Drive-In - recommended FM transmitter power?

[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

[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.

[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.

[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.

[Harold Hallikainen]

About 30 years ago, I wrote an article on low power FM broadcasting. Here's what I came up with at that time:

Unlicensed operation in the FM band is limited to a field strength of 250 microvolts per meter measured 3 meters from the antenna. This works out to 165.8 picowatts per square meter, which would be 18.7 nanowatts into an isotropic radiator.

At that time, an unlicensed FM station running 500 mW was fined $17,500.

Harold

[Scott Norwood]

What's the system that works like an induction loop that keeps the FM signal within the drive-in's property? Does anyone use that? Does it work well?

[Kenneth Wuepper]

The local drive in used a frequency in the AM radio band. They gave the transmitter to our church after they closed the drive in. It was unique and used a "Shorted loop antenna". Basically a loop of coaxial cable around the lot that was shorted to the shield at the end and the shield was not connected at the transmitter. It worked pretty well but the parking lot was not in sight of the pastor so the system never got used. It was only mono sound so not very exciting either.

One local cemetery has a low output portable FM unit that they deploy for grave side internment in bad weather. The family can stay in the car while the casket is lowered into the grave. It has a range of less than the limits of the cemetery. This is very handy now that people should not be close to each other or in groups.

[Stefan Scholz]

Carsten,

for Germany you do still need a license, issued by a govt board. This, in normal times, took 6 to 8 weeks, and in now, may take much longer.

For a transmitter, we have used products of the BW broadcast brand, that had the required signal processing and stereo coding built in.
A chinese DDS transmitter might go for 45 to 80$, at least, when they were able to deliver thru on-line sales, but did suffer from lack of dynamic range compression, and severe harmonics and unwanted side bands.
The BW transmitter offered very good signal processing, mainly dynamic compression, and optimal modulation within the allowed range, with proper output filtering to low harmonics.
The smallest version, a 5 watt exciter, was doing a very good job, and could be set to the requested low output. Pricewise they are in a good range, around 2500 Euros net in a package including a little radiator antenna and Neill connector cable.
One of our drive-in clients used the Chinese DDS type, and suffered from permanent complaints from the car users, and their inferior sound systems, not capable to handle the dynamic range of cinema, as well as the poor radiator that came with the unit, connected thru RTNC connectors (reverse threaded Neill Conzelman is not a standard in pro RF industry), that made it nearly impossible to adapt to something else.
For a limited time, you might get around with lo power and simply the use of a non used channel, without having a proper license. It is no longer a federal offense in Germany, and the 1990 times are gone, where we would have convicted you to 3-4 years in detention, just for owning the transmitter.

Power wise, with a small antenna, 6 ft above ground, you will end up in the 100 mW region.

[Marcel Birgelen]

Isn't the 87.5 to 89.0 MHz at the bottom of the "usable" spectrum reserved for stuff like personal transmitters? I guess in those times, if you operate in such a spectrum for a limited time with a wattage no more than necessary to get good coverage on the area you need, you'll probably get along for the time being. It's important you're not causing interference with any other station that you should be able to receive around your location.

Like mentioned earlier, you'll definitely will need a good compressor to make it work, your average car audio system isn't comparable to a normal cinema audio system.

[Carsten Kurz]

The small digital mixer we use for our openair screenings offers a multiband compressor. It will probably need a bit of training, but I think there is enough to be found on the web about typical compression schemes for radio applications.

I talked to a colleague today, and he said he received his transmitter license within two days.

- Carsten

[Barry Floyd]

We have two Broadcast Warehouse TX-5's at our place. One is 17 years old, the other is 15 years old. Both are rock solid. When we converted to digital in 2013, we had to get a compressor / limiter to go between our USL JSD-60 and the transmitters as the the new digital signals were blasting the transmitters with distortion. Sounds absolutely perfect now. If I remember correctly the TX-5's are no longer available. There is another rack mount unit made by DECADE (which I think is a Canadian manufacturer) and I saw one at the drive-in owners convention back in January, however it wasn't hooked up. We run ours at 1/10th of a watt and it covers the fields nicely and fades out about 1/2 a mile down the road from the theatre. The Broadcast Warehouse TX-5 has variable power (0-5 watts), and the TX-1 is fixed (I believe) at 5 watts).

[Christopher Markiewicz]

The TX-1 is fixed at 1 watt. When I was with West Wind we had 33 BW transmitters, TX-1s and TX-5 Series 2s. It was vital that the transmitting antenna length was tuned to the broadcast frequency to optimize SWR and minimize back EMF into the transmitter. The TX-5 display was useful in this, showing forward and reverse power. I also had a cheat sheet showing the correct antenna length for frequencies in the broadcast band.

[Jack Ondracek]

If I remember correctly the TX-5's are no longer available. There is another rack mount unit made by DECADE (which I think is a Canadian manufacturer) and I saw one at the drive-in owners convention back in January, however it wasn't hooked up. We run ours at 1/10th of a watt and it covers the fields nicely and fades out about 1/2 a mile down the road from the theatre. The Broadcast Warehouse TX-5 has variable power (0-5 watts), and the TX-1 is fixed (I believe) at 5 watts).

BW Broadcast has discontinued the V1 and V2 variants of this transmitter. Their new "V3" version uses a new exciter that is pretty much all digital in design. Otherwise, it has much the same feature set of the V2.

The BW transmitters are widely used in the broadcast industry for FM translators, low and mid-powered radio station transmitters an as exciters for higher-powered amplifiers. I use one of their 30 watt models as the exciter for a 25,000 watt commercial broadcast transmitter. Their large installed base in this industry makes them a reliable source for a drive-in's purposes.

The Decade transmitters work well, as far as they're designed. However, they're nothing more than the basic transmitter... no processing or web interface, and they're more expensive than BW Broadcast products of like size.

As you say, the TX5 is continuously variable from zero to 5 watts. The TX1 board serves as a fixed one-watt model (no adjustment), and it is used as the exciter for all the larger versions. They just add successively larger power supplies and amplifiers for the model selected.

They expect to reopen after the pandemic passes. you can get phone support now, but no sales or service shipping in/out.

[David Bird]

I need to take some time to research compression. Initially our sound was great after installing digital, I found later last year some of the louder scenes started to sound "muddy" to me, though still fine during normal scenes. I'm always very nervous tinkering with the mixer during the show, even though I can see the "clip" light flickering and the TX5 reading " ! ". Not sure whether the DCP's are outputting higher levels lately, but wasn't an issue before.

[Carsten Kurz]

Probably not easy to find the right compression scheme for cinema sound mixes. Unfortunately, the market is probably too small for anyone to develop an optimized scheme. For general radio applications, there are now digital radio broadcasting standards that convey higher dynamics - but these are not feasible for Drive-In's.

It's a pity that no one ever thought about developing a simple digital broadcasting standard over Bluetooth or WiFi bands.