Drive-In Theater FM Reception Issues with Large Crowds

[Jeremy Jones]

We run our drive-in movie theater sound with a fm transmitter (Decade FM-850). We've had it for about a year with no problems. We had our largest crowds this past Friday and Saturday, and we had 3 of our common repeat customers complain about loss of signal and static at times (in different locations). All our showings this week have been fine, but there were not nearly the amount of cars either. We've never had an issue like this before. Any fellow drive-in owners out there with any similar experiences or anyone with basic understanding of FM radio that could confirm that a large number of cars in our drive-in could potentially influence our FM transmission that much?
Thanks

[Barry Floyd]

Every now and then we'll have similar issues on crowded nights where everyone is parked close together. We'll get 5 or 6 cars all within the same vicinity come up and complain about static or no sound at all. 9 times out of 10 we've traced the source down to someone else parked within that same vicinity of the field has one of those "plug in the lighter socket" bluetooth FM transmitters that's set to broadcast on our frequency. I ended up making a 15 second static slide in DCP-O-Matic that's now part of our playlist every night that says "If you have a Bluetooth FM transmitter in your vehicle, please physically unplug it now". That slide literally took care of the majority of the issues.

It's probably not your transmitter... it's likely someone elses.

[John Eickhof]

i am not familiar with that fm transmitter you are using but most transmitter have adjustments for modulation (output) and signal level (volume unfortunately the newer digital soundtracks have a far greater dynamic range of sound and simple transmitters can be easily overmodulated, this combined with the usually poor quality antennas will combine to give you spots that will have poor coverage, the result is when your lot is crowded people park in some of the dead spots, the solution is not to increase modulation or the volume level as it will simply spread the coverage area outside of your lot and cause static in the lot because the signal is skipping outward, a better 360 degree antenna may help or repositioning the antenna, most antennas come with a chart to show the effective radiation pattern the best one i have found through the years is a simple vertical radiator with a single ground reflector at a 15 degree angle, this produces an elliptical pattern that will suit the typical drive in lt with the antenna place on the roof of the booth.the secret to good clean localized broadcast is 'low power' over powering is your nemesis. also depending on time of day the atmosphere can also be a factor especially with a full moon. perhaps the simple test is to have the customer move a few feet and see how it works, also most often fm systems are installed and adjusted during daylight, it is best to perform the set up adjustments at night! you will find the modulation is set too high in most cases. good luck

[Mark Gulbrandsen]

What power level are you running your transmitter at? Assuming the antenna is set up as John said, you would likely need about 1/2 to 1 watt of power to cover a 300 car drive in. I looked up your transmitter and IMHO it's questionable and not commercial grade. I always used BW Broadcast in the Drive Ins I serviced because they are FCC Type accepted and are commercial products., I installed new ones when we converted them to digital and there are two drive ins where I kept the old transmitter as a back up. All they have to do is move the cables over to it and turn it on. Everything else is preset. Note also, using a dipole results in a radiated pattern as is in the attached image. So you can see that unless the power is upped that you will have dead spots between the radiated "petals". I actually have a BW TX-30 at one location turned all the way down to 5 watts, and further attenuated with in line attenuators. BW used to make a nice 1 watt unit, but they discontinued it.

Also see: https://www.bwbroadcast.com/#

[Jack Ondracek]

Unfortunately, BW Broadcast is not currently an option, due to the demise of its owners over the past couple of years. There are rumors of a restart, but that depends on who you talk to.
Decade transmitters are pretty expensive, considering they're bare-bones, no-frills boxes. Without any internal goodies, you really should have some good audio processing before the transmitter input.
Beyond that, others have posted some good ideas. I use a Comet antenna here. It looks like an overgrown CB antenna and has worked pretty well over the years
I haven't experienced the holes in my sound coverage that you're reporting, but I did have a similar problem with my customer pagers when the fields were full.

It could very well be that Barry has the solution and your sound system is OK. I'd look for the simple fix first.

[Mark Gulbrandsen]

Not sure how Bluetooth could interfer... it's frequency is 2.4ghz. No where near the FM band. Even the Harmonics of 2.4 GHz are only.higher in frequency, not lower.
It's too bad about BW Broadcast. I used them because they had very good build quality and they are very easy to fix if there are issues. The TX-1 did include limited audio processing.

[Carsten Kurz]

Many of these Bluetooth audio forwarders have a small low power FM transmitter installed. The transmission often is BT from Smartphone to the transmitter, and FM from the transmitter to the car FM. And of course, it's the FM relay that causes issues, not Bluetooth The higher the attendance, the higher the possibility that someone operates one of these devices.

