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Author Topic: Transmitters for Drive Ins. Bernie Anderson Jr Master Film Handler Posts: 435 From: Woodbridge, New Jersey Registered: Apr 2000 posted 12-31-2003 12:52 PM                        Hey Guys, I need advice from those of you that run Drive Ins. I'm trying to set up an AM Transmitter in my house for transmitting signals to my old radios. I got frustrated with the kits and decided to use my theatre experience and look around for an AM transmitter from a drive in. Well I picked one up with the crystal too. Mad up by a Ham Operator. After a little bit I got it working, but I'm getting a 60 cycle hum transmitted through to the radios. I tried everything and I'm wondering if it's just a bad filter cap. Nothing seems to make it happy. I can lessen it but not by much. Have you had problems with that. Also, what type of antenna do they use? Is it burried in the ground? Is it a Leaky antenna? The other question is, do you guys ever remember having issues with the FCC and the amount of wattage that it's putting out to make it legal. All the radio hounds think that it's around 5watts which they say isn't legal with out a license. Thanks Bernie  |  IP: Logged Gary Crawford Expert Film Handler Posts: 200 From: Neptune NJ USA Registered: Nov 2003 posted 12-31-2003 12:59 PM                     Bernie, I would think that the AM 60-cycle hum you are picking up is everywhere. Power lines especially. DIs went to FM early on to get rid of this interference. Don't take this as gospel, but I believe a low-power installation would be either exempt or lightly regulated by the FCC as you are transmitting to an "enclosed" limited / dedicated area and not in danger of stepping on commercial broadcasts. Friends of mine are actively looking to establish a new drive-in and I've been researching some of these things for them.  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 12-31-2003 01:09 PM                        Some links about low powered radio transmitters: http://www.fcc.gov/mb/audio/lowpwr.html http://www.ruf.rice.edu/~willr/cb/index.shtml http://www.ruf.rice.edu/~willr/cb/cc.shtml http://www.arrl.org/tis/info/part15.html  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 12-31-2003 01:13 PM                     One thing to try is disconect the input to the transmitter and see if the unmodulated carrier contains the hum content If it does it is in the transmitter and is most likely a filter issue If not it is a ground loop issue on the audio input Most of the drivein transmitters I used had balanced inputs  |  IP: Logged Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 12-31-2003 01:47 PM                  I just recently upgraded my FM "station" to 1 kW. I used to go the AM route, but I like the stereo and Freq response of FM much better. I agree with the tips that Gordon suggested. >>> Phil  |  IP: Logged Bernie Anderson Jr Master Film Handler Posts: 435 From: Woodbridge, New Jersey Registered: Apr 2000 posted 12-31-2003 02:33 PM                        Yes, I've had it by itself with no audio input hooked up to it. with practically everything off in the house that possibly could create an interference. And I still got the hum/buzz. So I'm pretty sure that it is to do with the transmitter. I've tried every ground/antenna combo I can think of. There was a thing I discovered with it that by accident the antenna wire came off the terminal. The radio in the next room lost it's hum by at least half, although still present. When I went down stairs to turn that radio on, the hun was still present. It seems the further you go, the hum stays the same but the signal is weaker. Now, could this be possible. These transmitters sat for awhile. I got them and me and a friend worked on it and got it transmitting finally. That night, everything seemed fine. There was a slight buzz but nothing that annoying. It sounded like AM broadcast. When I moved it and since then, it got this buzz/hum, what ever you want to call it. Could a cap possibly stopped working over night? To me the buzz sounds like when you have a florescent light shine into a solar cell; a very familiar sound. At least on a projector , I know how to fix it. Thanks Bernie  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 12-31-2003 03:53 PM                        quote: I just recently upgraded my FM "station" to 1 kW . Ummmmmmmm.... is that right, Phil?.... one THOUSAND watts? Pretty good juice for an "experimental" broadcaster!  |  IP: Logged Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 12-31-2003 04:20 PM                  Yup Jack!... I want the POWER and I want it ALL NOW! Hahahahaha KGAY-FM...95.5 FM...Hollywood! >>> Phil  |  IP: Logged Bernie Anderson Jr Master Film Handler Posts: 435 From: Woodbridge, New Jersey Registered: Apr 2000 posted 12-31-2003 05:56 PM                        Hey guys. As I remembered when I was learning at the Bethlehem D-I in PA, I remember the AM transmitter antenna being a little one in the shape of a "T". Someone on the Radio Forum suggested it was something called a di-pole antenna where they came out of the Antenna terminal formed some kind of antenna shape and then returned to the ground terminal as a loop. And there was a moment that I shorted the two terminals. The hum was alot less, but I didn't get much transmission farther than the room. So that made me think that that is wrong. But I'm wondering now if an antenna of a certain length would fix the problem. Thoughts?  |  IP: Logged John Anastasio Master Film Handler Posts: 325 From: Trenton, NJ, USA Registered: Sep 2000 posted 01-01-2004 12:34 AM                        A dipole is not a loop, Bernie. It's two pieces of wire and at the frequency you're transmitting (1470Khz) a tuned dipole would be enormous. A loop would only act as a short on your set. It was designed to be used with either a long wire and a ground or "leaky" coax. It's entirely possible that the filter has died. When I got you up and running the other night, the unmodulated carrier was clean on the scope and the quieting factor on a receiver ten feet away was total...no sound. If you get a quiet, clean carrier, then the hum is coming from something other than the power supply. Your set uses high level plate modulation utilizing a transformer connected directly across a +8 dbm line, so it's unusual to get audio hum at the levels you're using. If you're getting hum all over the place with no audio being fed, check the filter cap.  |  IP: Logged Jeff Stricker Master Film Handler Posts: 481 From: Calumet, Mi USA Registered: Nov 1999 posted 01-01-2004 06:14 AM                     Bernie, I agree with John A. Sounds like you have some bad filter caps in the unit...  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 01-01-2004 12:31 PM                       Roger that, Jeff, John has it pegged. Also, unlicensed AM is only legal in a "carrier-current" application. I.E., it must use either the household powerlines or a buried "leaky" coaxial cable as an antenna. It cannot radiate a signal, therefore no aerial can be used. All rf reception must be from the induction field only. I believe the power level is limited to 20 watts output. FM is, strictly speaking, illegal for use in a drive-in application. That is because FM behaves differently; you can drive away the block and lose an FM signal, but then pick it up again 100 miles down the road. Therefore, unlicensed FM use is restricted - not by output power or ERP - but by field strength. The restriction is fairly small, too, something like 50 uV/m at 300 ft. Needless to say, I seriously doubt any drive-in is running FM legally, but the FCC seems to be unconcerned about it; especially since deregulation of the Commission (and the resulting staff cuts). Whichever system you use, however, one of the most important considerations is proper antenna matching! It can make all the difference in the world in transmitter performance and signal reception. quote: KGAY-FM...95.5 FM...Hollywood! Phil, you never fail to make me ROTFL!  |  IP: Logged Darryl Spicer Film God Posts: 3250 From: Lexington, KY, USA Registered: Dec 2000 posted 01-01-2004 12:56 PM                    Tim, I remember when the Lexington Drive-in had their FM transmitter. You could tune the sound in as soon as you crossed the county line into Jessemin county. I liked that drive-in. Wish it could have stayed open.  |  IP: Logged Jeff Stricker Master Film Handler Posts: 481 From: Calumet, Mi USA Registered: Nov 1999 posted 01-01-2004 04:52 PM                     Tim, Seems like there is a FCC 50 mW AM rule that says you can run 50 mW AM carrier to an antenna, provided the total length of feedline AND antenna didn't exceed 10 feet. My memory of all these regs is getting rusty...But that won't get you very far on the crowded AM band!  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 01-01-2004 06:13 PM                     Is the transmitter near anything that could be inducing a hum into the coils inside the transmitter? I have run into problems with hum trying to power some of the "cheep" FM transmitters with wall warts. Using a switching power supply crued the problem, but if your unit uses tubes, like John said, you would not be using a wall wart.  |  IP: Logged

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