My initial interest in Part 15 broadcasting originated as a means to broadcast the audio of my outdoor 16mm film showings to radios so that people had the option to sit in their cars and watch the movie.
It was strictly a hobby of mine, although I did consider creating a kind of redneck drive-in type of thing in a casual business sense, but realistically the royalty fees involved with doing it legally would most likely have resulted in spending more money for every event then what I might be able to take in.. So I just did it for fun.
Anyway, played several outdoor old feature films from my collection at least a couple weekends a month for several years, usually in Ellabell Ga. or on Tybee Island with groups of friends and cases of beer. Eventually, I bought myself a Rangemaster and instead of stringing the projector speakers outside, began transmitting the audio to AM. It worked very well, and it was pretty cool when someone could jump in their car and drive off to the store (usually for more beer), and still be able to listen to the dialog from the movie!..
I projected many a film using the same method you are considering. My transmitter installations were of course temporary in these instances; often times as simple as using a bungy cord to "hang" the transmitter, and a single 4 foot ground rod, which, after the showings could sometimes become become a competition object to see who was strong enough to pull it back out of the ground with a pair of vice grips!
Anyway.. If I were you, and could afford to; I would buy a Rangemaster. If not, then I would get the AM3000.. I advise this mainly because I do have both of these units and know first hand (same as most people on this forum) that both the Rangemaster and the AM3000 are high quality units and both have excellent sound quality.
However, I've only used my AM3000 around the house using just the wire antenna and no ground system - so I really have no idea what kind of range it's capable of.. I'm much more familiar with the Rangemaster.
SOOooo.. If I was trying to keep it cheap, my suggestion would to go ahead and buy that SStran AM3000 your looking at, use it's wire antenna, and run a ground wire to a 4 foot rod driven in the ground.
It would also be your simplest solution, then basically all you would need to do is plug the projectors outputs into the transmitters RCA jacks and your ready to go!
I highly suspect it will accomplish what you want it to do just fine, as well as probably being the most simple method of temporary installs.
I wouldn't worry about it not being stereo, AM will suit the situation well -IMOH.
"buy that SStran AM3000 your looking at, use it's wire antenna, and run a ground wire to a 4 foot rod driven in the ground."
I seriously doubt he will be able to slam a ground rod in the middle of a soccer field or parking lot.
Go with my mobile antenna idea for the best possible CLEAR coverage in the field/lot/whatever and the AMT3K for its price and praise by owners of its performance. There is also a video demonstrating how well it does work.
RFB
Before real estate values made suburban drive-ins too expensive to exist (and, well, there was the falling interest), many of 'em used AM transmitters. I remember one in particular whose screen could readily be seen from the roadside; tune to the right frequency, park and get a free movie!
Yes, skywave is going to present problems. Raecampus, what we're talking about here is the nature of the AM band to "skip" signals in from distant places after dark. Our Part 15 campus station can easily be heard a mile away during the day, but skywave interference knocks it down to a few hundred feet at night. But, hey, that's still pretty good for your drive-in project.
Yes, the Sstran AMT3000 is a nifty transmitter, and an easy kit to build. The audio quality is outstanding. For what you're trying to do, I wouldn't get too bogged down in transmitter efficiency, running AM stereo, etc. The challenge will be what sort of antenna you end up with. The wind-it-yourself base-loaded antenna plans shown on the sstran website can be a wee bit challenging to build, but that antenna works extremely well. It isn't necessary to go this route, but...preferred.
FM is limited in range when you follow Part 15 rules to the letter. That said, though, you won't have to worry about skywave interference (just choose a good blank frequency; go to http://www.radio-locator.com/cgi-bin/vacant). You could do stereo without any problems and, depending upon terrain, obstructions, etc., you might be able to get the 100 yards you're looking for. I would not be surprised, though, if the cars farthest from the transmitter had a few reception problems.
So, budget permitting...you could try both AM & FM or even a couple of FM transmitters (on well separated frequencies) located at either end of the field. Just make sure everyone knows to tune to whichever frequency best works for them.
Whichever way you go, it will likely take a little bit of experimentation, but if I were a betting man, I would put money on your success. Continue asking quesitons, read here and on other Part 15 sites, and don't be afraid to give things a try!
get an iAM and external ATU. costs 595 for the set. will cover 3000ft under worst conditions.
I'm just stating is the way it has worked for me on numerous occasions doing the exact same thing.. a drive-in movie type setting.
I seriously doubt he will be able to slam a ground rod in the middle of a soccer field or parking lot.
Come on.. it takes two minutes, and when you pull it back out it leaves a hole smaller than your pinky finger, which would be invisible the next day unless you knew the exact spot where to look.. and even then you'd have a problem finding it.
You talk like it's an outrages thing to do.
I assuming he wants to set up cheap, quick, simple, and effective.
Not knocking your suggestion RF, but my method fits the bill.
Pounding a ground rod into the ground is dangerous. You could puncture a gas line, water pipe or reach a buried electric cable. Boom, phshhh, pop!
There is a public service place that needs to be notified so they can make sure your ground intrusion is safe.
Sorry folks. The movie is canceled by the explosion and fire.
Pounding a ground rod into the ground is dangerous. You could puncture a gas line, water pipe or reach a buried electric cable. Boom, phshhh, pop!
Yes.. Good point. Fortunately I never ran into that problem.. and I sheepishly admit that thought never crossed my mind either.
So just be observant and careful before driving a rod into the ground.
