Post #3751424

2026-07-11 18:03 UTC

I have a #hamradio question. Is there a minimum power level required to make a transmission? Not legally, but physically, or well maybe not even physically, but practically speaking. Like could I broadcast an HF signal in my home that only reaches a few feet? If so is that minimum frequency dependent? I think the answer is no, because portable FM broadcaster for car radios exist, WiFi, exists, and interference from IF used to be a thing, but I am not sure.

Replies (4)

@flyingsaceur@ioc.exchange 2026-07-12 01:49

@trashpanda@m.alittlenook.net The concept you’re looking for is link budget: where all the energy goes between the transmitter and the receiver You have losses in your feedlines (connections to antennas) and even energy reflected back into your transmitter (measured by SWR) You have gain in the transmitting and receiving antennas You have attenuation from objects between antennas, and even in the general vicinity (fresnel zone) And then there’s free space path loss, the skin of rhe balloon, the inverse square law. (If you double the distance, the signal strength is one quarter, a reduction of 6dB which is roughly one unit on the S-meter (some restricitions apply)) Free space path loss is also proportional to frequency, because E = hf The crazy part is, because electromagnetic radiation is a reversible physical process, the gains and losses add up the same (in decibels) in either direction The minimum practical power to transmit is limited to the receiver sensitivity, factoring in those gains or losses. It is also limited by the ambient noise around the antenna, which togethe with the received signal strength determines the signal to noise ratio. Each modulation has a minimum SNR where the signal is intelligible That’s the one index card almost no math overview but I hope it helps you get pointed in the right direction

Open ##3751620
@allpoints@mstdn.social 2026-07-11 18:49

@trashpanda@m.alittlenook.net so many variables to play with. Two things to think about. The first is how much energy is radiated not how much you're generating. Putting one watt into a dummy load is much different than putting one watt into a well-tuned, long wire antenna. The other is frequency. The higher the frequency/shorter the wavelength, the more things become line of sight and are blocked by physical objects. One watt at HF into a good antenna will reach around the world with good conditions.

Open ##4330591
@nusse@chaos.social 2026-07-11 21:39

@trashpanda@m.alittlenook.net Any signal you transmit will interfere with all other signals everywhere, the question is just: how much. So yes, if you lower power low enough, your signal will be of negligible influence outside of your flat.

Open ##4330592
@DF1CN@social.darc.de 2026-07-12 11:56

@trashpanda@m.alittlenook.net the absolute minimum is -174dBm/Hz at the receiver. Thats the signal generated by components at room temperature. In practice the limit is higher due to noise and interferene.

Open ##4330600