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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

 

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 151 MHz FM wireless microphone

 

Frequency modulating a crystal oscillator for 147 MHz FM

 

Possible useful items for milliwatt VHF experiments

 

Disclosure: I receive a small commission from items purchased through links on this site.
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

 

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(c) Peter Parker VK3YE 1997 - 2020.

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without permission.