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I just recently upgraded my FM "station" to 1 kW .

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Ummmmmmmm.... is that right, Phil?.... one THOUSAND watts?
Pretty good juice for an "experimental" broadcaster!
 

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Posted by Phil Hill (Member # 371) on 12-31-2003, 04:20 PM:
 
Yup Jack!... I want the POWER and I want it ALL NOW! Hahahahaha

KGAY-FM...95.5 FM...Hollywood!

>>> Phil
 

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Posted by Bernie Anderson Jr (Member # 410) on 12-31-2003, 05:56 PM:
 
Hey guys.

As I remembered when I was learning at the Bethlehem D-I in PA, I remember the AM transmitter antenna being a little one in the shape of a "T". Someone on the Radio Forum suggested it was something called a di-pole antenna where they came out of the Antenna terminal formed some kind of antenna shape and then returned to the ground terminal as a loop. And there was a moment that I shorted the two terminals. The hum was alot less, but I didn't get much transmission farther than the room. So that made me think that that is wrong. But I'm wondering now if an antenna of a certain length would fix the problem. Thoughts?
 

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Posted by John Anastasio (Member # 607) on 01-01-2004, 12:34 AM:
 
A dipole is not a loop, Bernie. It's two pieces of wire and at the frequency you're transmitting (1470Khz) a tuned dipole would be enormous. A loop would only act as a short on your set. It was designed to be used with either a long wire and a ground or "leaky" coax. It's entirely possible that the filter has died. When I got you up and running the other night, the unmodulated carrier was clean on the scope and the quieting factor on a receiver ten feet away was total...no sound. If you get a quiet, clean carrier, then the hum is coming from something other than the power supply. Your set uses high level plate modulation utilizing a transformer connected directly across a +8 dbm line, so it's unusual to get audio hum at the levels you're using. If you're getting hum all over the place with no audio being fed, check the filter cap.
 

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Posted by Jeff Stricker (Member # 224) on 01-01-2004, 06:14 AM:
 
Bernie, I agree with John A. Sounds like you have some bad filter caps in the unit...
 

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Posted by Tim Reed (Member # 137) on 01-01-2004, 12:31 PM:
 
Roger that, Jeff, John has it pegged.

Also, unlicensed AM is only legal in a "carrier-current" application. I.E., it must use either the household powerlines or a buried "leaky" coaxial cable as an antenna. It cannot radiate a signal, therefore no aerial can be used. All rf reception must be from the induction field only.

I believe the power level is limited to 20 watts output.

FM is, strictly speaking, illegal for use in a drive-in application. That is because FM behaves differently; you can drive away the block and lose an FM signal, but then pick it up again 100 miles down the road. Therefore, unlicensed FM use is restricted - not by output power or ERP - but by field strength. The restriction is fairly small, too, something like 50 uV/m at 300 ft.

Needless to say, I seriously doubt any drive-in is running FM legally, but the FCC seems to be unconcerned about it; especially since deregulation of the Commission (and the resulting staff cuts).

Whichever system you use, however, one of the most important considerations is proper antenna matching! It can make all the difference in the world in transmitter performance and signal reception.

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KGAY-FM...95.5 FM...Hollywood!

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Phil, you never fail to make me ROTFL!
 

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Posted by Darryl Spicer (Member # 711) on 01-01-2004, 12:56 PM:
 
Tim, I remember when the Lexington Drive-in had their FM transmitter. You could tune the sound in as soon as you crossed the county line into Jessemin county. I liked that drive-in. Wish it could have stayed open.
 

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Posted by Jeff Stricker (Member # 224) on 01-01-2004, 04:52 PM:
 
Tim, Seems like there is a FCC 50 mW AM rule that says you can run 50 mW AM carrier to an antenna, provided the total length of feedline AND antenna didn't exceed 10 feet. My memory of all these regs is getting rusty...But that won't get you very far on the crowded AM band!
 

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Posted by Bruce Hansen (Member # 281) on 01-01-2004, 06:13 PM:
 
Is the transmitter near anything that could be inducing a hum into the coils inside the transmitter? I have run into problems with hum trying to power some of the "cheep" FM transmitters with wall warts. Using a switching power supply crued the problem, but if your unit uses tubes, like John said, you would not be using a wall wart.
 

