Total posts : 45366
MLR is correct. A Part 15 FM transmitter operating in the FM broadcast band (88-108 MHz) can have a field strength of no more than 250 uV/m (microvolts per meter) at a distance of 3 meters from the antenna.
The “microvolts per meter” means that a tuned and calibrated field strength meter (not available to most folks) with an antenna that is one meter long will read the specified voltage at a certain distance from the radiating antenna. You have to orient the FSM antenna for the maximum voltage reading. Usually, this means that if the radiating antenna is vertical, the FSM antenna has to be vertical, and so forth.
This is why most Part 15 FM transmitters come as a single unit with a permanently attached antenna. The manufacturer designs the entire unit to meet the field strength limit, then stamps out copies.
pr_guy is probably close to the actual power output of a P15 FM box used with a 1/4-wave vertical antenna. However, as pr_guy also said, field strength depends on both power into the antenna and antenna efficiency (length vs. wavelength, height above terrain, construction, etc.).
I also agree with MLR that Part 15 AM is more useful than P15 FM. MLR is taking advantage of phase-synchronized transmitters on the same frequency to increase coverage area. However, even one P15 AM box with a good antenna will cover considerably more area than a P15 FM rig.
On P15 AM, you should go high in the band, at least above 1500 kHz and preferably above 1600. Your antenna will be more efficient, and above 1600 you will probably find more open spots and less competition. For an out-of-box P15 AM setup, one good value choice is the Gizmo or Gizmo Metzo transmitter and their vertical antenna.
Shameless Plug 1: See the “Digest of Part 15” (by yours truly on this site) for more information.
Shameless Plug 2: See “Receiving Part 15 Transmitters” (guess who and where?) for some ideas. Part 15 is like early radio – transmitters with low to moderate power mean some effort on the other end, both receiver and antenna.
For P15 AM fringe reception, encourage your listeners to get either a manufactured passive tuned loop antenna or a GE SupeRadio III. There are three manufactured loops available (new and used, especially eBay):
* The Select-A-Tenna (get one that goes to 1700, some older ones don’t)
* The Terk AM-1000 Advantage
* The Radio Shack 15-1853 (discontinued, available used but seen less than the others)
The GE’s internal antenna is good enough that the loops I mentioned may not do much more for the SupeRadio. You may need to move up to a 2-3 foot diameter or larger loop to make a difference with the SupeRadio. Larger loops are usually home-brewed, but they’re not terribly difficult projects. Someone was selling large AM loop kits on eBay.
If you’re logged into Part15.us while reading an article on the site, you should have an option to comment on the article. Unload about the shameless plugs – good, bad, or otherwise.