Total posts : 45366
PhilB wrote: I don’t agree with your strict interpretation of the part 15.219 rule. The rule is too vague to interpret strictly. The rule doesn’t define “ground” and it doesn’t define a “ground lead” …
Part 15.219 doesn’t define what is meant by, or required of an antenna ground, but physics does. A perfect r-f ground is one with an infinite current source capability for all r-f frequencies, always at a potential of zero volts. The physical earth is as close as we have to a perfect r-f ground fitting this definition.
Here’s part of what Wikipedia says about this: “An electrical connection to earth as a reference potential for radio frequency antenna signals. … An ideal signal ground maintains zero voltage regardless of how much electrical current flows into ground or out of ground. The resistance at the signal frequency of the electrode-to-earth connection determines its quality,..”
A true r-f ground (the earth) does not radiate. Unshielded conductors leading from the earth, and carrying r-f current do radiate. That is why the FCC includes the length of such conductors in the wording in 15.219, whether those conductors are called ground leads, ground wires, “lightning grounds,” or anything else, and are used alone or connected together. Even the outside surface of the shield of a coaxial or shielded cable can radiate unless means are provided to prevent r-f current from flowing there.
When a short ground lead is connected from a Part 15 AM tx chassis to a long, “massive” ground wire/flapole/mast/billboard steel or whatever, then that whole assembly radiates — often far more than the 3-m whip attached to the tx r-f output connector. These are facts of physics that need to be known and considered by Part 15 operators when they decide on their system configuration (and possible FCC risks).
We all know an antenna won’t work without a ground, …
There are many antenna configurations that need/use no reference to an earth (r-f) ground for efficient radiation, as Daniel mentioned. Unfortunately they are not practical for medium wave frequencies.
A Part 15 AM system will operate without any wires connected to the tx but the 3-m antenna (assuming it has an on-board power and program source). It won’t have the coverage range that most “community broadcasters” want, but it may serve the goal of a user to pick up the signal on a nearby radio, which probably is closer to the purpose of allowing “Part 15” uses in the first place.
If we want the FCC to continue to allow part 15 broadcasting, it is our responsibility to reasonably, but not necesarily strictly, comply with the rules.
Of course this leaves the decision as to what may be reasonable up to each person to decide. People need to understand the rules before they can make a decision about whether or not they will “reasonably” follow those rules. Understanding them takes some technical background.
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