Drive-In Theater FM Reception Issues with Large Crowds

[Barry Floyd]

We had the exact same thing happen to us on Friday night. Someone on the lot had a Bluetooth FM transmitter plugged in their car and its FM signal canceled out the left side of the front 4 rows. I literally stopped Top Gun long enough to make an announcement to people to physically unplug the transmitter from their dash. They never did. We weren't sold out so we were able to move folks around and found a spot that worked for them. After intermission - and 1/3 of the crowd left, the sound issue magically disappeared.

Saturday night, completely sold out every available parking space for Top Gun. We even "invented parking spaces". Not a single sound issue at all.

For what it's worth, we use a USL JSD-60 cinema processor which feeds into a DBX 166XS compressor / limiter, then directly into a Broadcast Warehouse TX5 transmitter with a "J"-pole antenna. Both screens have the same identical setup. Our transmitters output is set to 0.1 watt, which covers the field fully and fades out about 1/4 of a mile away from the drive-in.

Our frequency for Screen 1 is 87.9 FM which Van Dalton correctly mentioned is the default for these Chinese junk FM Bluetooth transmitters.

[Jeremy Jones]

Wow Barry, what a pain. Yeah, our biggest crowd when we had the issue was with Top Gun as well. Our frequency is at 90.3.. I hope that keeps us partially out of the Bluetooth transmitter fiasco since we're not at the default setting?! Thanks for your sound setup details. I'll look into the DBX 166XS compressor to potentially go along with our current setup.

Hey Mark, Yes, Based on your pattern, it looks like I might be better off changing the angle of the antenna as well. Anything that you'd recommend to help identify signal strength? A RF spectrum analyzer was brought up for identifying a clear fm channel, but I wasn't sure if it would also help me on getting an idea of the actual signal strength for my transmitter in different areas of the drive-in. Help to identify weaker spots that would be more prone to disturbances during bigger crowds?

Dave, Since you install Decade transmitters, what sound compressor would you recommend for use with the Decade?

Getting a lot of useful advice here. Thanks everyone!

[Gordon McLeod]

The first decades we installed we used the smart afterbuner wit them as a compressor limiter later dbx compresors

[Jim Cassedy]

One of the things I keep in my "bag of tricks" is this GQ EMF-390
RF-Sniffer & EMF Detector. It has several modes, but it's basically
a "poor man's rf spectrum display" I think I paid abt $120usd for it a
while back, but I've found it very useful in locating mystery RF sources.

The freq range is appx 50Mhz-5.5Ghz in 7 bands. Took this pic on a
table in my living room, the big peak is probably from the wi-fi access
point in my apartment, and the smaller peak is most likely my neighbor's
wi-fi router which is just on the other side of my living room wall.
RF_Meter02.jpg

This mode gives several 'rotating' read outs of nearby RF signals, including
suggestions as to possible source. In this case, a "Cell Tower" - - and there
is indeed a cell site on the roof of a building about a ½ block down the street.
RF_Meter01.jpg

GQ Electronics makes some interesting instruments. I've got 3 of their Geiger Counters.

[Jeremy Jones]

That looks awesome Jim! I just ordered it.
I can't wait for my new toy to come in.
Thanks

[Martin Brooks]

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.

It's not the Bluetooth. Bluetooth is used to pick up the signal from the phone (although some of these simply use the phone's headphone jack if it has one or other port) but then it transmits the signal on an FM frequency that can be picked up by the car radio. It's used in older cars that don't have Apple CarPlay or Android Auto and also don't have an auxiliary input. I've used them in my 2003 Honda Accord which has none of these features (although the transmitters I have don't work very well).

So if someone is having this problem at a drive-in and the on-screen message doesn't work, look for older cars.

Years ago, I was driving somewhere and all of a sudden my car radio was playing Sirius/XM and I don't have Sirius/XM. I realized that there was a car next to me using one of those transmitters out of their Sirius/XM tuner.

[William Kucharski]

FWIW the SiriusXM ones are usually designed to be directly wired into the FM antenna signal path, but they were ordered to recall their first few generations of Onyx receivers because they featured onboard FM transmitters that broadcast at too high a signal level.

The BT transmitters are almost all inexpensive Chinese models from Amazon and as you might expect their makers don’t care much about FCC regulations.