[John Eickhof]

thats interesting carsten, it actually ma be phase cancelling with any given vehicle causing signal blanking? im sad to hear of bw too, i used many of them through the years i also used many blonder-tongue commercial modulators with excellent results

[Stefan Scholz]

Even though I personally preferred BW broadcast for the fine signal processing and the legal listing in this application, actually the Decade 850 seems to be decently built, and is not a bad choice, like the cheap $ 50 to 100 chinese "church lot" transmitters.
It's not Digital sound , when the problem with dynamics started. Even in the time of film prints and Dolby decoded soudtracks, the dynamic range was far too big for FM broadcast. You always needed signal processing before modulating the transmitter. If it otherwise works fine, on lesser occupied days, the reason must be in the patrons cars.
Barry Floyds idea of those Bluetooth to FM gateways is really a thing to dig into, these are not known for FCC conforming modulation and rejection of spurious signal components. The Decade line at least does that to an acceptable value of - 60dB. Go, and look for interference creating hardware in cars.

[Mark Gulbrandsen]

The Decade is Canadian made and meets Canadian requirements, so it's probably pretty well made. Unfortunately, the Canadian and FCC approval are not interchangeable in each other's countries. So you would be using an illegal transmitter. BW was type accepted, at least the three different models I used, by the FCC, and that's not a cheap process to go through. Low power FM stations are authorized for noncommercial educational broadcasting only, no commercial operation, and operate with an effective radiated power (ERP) of 100 watts (0.1 kilowatts) or less, with maximum facilities of 100 watts ERP at 30 meters (100 feet) antenna height above average terrain. Regardless of popular misconceptions, it is not legal to broadcast on FM at low power, or at any power if not licensed by the FCC, even with less than 1 watt. Not sure how our neighbors up North handle it up there, but perhaps one of our Canadian friends could chime in.

Just sayin!

[Dave Macaulay]

We install Decade. In Canada. Lots of them in the DI boom during lockdown. Max power is 1.8W and if you get their recommended antenna and cable, then tune the antenna either with the instructions or a SWR meter, coverage is really good. Several km in "the country" and I have not had problems covering any DI lot yet. Licensing... don't know the law but we just find a clear channel and use that, no license that I know of.
Yes you should have a compressor for best sound but few do.
In Toronto there aren't really any clear channels. One client used an rf spectrum analyzer to find the cleanest one but they already had the analyzer. We just go through all of them with a radio.

[Gordon McLeod]

We installed many of the Decade units very reliable and well built. We always installed a compressor limitor on the input.. To my knowledge the decade was approved in the usa

[Jeremy Jones]

Thanks Barry for the heads up on the bluetooth transmitter. I did not even realize something like that could be an issue.

Hi John, The antenna that we use is an Alan Dick Jampro JFWD Broadband Folded Dipole FM Antenna. I will definitely try lowering the power on the transmitter & testing it at night. Thanks a lot!

Mark, We have the power setting for the FM850 on the higher end, but will try lowering it based on John's suggestion.

Hey Jack, We are using an older model Christie CP2000 Projector with a Doremi AUD-D2A audio processor. Sound is definitely not cutting edge, but we are working with an older model projector and have a limited budget, what would you recommend?

Dave, Yes, I'd like to be able to check the signal strength of our transmitter in different areas of the drive-in. Would a rf spectrum analyzer be the right tool for the job or is there something else more fitting?

[Van Dalton]

Many of these Bluetooth audio forwarders have a small low power FM transmitter installed. The transmission often is BT from Smartphone to the transmitter, and FM from the transmitter to the car FM. And of course, it's the FM relay that causes issues, not Bluetooth The higher the attendance, the higher the possibility that someone operates one of these devices.

thats interesting carsten, it actually ma be phase cancelling with any given vehicle causing signal blanking? im sad to hear of bw too, i used many of them through the years i also used many blonder-tongue commercial modulators with excellent results

A sidebar, but worth mentioning. Sirius and XM also used to integrate such devices into some of their portable SDARS receivers to make them operate with car and home stereo equipment. They may still do for all I know; I don't u$e either of tho$e wa$te$ of money so I have no idea of their current status.

If you live near a major highway (state or interstate) or high-traffic local arterial/collector road, or even when driving on one, park your radio on 87.5, 87.7 or 87.9 MHz, the default output frequencies for a lot of these Chinese plastic junk FM microtransmitters, and see what you hear. (Protip: program all of them into presets and set the radio to scan!) 87.9 tends to be the "sweet spot" for them. These three frequencies will let you receive probably 80% of the consumer FM transmitters in operation including the Bluetooth ones.

[Mark Gulbrandsen]

Thanks Barry for the heads up on the bluetooth transmitter. I did not even realize something like that could be an issue.

Hi John, The antenna that we use is an Alan Dick Jampro JFWD Broadband Folded Dipole FM Antenna. I will definitely try lowering the power on the transmitter & testing it at night. Thanks a lot!

Mark, We have the power setting for the FM850 on the higher end, but will try lowering it based on John's suggestion.

Hey Jack, We are using an older model Christie CP2000 Projector with a Doremi AUD-D2A audio processor. Sound is definitely not cutting edge, but we are working with an older model projector and have a limited budget, what would you recommend?

Dave, Yes, I'd like to be able to check the signal strength of our transmitter in different areas of the drive-in. Would a rf spectrum analyzer be the right tool for the job or is there something else more fitting?

Well, you saw the image of the radiating pattern of the dipole, so hopefully that might give you something to go on. As you raise or lower power that pattern is going to get larger or smaller...