Drive-In - recommended FM transmitter power?

[David Bird]

I've had the pastor of a church asking me about transmitters and the legalities. I'm no engineer of course and remember Jack's recommendation of the TX5 years ago and that's what we have. I ran it at 1W when we first got it which was WAY more than we needed, but I was overcompensating, our previous "transmitter" turned out to be a unit more meant to feed a cable-tv system with the signal and we actually had spots on the field you couldn't hear it. After hearing our soundtrack one night about 8 miles away, I've turned it way down. Back to the preacher, "Industry Canada" guidelines popped up when he went to order a small transmitter. They're "murky" to read, many low-power, limited-duration broadcasts are exempt, however there's also something there that says you need to be under 1 microwatt (output). Now, again, I should ask my son taking electrical engineering, but I read that as one one-millionth of a watt. He's buying a unit with settings of 1 or 5 W. I've warned him to stay away from commercial frequencies and realistically, this won't leave the little valley our town is in. And it's an hour each Sunday morning. He's hoping to do some drive-in services in the large parking lot across from the church. I was able to find a small belt "clip-on" transmitter with mic that claims to "throw" 300 feet or so at under a microwatt, but he's going with something like a small self-contained "Ramsey" unit with attached telescoping antenna that he can feed a mixer into.

[Sam Chavez]

I had good luck with the TX1 in a Dodge City DI a few years back. The TX5 sounds like a treat with a web interface. I got a notice a few days ago that they've ceased operation due to the virus. Temporary I hope.

[Jack Ondracek]

What would be the recommended VHF-FM transmitter power for a single screen medium sized parking lot?

I saw a few numbers in older threads here dealing with Drive-In transmitters, but no general rule of thumb.

- Carsten

Because of variables, there is no "rule of thumb" that fits all. In the US, drive-in transmitters do not fall under FCC part 15 rules (not enough allowed power to cover the field). We operate in a gray area that dictates you use just enough power to adequately cover your theatre.

I've seen extreme cases over the years. One was where an owner used his FM to entertain much of his community, running mostly advertising for his theatre, but also playing a few ads for friends and other local businesses. He was running a couple hundred watts with an antenna that was mounted on top of his screen (the highest structure on the property). That was so far outside the rules, it did get the FCC's attention. Ironically, he called me to ask what he should do when he heard a field inspector was on the way. "Turn it off now and separate the coax from the antenna", was the easy response.

The trick is the combination of the antenna, how high you mount it and then the transmitter power you select. An antenna with "gain" multiplies the power you give it, meaning an antenna with a gain of 3 will effectively give you 15 watts if you feed it 5. A single-bay circularly-polarized antenna usually has a gain of about half, so feeding that one 5 watts would give you an effective output of a little under 2.5.

All power aside, most combinations with the antenna on the booth roof will give you all the coverage you need, running somewhere between .25 (one quarter watt) and 1/2 watt. A large field with a noisy FM band might need a little more. I've never needed more than 3/4 watt, after considering gain.

The point is that you should use only enough power to cover the corners. At that, you will bleed out more than you might imagine.

The BW Broadcast TX5 is a popular transmitter because, unlike their TX1, the power output is variable from a low of zero. The new ones include a web interface and basic audio processing. You would never, ever, put your foot to the floor with this box. 5 watts with a gain antenna could get you upwards of 10 miles, depending on how high your antenna was mounted.

[Carsten Kurz]

What would be the recommended VHF-FM transmitter power for a single screen medium sized parking lot?

I saw a few numbers in older threads here dealing with Drive-In transmitters, but no general rule of thumb.

- Carsten