Keith,
That is useful information. We are fortunate that 15.219 can be used in lieu of 15.209 since as you mentioned this gives higher allowable field strength and it is a lot easier to measure power in than to measure field strength.
Re the lightning protection, our local building code and I believe the NEC requires antenna masts to be grounded. I don't think the FCC had protective grounding in mind when they formulated 15.219 since they include the phrase "ground lead (if used)". They probably anticipated that a ground lead could contribute to the field strength and therefore set limits on this. Since an outdoor transmitter needs a protective ground, it is not optional as implied in the rules.
Thanks again,
Neil
For general safety and the quality of our unit we never considered not including lightning protection and for the lightning protection to work the antenna circuit must be properly grounded, same is true with all the other certified transmitters out there.
However with a suitable tx system design, a proper safety ground for the purpose of lightning protection can be implemented without forcing Part 15 AM users into non-compliance with the 3-meter radiating length limit in 15.219 when they install the system on a rooftop, tower, billboard etc.
Commercial AM broadcast stations use "static drain chokes" for that purpose, which as far as r-f is concerned don't connect directly either to the chassis ground of the tx or the antenna system. These chokes and conducting paths to a real ground buried in the earth don't affect the radiating characteristics of the antenna, as will a direct connection of a ground wire without a choke.
This approach would add little cost to a Part 15 tx, and remove the issue of non-compliance to the 3-meter rule in 15.219 for elevated systems and the long, direct ground wires suggested or required by some Part 15 AM tx manufacturers.
Compliance with this rule is a user responsibility, but it would be helpful to users if the tx manufacturer had provided for it in the original design.
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True, a choke can be used if an engineer has a problem with RF getting into his ground system, in fact we stock a choke for just that purpose if a customer needs it. We just didn't design one into the product, not all situations, (we feel most) would need a choke added.
True, a choke can be used if an engineer has a problem with RF getting into his ground system, in fact we stock a choke for just that purpose if a customer needs it. We just didn't design one into the product, not all situations, (we feel most) would need a choke added.
But a caveat here is that using such r-f chokes in the ground and other wires connected to an elevated Part 15 AM tx can add several hundred ohms of r-f resistance in series with the r-f current trying to flow in the "official" ~ 3-meter monopole antenna. That will reduce system radiation efficiency -- which isn't very good even under the best conditions.
A different antenna system not needing a connection to an r-f ground in the physical earth can be used in elevated installations (flagpole, tower, rooftop, billboard etc). One possibility is described in this thread: http://part15.us/node/926 .
A side benefit of the antenna form in the above thread is that it can greatly reduce the r-f losses in series with the monopole, which at radio frequencies can be tens of ohms in a typical Part 15 AM setup using a ground wire connected to a few ground rods, a cold water pipe, or buried radials. Of course the loading coil losses will still be there.
All of this may be controversial, because even though a choked ground lead won't radiate, it is still a lead to ground, and that's the relevant terminology in 15.219. Or maybe it can just be ignored, because even long, unchoked ground leads and other wires seem to be acceptable to FCC inspectors (from what is posted on some Part 15 websites).
This would be a good R&D project for the manufacturers of Part 15 AM transmitters, so as to supply a complete "system" answer, and possibly increase product sales.
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