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Experiments with low power 27 MHz and VHF transmitters
Many countries have 27 MHz, VHF and UHF frequency allocations reserved for short range uses. Power limits for such devices are measured in milliwatts and there is
typically no protection against interference from higher power transmitters that operate on or near these frequencies. Low power transmitters go by different
names in different countries, eg 'FCC Part 15', 'short range devices' or 'low interference potential devices' etc.
Applications include remote controlled toys, garage door openers and low powered walkie talkies. While this equipment may have ranges measured in the tens or hundreds
of metres, a lot of this is due to the very modest receivers and receiving antennas used. If you are able to use a high quality receiver and directive receiving antenna
you can cover somewhat longer distances without breaching output power or EIRP limits.
A good source of educational material on low power VHF transmitters (typically using the FM broadcast band) is at
Talking Electronics. Regulations for milliwatt-class VHF transmitters vary across the world. Users of them should be aware of the frequency
and power limits applying in their country. The videos below discuss transmitting range, circuit design and possible applications for small VHF transmitters.
Comparison and range test of 1982 vs 2012 cheap walkie talkies
Another comparison with an even older 27 MHz walkie talkie
Range test with 2 milliwatt 27 MHz walkie talkie
Things to do with cheap wireless door chimes (actually UHF not VHF)
More things to do with cheap wireless door chimes (actually UHF not VHF)
Range testing a low power 1 watt FM transmitter ('Surf FM' 87.6 MHz)
Milliwatt LIPD transmitter - range test
CMOS gated oscillator and use to control a radio beacon
1 transistor milliwatt beacon
Tracking down the Easter Bug
1.5 volt 1 transistor milliwatt beacon
Low power milliwatt beacon inside a solar garden light
Another VHF milliwatt pulsing radio beacon
Even better solar VHF milliwatt pulsing radio beacon
Milliwatt VHF beacon range test and more LED experiments
VHF transmitters from crystal oscillator modules
A 53.2 MHz crystal oscillator module transmitter
Crystal oscillator modules - how far will they transmit?
Simple VHF voice beacon with a crystal oscillator module
Range testing a 55 MHz FM walkie talkie
Range testing a 151 MHz FM wireless microphone
Frequency modulating a crystal oscillator for 147 MHz FM
Field range test with 2.4 GHz walkie talkies
Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.
For more on small transmitters see Experiments with low power UHF modules.
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Books by VK3YE
Ham Radio Get Started (USA)Australian Ham Radio Handbook (Aust) More Hand-carried QRP Antennas 99 things you can do with Amateur Radio Getting back into Amateur Radio Illustrated International Ham Radio Dictionary Make your Passion Pay (ebook writing)
All material on this site |