Even if the Op wanted to take time to pound a ground rod into the football field or soccer field, I doubt the college athletic board or even administration would allow it.
And did I miss something here or did the Op want something that was "mobile" since he operates at various locations?
As far as fast, quick, cheap, the ground rod way isnt exactly fast, tho would provide the ground needed..but so would laying down even 5 10 foot lengths of ground wire in a spoke pattern with the TX, antenna/coil mounted on a tripod at the center, and be far more mobile and faster to tear down than to be yanking ground rods out of the dirt.
Chances of running into a gas pipe or water pipe right underneath a football field or soccer field is slim.
But I think the main roadblock to the ground rod thing is getting permission to do it, then if permission is granted, find a method for this "quick" approach to pounding the rod into the dirt, running the wire, tuning up, do the shows, tear down and yank that rod out of the dirt and go pound it into the next location's dirt.
Seems more steps than necessary to me anyway. Not knocking your method there Rich, but for all practical purposes to the Op's operations, my method fits the bill for the sake of mobility and saving setup/tear down time, can overcome Carl's worry about night sky wave since the antenna system will actually have a radial ground system to work off and not just a single wire attached to a short ground rod, and any worry about people tripping over the ground wires, well heck lay down a section of artificial turf or green carpet over the radials with a hole in the center for making the connection to the radials.
But hey..who am I to stand in the way of unnecessary labor..or worry!
RFB
What do you need for range? How far from the movie screen do you need to get the signal out?
A high quality Part 15 FM transmitter can usually get 150-200 ft. in stereo (assuming it's important for your audience to hear the movie's sound track in stereo).
If that's adequate, then stick the TX and antenna as high up as you can get it, maybe the top of the movie screen (depending on what supports it). The higher the better, and nothing in it's path to the audience's radios. You could mount it before you raise the screen, allowing enough shielded audio cable to hang down and get to your movie's audio source. You'll need to find a blank FM frequency (not so easy in some areas) and tune your antenna length for best reception.
If you need more range, then in this case you will have to go with mono and an AM transmitter solution as brought up here (I guarantee no one in your audience will have an C-Quam stereo AM radio receiver). The problem with AM signals is more critical tuning and skywave interference after sunset.
If it was me doing this and I needed some range, I'd use my spare $50 Talking House TX and it's wire antenna, making sure your audio cable has good ground through the projection system. I'd broadcast using this system with the unit and wire antenna, letting it auto tune the wire antenna for me. That's a really good advantage when you have time constraints setting up.
Last experiment, on a second floor, I tucked the TH's wire antenna in the small gap up and over an arched window on the inside of the building. On a frequency of 1610 kHz, I could hear the sound quite clearly in a car radio a solid 600' and could still pick it up nearly 1/4 mile away. I wonder what it would have done if I'd put it outside the window.
Next experiment will be to run the TH on an inverter in the Astrovan Taxi (the TH uses very little power, I run my TH on my boat on the inverter during power outages, even with some lights on, never ran down the batteries), and mount it near the ceiling at the back doors, run the wire out through the rubber molding and wind it horizontally around 4 suction cups about 2' on a side, a few inches off the roof (it's ground plane).
Just some ideas to float out there 😉
Use one of these CZH 05 with the included whip and it will cover your football field, soccer field, golf course, parking lot just fine.
Cheap, fast, convenient, puts out a clean signal, and is power adjustable in case the default setting is too high.
No kits to assemble, no antennas to build, no ground rods to pound and clean crisp FM stereo sound.
Plenty of options available to you. Just pick one and go with it.
RFB
why run the THII off an inverter? it runs off of 12VDC positive tip. you can plug it directly to cigarette lighter jack using a barrel plug on one end and a cig plug on other end.
If it was me doing this and I needed some range, I'd use my spare $50 Talking House TX and it's wire antenna, making sure your audio cable has good ground through the projection system. I'd broadcast using this system with the unit and wire antenna, letting it auto tune the wire antenna for me. That's a really good advantage when you have time constraints setting up.
I've reconsidered.. I still think my idea is good, reasonable, and simple, but I have to agree Ken on this one, you would have the least amount of hassle. You can get a Talking House on ebay for cheap.
Of the six drive-ins around my area(before they went belly up),five were FM and worked fine,the sixth one on AM was horrible due to skywave interference. I would use an EDM-10($100 class kit),it will cover a football field no problem from a simple whip antenna,BT-DT. Forget AM stereo,no newer cars can recieve AM stereo,that concept failed and died long ago...
(Pullquote from RFB)... Part 15 AM can give you a coverage area of well over 100 yards ... whereas in FM, the field strength is ... about 300 feet. (End quote)
RFB, doesn't 300 feet = 100 yards? Except for the qualifiers "well over" and "about", the distances appear to be roughly the same.
A bit more clarification is needed here. Thanx.
Don't take the following personally...
What needs clarifying in comprehending AM goes way beyond 100 yards and FM covers about 300 feet?
Oooook. AM can go BEYOND 300 feet...aka 100 yards.
FM can cover (legally) 300 feet, or 100 yards.
Didn't I already say that before? (hint=just worded differently)
The term "well over" means more than..greater..further.
Example: "The store is well over a mile from here".
Or it can be worded like this: "The store is more than a mile away".
Or like this: "The store is about a mile and half from here".
All mean more than a mile. Basically saying the same thing, just worded differently.
I'm no English teacher but this is elementary English language comprehension.
Sorry...not in good moods. But I hope that clarifies things.
RFB