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I currently own a 7 Watt Frequency Modulation (FM) transmitter that I purchased from eBay some years ago.

We are planning a drive-in cinema setup where the transmitter will be placed 5 meters above ground at this location.

A technician I interacted with informs me that considering the number of cars that might show up, about 300, the transmitter signal will severaly degrade as the cars will pretty much cause interference and signal power will have to be boosted.

Is there a formula / mathematical calculation that can help me determine how many cars can tune into the assigned frequency without experiencing a drop in audio quality?? Or a formula to help determine how much transmission power is needed?

picture from online auction of a monopole whip antenna

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    $\begingroup$ Welcome to hamSE. Please, tell us what kind of antenna you will connect to the transmitter. Will the antenna be located near the center of cars, or somewhere else? What ground area will the cars occupy? $\endgroup$ – Brian K1LI Feb 16 at 10:29
  • $\begingroup$ The type of antenna is omni direction and it can be found at the eBay link in the question. As for the ground, it is a flat circular space (please refer to location PIN in the question) and the transmitter would be mounted at one end of the circle where the big screen will be mounted @BrianK1LI . the cars will cover a radius of roughly 200metres from the centre of the circle $\endgroup$ – Manny265 Feb 16 at 13:54
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    $\begingroup$ Rather than perform a detailed analysis and present as an answer, experience tells me that this should work, as-is. With the antenna so high above the cars and assuming line-of-sight from the antenna to the cars, it's hard to imagine a problem with 7-W around 88-MHz. The only challenge might be the antenna; it could be helpful to place the system on top of a conductive surface about 1-m diameter, electrically connected to the transmitter case. $\endgroup$ – Brian K1LI Feb 16 at 14:41
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    $\begingroup$ Years ago a typical college FM radio station would run 10W and could be heard for a couple km, although their antennas were higher than 5m. You don't say where you're located, but also make sure you're not violating your country's licensing requirements. $\endgroup$ – Duston Feb 16 at 15:43
  • $\begingroup$ @Duston I have interacted with the regulator some years ago but to my surprise they couldn't determine whether my transmit power was breaking the law, I cannot find anywhere in their Acts of Law that state the limit for transmit power. Follow this pin for the proposed venue of the event goo.gl/maps/tPffnyfQ8r8WKLE57 $\endgroup$ – Manny265 Feb 17 at 9:02
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The number of cars is not an issue. Driving in a city with thousands of cars in your vicinity still allows FM reception. Engines are running, which is probably not the case when watching a film in a drive-in cinema.

For good stereo reception (26 dB audio signal to noise) you need a field strength of 100 µV/m or more. In a Rayleigh path with reflections you need about 26 dB reserve to guarantee that 100 µV/m is reached at 97 % of the positions. In case of no reception the car must be moved a few centimeters. The statistics are from experience; not from mathematics. This is valid for a receiver with a single antenna. Antenna diversity in the receiver is another situation; much better reception.

So design your system for 2 mV/m for the largest distance on the parking of that cinema. I assume the screen size does not allow more than 200 meters distance.

See: https://electronics.stackexchange.com/questions/284699/formula-for-field-strength

and then you see that 5 mW TX-power can give that reception. The mentioned 7 watts gives acceptable reception in 6 to 7 km.

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In my experience, RF noise from vehicles comes from engine ignition systems (distributors, ignition coils, spark plugs, and spark plug cables) of gasoline-fueled vehicles, but ignition systems in cars made over the last 30 years or so are much better shielded than older cars' ignition systems. Diesel-fueled vehicles don't have ignition systems at all. The customers of your drive-in cinema will hopefully stop their engines to watch the films, anyway.

I doubt that you will have a problem. Good luck, and please come back and tell us how things went.

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    $\begingroup$ 30 April 2021, I will definitely add comments on what we experienced after the event. Thanks. @rclocher3 , we plan to encourage others to bring portable radios or use the radio app on mobile phones with earphones $\endgroup$ – Manny265 Feb 17 at 9:00
  • $\begingroup$ Are you in the US? Please note that those 10 watt FM transmitters you can buy from china on ebay are highly illegal for use here. Supposedly, the first-time offender fine if you get caught by the FCC using one of those is $10,000. Sorry, I see you are in Malawi! Bear in mind it is not hard to fashion a directional antenna which will "spray" your signal out across a fan-shaped segment of the car park, which will improve reception. These can easily be fashioned out of old-school rooftop VHF TV antennas using designs you can find on the web. $\endgroup$ – niels nielsen Feb 21 at 5:46

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