When I was in high school I built an AM Broadcast band transmitter from a kit. It had one tube and a very short range since it operated under FCC Part 15 (Wiki) rules. While looking for a similar device I came across the Ramsey AM25 kit and information on how to extend it's range a little. But for much less money you can get a used Talking House AM transmitter that appears to be a synthesized AM Broadcast Band (Wiki) transmitter with built-in digital memory for the outgoing message as well as provision for "live" am broadcasting from an external microphone or other audio source.
These are placed inside a house for sale with a sign at the curb announcing the AM radio frequency to tune in order to hear a 5 minute sales pitch. I expect they are sold to the homeowner at full list price then end up on eBay at a steep discount.
Allows for unlicensed transmitter operation with two types of rules. Specified output power and antenna length or specified field strength. (FCC.pdf) For the AM broadcast band 525 - 1705 kHz 100 mW and 3 meters.
For the FM broadcast band (88 - 108 MHz) limited to 250 uV/meter at 3 meters from the antenna. Common uses of Part 15 transmitters (Wiki) includes Talking House transmitters.
Made by i.AM.Radio who is owned by Radio Systems. They also make a number of low power transmitters, like used for carrier current AM (Wiki). In both cases one of the challenges is coupling the transmitter output to the antenna or power grid and that's the subject of most of the patents below. Brochure.pdf
Synthesized TransmitterFrequency: 530 - 1700 kHz
Motorized automatic antenna tuning.
Because of the restriction on antenna length
REL branded model TH-2000.
Measured Output: 15 VDC (Brochure says 12 Volt, back panel says 18 Volts)
Size: 3-3/4" h x 2-1/4"w x 1-3/4" d
Plug: standard 5.5 mm DC plug.
Maybe Jameco: 2225967 15V, 40 Watt (A better choice would be Jameco p/n: 2225975 18V, 40 Watts.
The supplied adapter looks like the old fashioned transformer type. The Jameco is modern world AC switching type (lighter and smaller).
Model # IAM-Indoor Antenna
Open space terminal on one end.
Length: 118" (3 meters)
Model # IAM-Mic
External Waterproof box with whip antennaModel # IAM-OutdoorTransmitter
The main problem for me is that as I drive out the driveway when the car is at the street there are overhead power lines and the noise is so strong it blocks the signal. Maybe there's a problem with the transformer near my house?
Fig 1 Almost new in the box (propably used one time)
Fig 2 Included wall wart and antenna
Fig 3 Powered up at 1370 kHz with a message.
When power first applied you can hear the antenna
Fig 4 Rear panel
Fig 5 Spectrum plot 1370.08125 kHz
Nice strong signal.
Fig 6 Spectrum Plot at 1700 kHz.
KUKI is at 1400 (Wiki)
1520 kHz is the only open frequency.
This plot takes about 5-1/2 hours.
Fig 7 Bottom Labels
Shows patent number 6295443
4541119 Portable broadcast band information transmitting system, John R. Cooper, Richard G. Sawyer, Priority: Oct 3, 1984, Pub: Sep 10, 1985, 455/526, 455/128, 455/127.1, 455/18, 455/129, 340/905
Based on the FCC AM broadcast 100 mW & 3 meter antenna rule. Aimed at selling houses. Uses an indoor cassette tape recorder and simple single frequency transistor AM transmitter built into the sign(s).4817193 AM transmitter having single tuning means for oscllator, carrier isolator and antenna, Scott C. Matthew, Realty Electronics, Inc., Mar 28, 1989, 455/108, 334/78, 455/120, 334/83, 455/125, 455/121
Similar to 4541119 except has digitral readout of Tx frequency and a manual antenna tuner.6295443 Automatic tuning AM transmitter, Scott C Matthew, Tony Robert Esdaile, Michael Robert Milne, priority: Nov 30, 1998, Pub: Sep 25, 2001, 455/115.1, 455/121, 455/120
Similar to above except uses microprocessor, and motor driven antenna tuner.6973294 Automatic tuning AM transmitter, Scott Matthew, Tony Esdaile, Michael Milne, Radio Technologies, Llc., Priority: Nov 30, 1998, Pub: Dec 6, 2005, 455/115.1
Almost identical to 6973294 - There is a difference in the class numbers7437130 Smart tuning AM transmitter, Scott Matthew, Tony Esdaile, Michael Milne, Broadcast Marketing Llc, Priority: Nov 30, 1998, Pub: Oct 14, 2008, 455/115.1, 455/121, 455/120
Almost identical to 6973294 - There is a difference in the class numbers
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