AM BROADCAST
TRANSMITTER KIT
Ramsey Electronics Model No. AM1
Ever want to be your own disc jockey or talk show host ? Build
and run your own AM broadcast station with this exciting kit !
The AM1 transmitter is designed specifically for simplicity of
construction and use - an ideal first kit !
• Great for transmitting your tape deck, CD player, or voice
throughout the house, yard, or car !
• Powerful enough for high school, college, or neighborhood radio
station - in use the world over !
• Easily tunes to any clear channel on the AM broadcast band. - from
530 to 1750 KHz.
• Operates on the same principle as the “big-boys”, learn how
commercial transmitters work as well !
• Superior performance - transmits up to 1/4 mile !
• Adjustable input level - can be configured for line level or
microphone input.
• Unit runs on 9 - 12 volts DC.
• Handy information on FCC rules and antenna hints.
• Complete and informative instructions guide you to a kit that works
the first time, every time - enhances resale value, too !
AM1 • 1
PARTIAL LIST OF AVAILABLE KITS
RAMSEY TRANSMITTER KITS
• FM10A FM Stereo Transmitter
• MP3FM Synthesized MP3 Transmitter
• FM25B, FM100B Synthesized FM
Stereo Transmitters
• AM25 Synthesized AM Transmitter
• AM1 AM Transmitter
RAMSEY RECEIVER KITS
• FR1 FM Broadcast Receiver
• AR1 Aircraft Band Receiver
• AA7 Active Antenna
• SC1 Shortwave Converter
RAMSEY HOBBY KITS
• SG7 Personal Speed Radar
• SS70 Speech Scrambler
• TT1 Telephone Recorder
• SP1 Speakerphone
• MD3 Microwave Motion Detector
• PH10 Peak hold Meter
• LC1 Inductance-Capacitance Meter
RAMSEY AMATEUR RADIO KITS
• HR Series HF All Mode Receivers
• QRP Series HF CW Transmitters
• CW7 CW Keyer
• CPO3 Code Practice Oscillator
• QRP Power Amplifiers
RAMSEY MINI-KITS
Many other kits are available for hobby, school, scouts and just plain FUN. New
kits are always under development. Write or call for our free Ramsey catalog.
AM1 AM BROADCAST TRANSMITTER KIT INSTRUCTION MANUAL
Ramsey Electronics publication No. MAM1 Revision 1.2 First printing: August, 1994
COPYRIGHT
14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.
1994 by Ramsey Electronics, Inc. Fishers Station Drive, Victor, New York
AM1 • 2
Ramsey Publication No. MAM1
Manual Price Only $5.00
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
AM BROADCAST
TRANSMITTER AM1
TABLE OF CONTENTS
Introduction to the AM1 ................. 4
AM1 Circuit Description ................ 4
Schematic Diagram ....................... 6
Parts Layout Diagram ................... 7
Parts List ........................................ 8
Assembly Instructions ................... 9
Antenna Considerations ............... 13
Alignment Procedures .................. 14
Experimental Broadcasting .......... 14
Troubleshooting ........................... 15
FCC Information ........................... 16
Antenna Experimenting ................ 18
Ramsey Kit Warranty ................... 19
AM1 • 3
RAMSEY ELECTRONICS, INC.
590 Fishers Station Drive
Victor, New York 14564
Phone (585) 924-4560
Fax (585) 924-4555
www.ramseykits.com
INTRODUCTION
Many times we wish to be heard via wireless communications. Whether
transmitting music or voice, what’s required is a stable transmitter tunable to
an unused frequency on the broadcast band. Here at Ramsey, we have
produced several models of FM wireless units for years. Recent requests for
an AM type transmitter have been answered with this kit.
The Ramsey AM1 transmitter is a true broadcast transmitter, which any
person may build and use in accordance with the rules of one’s national
telecommunications authority. For U.S. residents, that authority is the
Federal Communications Commission (FCC). The AM1’s low power
broadcasting capabilities make it practical for many uses, but one should
remember that this transmitter is definitely not a toy. We will refer to the FCC
regulations frequently in this manual and provide you with some information
necessary to enjoy the AM1’s capabilities in accordance with the law.
Typical uses for the AM1 include:
• Student operated school radio stations.
• Re-broadcasting received audio for shortwave or HAM radio use.
• Listening aid for auditoriums, churches, or other events.
• Short range experimentation.
We think that you will be very pleased with the transmitting range, audio
quality, and frequency stability of this build it yourself AM transmitter. If you
follow our assembly instructions carefully and use your AM1 in accordance
with applicable FCC rules, a whole new world of sharing news, views, and
music with friends and neighbors awaits you.
AM1 CIRCUIT DESCRIPTION
Before we dive into the circuit description of the AM transmitter, first we’ll
describe exactly what we mean when we say “AM”. An (A)mplitude (M)
odulated signal is actually a combination of two signals. The high frequency
carrier is the frequency that one will tune on the radio receiver’s dial, from
530 to 1750 KHz. The modulation is the audio information that rides “on top”
of the high frequency carrier, resulting in a changing of the level, or
amplitude, of the output waveform. Take a glance at the schematic and
follow along at how we produce this AM waveform.
The RF oscillator consists of Q6 and associated components. The
frequency of operation is determined by selecting the proper values for C9
and C10, and adjusting the inductance of coil L2. The “buffer” amplifier (Q5)
is connected to the base of Q6 in order to use the undistorted oscillator
AM1 • 4
Low Frequency Audio Waveform
High Frequency Radio Carrier
Amplitude Modulated Signal
output for the RF carrier frequency.
The audio input path is routed from J1, the audio input source, to transistor Q2
to amplify the incoming signal. Notice that the transistor is biased to be linear
using resistors R3, R5, and R6. The incoming audio signal is therefore
amplified undistorted (for great sounding audio). Optional capacitor C4 is used
only when a microphone input is used to provide additional gain from transistor
Q2. The audio input level to the amplifier can also be adjusted using R12, the
input level adjustment.
The resulting audio output is fed to transistor Q1, which does not provide any
gain, but supplies enough current to modulate the RF carrier. Inductor L1
allows the low frequency audio to pass through but “chokes” the RF signal and
does not allow it to get “back into” the audio circuitry.
Transistors Q3 and Q4 comprise the “power amplifier” section of the circuit.
Their collector supply voltage is furnished by Q1, thus producing an AM output
waveform. This signal is then low pass filtered using C13, C14, and L3.
Notice also that the audio information is applied at the power amplifier stage.
This is referred to as “high level” modulation, and is commonly used for high
power AM broadcast stations. The distinct advantage to this is that the RF
amplifiers need not be biased for linear operation. It is much cheaper to
manufacture a linear amplifier for the relatively low frequency audio, than to
produce the AM waveform at a low level and amplify it to a higher power level
without distortion. The main disadvantage of high level modulation is that the
audio modulator’s power must be half that of the final transmitter, not too tough
for our low power kit, but try to imagine the amplifier for a 50,000 watt AM
broadcast station! Boy, that audio amp would sure crank the ‘ol car stereo!
It should also be stated that, due to the linear operation of the amplifiers in this
circuit (transistors Q1 and Q2 biased “on”), this circuit will consume some
power. It is not recommended that a common rectangular 9V battery be used
to power this kit. Instead, a battery “pack” consisting of eight 1.5 volt cells, a
12V sealed battery, or other external 12V DC supply may be used.
AM1 • 5
AM1 • 6
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