A lot of countries allow the use of low power transmitters at certain frequencies without the need of a licence. In this post, we'll look at the low VHF frequencies available in Australia with the LIPD (Low Interference Potential Devices) Class Licence.
Why Low VHF? With the annual Summer Sporadic-E season and at the peak of the 11-year sunspot cycle, these very low power transmitters have the potential to be heard over many thousands of kilometres.
Frequency Range - Width of band - Maximum Power - My notes
29.700 to 29.720 MHz - 20 kHz - 100mW or -10dBW EIRP* - This starts just above the 10m amateur radio band
30.000 to 30.0625 MHz - 62.5 kHz - 100mW or -10dBW EIRP* -
30.3125 to 31.000 MHz - 687.5 kHz - 100mW or -10dBW EIRP* -
36.600 to 37.000 MHz - 400 kHz - 100mW or -10dBW EIRP* -
39.000 to 39.7625 MHz - 762.5 kHz - 100mW or -10dBW EIRP* -
40.250 to 40.660 MHz - 410 kHz - 100mW or -10dBW EIRP* -
40.660 to 41.000 MHz - 340 kHz - 1W or 0dBW EIRP* - 1) Note the higher power 2) This overlaps with the ISM (Industrial, Scientific, Medical) band of 40.660 to 40.700 MHz which is implemented in most countries.
54.000 to 56.000 MHz - 2 MHz - 2.5mW or -26dBW EIRP* -
70.000 to 70.24375 MHz - 243.75 kHz - 100mW or -10dBW EIRP* - Note that this overlaps with the 4m amateur radio band
77.29375 to 77.49375 MHz - 20 kHz - 100mW or -10dBW EIRP* -
*EIRP - EIRP means equivalent isotropically radiated power. This is the radiated power from a theoretical point source with the radiation pattern shown as a sphere.
A worked example might be as follows...
Example 1: The antenna is a half-wave dipole with a gain of about 2.1dBi. The coax loss is 1dB. The radio transmitter would need to be limited to about 750 milliwatts to comply with the 1-watt EIRP limit. If the limit is 100-milliwatts then the power should be kept at 75-milliwatts.
Example 2: If a long length of coax was used to feed the antenna and the loss was 2.1dB. This would cancel out the antenna gain and the transmitter can be used at 1-watt or 100 milliwatts depending on the frequency.
Restrictions: Other than the EIRP, there are a few restrictions such as avoiding radio astronomy sites. The licence clause also states... "The transmitter, whether on its own or in operation with one or more other transmitters, must not cause interference to the operation of radiocommunications services" and "A receiver tuned to the transmitter will not be afforded protection from interference caused by other radiocommunications devices."
Experiments: In January of 2023, I had a post up about how radio experimenters were using very low power transmitters on the 40 MHz band and how a 5-milliwatt transmitter reached almost 6000kms. See post HERE
Links: 1) There is a small Facebook group called LIPD class licence beacons in Australia if you want to join.... https://www.facebook.com/groups/1269404663238415
2) If I come across any more low band VHF experiments, I'll keep a list of the posts on my 40 MHz page.
