt indicators
C5 .56uf RFC2 100uh
C11 .1uf RFC3 19uh

Extras: 12 volt 200 milliamp DC power supply cube, Ferrite toroid (RFC1), modulation transformer (T1): 17:42 ohms, 1/8-inch audio jack (3.5mm), SO-239 coaxial antenna jack

Get the schematic in PDF: pdf

"On the 'scope, the output of this TX was very clean. Anyway, good luck building!"