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This topic comprises 3 pages: 1 2 3
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Author
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Topic: Drive-In - FM transmission coverage issues
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Jack Ondracek
Film God
Posts: 2348
From: Port Orchard, WA, USA
Registered: Oct 2002
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posted 07-07-2016 05:38 AM
Monte, you have a fairly low-power system there. Those modulators were originally intended for inserting FM stations in cable systems. Yours could possibly manage as much as 100mW (1/10 watt), but there's no way to tell if it's actually doing that without measuring it there. It could be much less.
From your pictures, it doesn't look like antenna placement is the problem.
If this thing ever did work well and the modulator hasn't conked out, I'd still have a look at the coax and antenna connections. If they're OK, then I'd try moving the frequency. Your FM band is really full there, but at the bottom of the dial is 88.1 (K201EO). It's putting out only 10 watts and its main contour just barely touches the northeast corner of Boise, some 20 miles from your drive-in. Try moving your transmitter to 88.3 & see what you get. On the dial, it's close to the translator, but farther away from KOAY, which has more power on 88.7. If your problem is proximity to other stations, this might help.
If none of that works, then I think you're a candidate for a new transmitter and antenna.
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David Buckley
Jedi Master Film Handler
Posts: 525
From: Oxford, N. Canterbury, New Zealand
Registered: Aug 2004
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posted 07-07-2016 08:55 AM
Although I'm far from an expert in this area, I know a bit about this stuff as I do tech support for a community radio station with a 2KW EIRP transmitter on a small mountain, which covers a fairly large area.
quote: Monte L Fullmer
Some portions of the field receives the signal with no issues, and some areas, the signal comes in rather sketchy, or none at all. We have a side yard that carries the overflow and also experience broadcast issues.
"Sketchy" to me sounds like insufficient signal strength. For FM to work well, the signal level at the receiver has to be sufficient to ensure the receiver is "fully quieting", once this signal level is reached then the amplitude modulations caused by stuff like car moving, people walking in front of the antenna etc do not cause the signal level to drop, and the audio to become scratchy. Theres a number that it is considered one has to reach to get acceptable performance, and that is 66 dBuV/M. There are tools that given the parameters can calculate the coverage. But for a problem the size a field, the coverage prediction tools are a bit of a sledgehammer to crack a nut.
The Inverse square law applies to the signal propgated from transmitting antenna, and - all things being equal - as the distance from the transmit antenna to your receiver increases, the signal strength drops. As some of your receivers are close, and others are relatively far, the drop-off is quite pronounced that close in.
Solutions: The obvious is more transmitter power, with the caveat that any time anyone transmits anything anywhere there may be legal implications and licensing to deal with, don't know how your rules work in the USA. Less obvious, a directional antenna to take the same transmitter power and "shape" it in a specific way, so that the effective power (EIRP) goes up in the direction you need it to, whilst reducing radiation in directions you don't require it.
Given you've got a transmitter the local folk regard as dodgy, then a new transmitter is the obvious solution.
Mark's already mentioned BW broadcast; their current range of transmitters, the V2s, also have on-board processing; Jack mentioned that the dynamic range of digital cinema is far too wide for FM radio to handle sensibly, and so this is a perfect one box solution. You can easily spend far more than the price of a low power transmitter on a processor, and especially if you want a big brand processor like an Optimod.
Sadly BW no longer seem to do "small" transmitters, the TX50 being the smallest, but it turns down to 5W; If 5W is still too much power, then a simple resistive dropper will reduce it still further.
Make sure the antenna - whatever it is - is impedance matched to the transmitter, which, unless you want to spend more time tuning antennas than showing movies, means a 50R antenna.
The transmitter (as is any half-decent FM transmitter) is stereo; to take advantage of this for the best audio to the vehicle, you'll need to arrange a stereo + centre feed to the TX from cinema audio system. Of course, dome folks will have their stereo backwards, so the screen movement will be opposite the audio...
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David Buckley
Jedi Master Film Handler
Posts: 525
From: Oxford, N. Canterbury, New Zealand
Registered: Aug 2004
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posted 07-07-2016 05:49 PM
quote: Jack Ondracek
Hi, David. Are you sure about the TX5 being discontinued?
My bad Jack; there is a TX5 V2, webpage. I was sure it had been killed off, turns out I've remembered it wrong. Sometimes, its good to be wrong ![[Smile]](smile.gif)
This looks the perfect solution for a drive-in, power level is variable from 0.05W (ie 50mW) to 5W, and its got the integrated audio processing the rest of the V2 series has.
quote: Dave Macaulay
I've never come across a BW, here in Canada
That's odd; they are available all over the world, even here in New Zealand. I wonder if Canada has some regulatory weirdness that BW kit is not certified for. You could order online from Broadcast Warehouse, they ship all over the world.
quote: Dave Macaulay
I only see Decade transmitters
Never come across these, but I note they don't have an integrated audio processor, which is the big win with the BW V2 series. Add a quarter-decent external processor, at over a grand, and suddenly the BW V2 kit looks very value priced
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Jack Ondracek
Film God
Posts: 2348
From: Port Orchard, WA, USA
Registered: Oct 2002
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posted 07-07-2016 07:10 PM
quote: Gordon McLeod
Currently the only legal one in Canada is the Decade
Last I checked, and it was a while back, the Decade transmitter was solid, but a bit expensive for the value received. the BW gear now includes a raft of internal features... many of which might not be useful to a drive-in, but I put a lot of these in US translator installations. The processor is a bit lightweight, but certainly suitable for a drive-in. It will also auto-shutoff after so many minutes with no audio. It also has a built-in web interface, which makes any adjustments easy.
For the price, the Decade models I last looked at had none of this. A solid exciter... but not much else. I did check the web site and, after paying $1,600 for their most-comparable transmitter (but only 1.8 watts), you'd need to add some kind of processor/limiter for an effective drive-in box. That'd put it in the $2,000+ range. The UK price (from their site) for the TX-5 V2 is $1,500, fully loaded.
Gordon, If you're following US and Canadian legalities, the only Decade box that an unlicensed Canadian drive-in (or US one, for that matter) could legally use would be their MS-100. The power level of that product is so limited that you probably couldn't get to the screen with its signal. It claims 150 feet for the Canadian power level, which is more than what's allowed by FCC part 15. In order to comply, I think you're also required to use the box's internal antenna.
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