Of course today almost all remaining drive-ins use FM. There are several models to choose from made locally. The problem is with an unlicenced 50 milliwatt transmitter you are at the mercy of a new licence.

Say you choose 91.1 FM and do all your slides and signage for that frequency, then a new licenced station opens up within 75 miles on the same frequency and before you know it on some nights you hear Slim Dusty instead of Mel Gibson!

You can of course buy a 10 watt unit and blow anything else away, but that brings another set of problems - you affect others who have paid big $ for their licence. The key is to use a low power unit and choose a frequency that is in use already, but a long distance away. Your low power unit will be OK if the station is 150 - 200 miles away and they will not licence another station on the same frequency in the same state - applies to Australia anyway.

I think I'll go down to the garage and seek out my original blue Cine-Fi hire radio. I wonder how many ramps still have Cine-Fi clips hanging on them???

Bill, the radio station I worked at has an old volumax on AM and an Optimod 8100A on the FM. Memories......

Has anyone heard of Broadcast Warehouse? They make a rack mount 100 mW FCC legal (part 15 reg.) transmitter for FM. About $300-400. Comet antenna for around $100.

I keep hearing about a Skycaster unit. Anyone have info on this?

I might go with an AM and FM config. (for redundancy-one blows up the other's similcasting) AM still looking...FM Decade?

I believe Steve ______? (I forget his last name) at the Swan Drive-In in Blueridge, Georgia uses one of these at his theatre. Worked very well when I was down there in September. Give him a call or email him.

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anyone else heard that one?

anyway, regarding FM Drive-in transmitters, I f you are unlicensed in the U.S. your maximum output power is limited to around 100 microvolts at 10 feet(I think, It's been a while.) Regarding compression, it wouldn't hurt. The main reason why commercial broadcasters use it is to prevent overmodulation, or for FM, carrier deviation over 75 KHz. The reciever will detect such an occurance as clipping of the audio signal. Overmodulation also is against the FCC's rules. IMHO if you put a harmonic free signal into the air with proper compression, you should be able to use as much power as you want.

Josh

This post is several years old but for those who have the desire to try and build their own drive-in fm transmitter here are some very cheap (and Australia) kits that can help. Halve the prices to get $US
http://www.ozspy.com.au/kits/kits.htm (this one is illegal in Australia methinks as it puts out 200 microwatts)
http://members.ozemail.com.au/~auselec/commun.htm
http://ieee.uow.edu.au/~daniel/projects/radio_link/ (super cheap)

Slightly more expensive but currently on special in the US is
http://www.mp3playerstore.com/buy_it_now__/link-it.htm

cheers Peter

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Run the Dolby Cat69t taht is 50% modulated and set the gain stages for the transmitter to that and you will not have to worry about overmodulation

Since the transmitter will be recieving Lt and Rt they will transmit the centre and surround channels in a standard matrix mode which some car units now decode

Sony has a consumer unit now out on the market - MDR-DS5100 Wireless Surround Headphones. It will take any encoded L/R outputs, as well as a 5.1 input; it then outputs a L/R signal that has been processed to create the Virtual Surround that Dave is talking about. Obstensibly its purpose is to broadcast the processed signal to its complimentary headset receiver, but the output is easily be sent to any line level input such as the FM stereo transmitter. The resultant processed signal creates a psychoacoustical soundfield that simulates 5.1 surround sound in any 2 channel playback system, be it the headset or a standard stereo car receiver. In the headset it sounds quite impressive, but I have heard this same system used in some TV sets and computer 2 channel playback system and even though it requires a person sit equidistant between the two speakers in the close-field "sweet spot," a car environement puts the occupants close enough to the sweet spot that surround effects will still be perceived.

And you are totally right Dave, what a great marketing tool it would be to be able to claim that you are the first Drive-in in the world to have Dolby Virtual 5.1 Surround Sound! Throw this up on the screen using the Dolby Surround logo snipe and it will be the talk of the town. It should even get a few free newspaper stories written about it.

Frank

Also, commercial broadcasters transmit horizontally while the Ramsey antenna punches out vertically. If you use it on low power your interference to TVs and other broadcasters should be minimal. And the "capture ratio effect" will allow the licensed broadcaster to win out in any conflict. Unless of course your nextdoor neighbor is trying to bring in a distant signal from 75 miles near the horizon then you could be considered "interference".