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This topic comprises 2 pages: 1 2
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Author
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Topic: Transmitters for Drive Ins.
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Jack Ondracek
Film God
Posts: 2348
From: Port Orchard, WA, USA
Registered: Oct 2002
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posted 01-01-2004 08:10 PM
Tim is close, but not quite "right on". The FCC does have rules regarding unlicensed usage of AM and FM frequencies. The AM rules are well defined, given that the Commission has more years of dealing with this medium (and quite a few years ignoring private FM usage). However, things like "Mr. Microphone"-type transmitters and "real estate radio" boxes have promoted certain rule changes to address these circuits... and for FM they're restrictive to the point of being practically useless. I don't have the numbers memorized. However, with both mediums you CAN radiate a certain amount of energy legally. That formula is based on signal strength at a certain distance from the antenna. To be certain you're within spec, you'd have to have a field strength meter... an expensive tool for the hobbyist. You can have a very small free-radiating antenna on an AM box... something like 3 inches or the like. An FM box can not legally be sold on the hobbyist market if it is pre-assembled with a socket which allows an external antenna to be connected, leaving things like self-enclosed telescoping sticks as among the few "legal" configurations (how many drive-ins have you seen that use those?). The "real-estate" boxes go about 100 feet from the antenna, and fairly poorly at that. Those who are buying current model broadcast-quality transmitters of 1/4 watt and up... and those adventurous entrepreneurs who have made good use of recycled FM exciters are nowhere close to compliance. Those things put out too much power just free-radiating into an empty output socket! Use of low-power circuits like FM modulators and Rat-Shack antenna amplifiers usually require gain antennas, which are likely to put you right into the same bucket as the guy who uses a 6 inch stub and 5 watts of power.
Delving into reality for the moment; the sad truth (and where Tim has his info right on) is that the letter of the FCC's law does not generally give you enough signal level to provide adequate coverage to the average drive-in... whatever that may be. Because of that, most... if not all drive-ins are not strictly compliant with FCC regs when it comes to FM radiation levels. AM has also become hard to do within the regs, mainly because of the number of signals out there, compared to say... 20 years ago. You just about have to jam more power out the door just to deal with the night-time skip.
The "leaky" concept with AM and existing field wiring is an option, though not usually consistent. There are a lot of drive-ins with field wiring in such bad shape that this doesn't always work anyway. Digging real "leaky coax" into the ground is an expensive way to provide a sound format that nobody younger than 40 uses anymore.
So, in a nutshell, here's the truth of the matter. Every drive-in out there is probably outside of strict FCC compliance, or is either smaller than, say, 4 acres (about 100 cars) or has some serious hole in its coverage area. Is that a concern? Strictly speaking, yes. Practically speaking, probably not. The Commish has had their hands full for decades, just dealing with fully licensed stations, and the occasional pirate (ooops... I meant "experimental") operation that gets a little too much "in-your-face" to ignore. I personally have NEVER heard of any FCC action against a drive-in operator, and I notice that those who really need to use FM for their businesses are doing it thussly: First, go out of your way to find the quietest, most unused channel location you can find. That rule works for both you and any station you might (illegally) cover (a very huge no-no). Quiet frequencies are far easier to get good results out of with very little power, and the worst possible thing you can do is interfere with a popular local station. Second, use only enough power to provide good quality coverage within the confines of your theatre. Making a conscious effort to be heard 5 miles away from your theatre is only inviting unwanted attention from a governmental agency that has the authority to shut you down entirely... possibly leaving you with no sound system at all.
I'd guess you can draw your own conclusions from here. None of the above is in any way a suggestion or encouragement by me, my agents or occasional cohorts to violate Federal Communications Commission rules and regulation in any way, shape or form.
... so there!
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We were able to get our signal out about 15 miles weather permitting. 24 hours a day, alternative rock programming. Took the boys in blue (suitcoats) 5 years to track me down and hit me with a $1000 fine. Of which, the community shelled out the funds in contributions. Nice area that I live in! ![[Smile]](smile.gif)
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