Field Strength

Maybe you're joking, NounosSon, but I'll bite.

First of all, field strength is in uV/m, not uV. Field strength is not a voltage, but a voltage gradient. I've seen many misunderstandings and arguments about this point on Part 15 forums. Suffice it to say that the chart is supposed to be in uV/m, not in uV.

Where RadioBrandy uses yellow highlighting represents an excellent receiver with a sensitivity of about 2 uV/m to a monaural signal. Range is considerably less for stereo reception. Many receivers have much poorer sensitivity than 2 uV/m for mono. The lower field strengths shown in the chart would require a receiving antenna with gain, such as a Yagi.

Oh ok i get it now. lol. Thank You for the help Ermi. I think I might just stick with am broadcasting because the rules are less confusing.

Might be wise, but AM has some potentially confusing rules as well. The FCC can come at you for excessive field strength in broadcast band AM radio too.

If you run the numbers for legal field strength for Part 15 FM, you'll see you can't get a practical signal at much more than 200 ft. or so. Beyond that it drops of quickly and dramatically. But, again, a great deal depends on the RECEIVER ... and also whether or not you broadcast a stereo signal ... because as you'll find in other threads, the stereo pilot signal sucks up a lot of energy.

Compare to AM which drops off slowly and has greater (I would say much greater) potential range. Antenna tuning for AM radio is very critical as is a good ground (my gear is on my boat grounded to the sea), and even then the signal is more prone to interference, everything from Christmas LED lighting to auto traffic to buildings to power line boosters to atmospheric conditions, and more, it's still what I would consider 'acceptable' at ranges up to nearly a mile in certain conditions.

Most modern auto radios work well. If I go down most streets in town, I can hear it well enough, but when I turn a corner I can lose it for a bit, or when I pull into a parking space next to the grocery store, which has hundreds of florescent lights and blocks the signal, it gets a lot of noise. But if it were legal Part 15 FM, the signal would barely get off the dock. AM is more difficult to deal with, but again the payoff is more overall range.

A couple of other things about that chart. A field strength of 1.9uv/m does not translate to a 1.9uv signal into the radio. There were a series of posts a while ago describing the somewhat complex math to compute the actual signal strength, but it would certainly be less than 1uv for this case. And that is for absolutely optimal conditions, i.e., direct line of sight to the antenna (no obstructions), broadcasting in mono, no other losses in the system (such as cable runs), etc. I don't know of any radios, other than expensive communications type receivers, that have that kind of sensitivity - and not your run of the mill car radio.

The graph is actually for the field strength in uV/m, not the voltage applied to the antenna terminals of the receiver. Not including "/m" in the graph was an error. An impedance-matched lossless half-wave dipole in free space at about 100 Mhz has an effective height of about 1m, making a field strength of 1.9 uV/m produce about 1.9 uV across the antenna terminals. This is a unique situation where the field strength and the applied voltage are numerically about the same. In most cases, the effective height is not 1 m, and the field strength and applied voltage have different numerical values.

Incidentally, the "effective height" is not the same as the length of the antenna.