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Posted by Jack Ondracek (Member # 1466) on 01-01-2004, 08:10 PM:
 
Tim is close, but not quite "right on". The FCC does have rules regarding unlicensed usage of AM and FM frequencies. The AM rules are well defined, given that the Commission has more years of dealing with this medium (and quite a few years ignoring private FM usage). However, things like "Mr. Microphone"-type transmitters and "real estate radio" boxes have promoted certain rule changes to address these circuits... and for FM they're restrictive to the point of being practically useless. I don't have the numbers memorized. However, with both mediums you CAN radiate a certain amount of energy legally. That formula is based on signal strength at a certain distance from the antenna. To be certain you're within spec, you'd have to have a field strength meter... an expensive tool for the hobbyist. You can have a very small free-radiating antenna on an AM box... something like 3 inches or the like. An FM box can not legally be sold on the hobbyist market if it is pre-assembled with a socket which allows an external antenna to be connected, leaving things like self-enclosed telescoping sticks as among the few "legal" configurations (how many drive-ins have you seen that use those?). The "real-estate" boxes go about 100 feet from the antenna, and fairly poorly at that. Those who are buying current model broadcast-quality transmitters of 1/4 watt and up... and those adventurous entrepreneurs who have made good use of recycled FM exciters are nowhere close to compliance. Those things put out too much power just free-radiating into an empty output socket! Use of low-power circuits like FM modulators and Rat-Shack antenna amplifiers usually require gain antennas, which are likely to put you right into the same bucket as the guy who uses a 6 inch stub and 5 watts of power.

Delving into reality for the moment; the sad truth (and where Tim has his info right on) is that the letter of the FCC's law does not generally give you enough signal level to provide adequate coverage to the average drive-in... whatever that may be. Because of that, most... if not all drive-ins are not strictly compliant with FCC regs when it comes to FM radiation levels. AM has also become hard to do within the regs, mainly because of the number of signals out there, compared to say... 20 years ago. You just about have to jam more power out the door just to deal with the night-time skip.

The "leaky" concept with AM and existing field wiring is an option, though not usually consistent. There are a lot of drive-ins with field wiring in such bad shape that this doesn't always work anyway. Digging real "leaky coax" into the ground is an expensive way to provide a sound format that nobody younger than 40 uses anymore.

So, in a nutshell, here's the truth of the matter. Every drive-in out there is probably outside of strict FCC compliance, or is either smaller than, say, 4 acres (about 100 cars) or has some serious hole in its coverage area. Is that a concern? Strictly speaking, yes. Practically speaking, probably not. The Commish has had their hands full for decades, just dealing with fully licensed stations, and the occasional pirate (ooops... I meant "experimental") operation that gets a little too much "in-your-face" to ignore. I personally have NEVER heard of any FCC action against a drive-in operator, and I notice that those who really need to use FM for their businesses are doing it thussly: First, go out of your way to find the quietest, most unused channel location you can find. That rule works for both you and any station you might (illegally) cover (a very huge no-no). Quiet frequencies are far easier to get good results out of with very little power, and the worst possible thing you can do is interfere with a popular local station. Second, use only enough power to provide good quality coverage within the confines of your theatre. Making a conscious effort to be heard 5 miles away from your theatre is only inviting unwanted attention from a governmental agency that has the authority to shut you down entirely... possibly leaving you with no sound system at all.

I'd guess you can draw your own conclusions from here. None of the above is in any way a suggestion or encouragement by me, my agents or occasional cohorts to violate Federal Communications Commission rules and regulation in any way, shape or form.

... so there!
 

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Posted by Don Anderson (Member # 397) on 01-01-2004, 09:01 PM:
 
Gee whiz Bill, 1000 watts.....I got busted by the FCC for running a 100 watt FM station back in the 1990's. We were able to get our signal out about 15 miles weather permitting. 24 hours a day, alternative rock programming. Took the boys in blue (suitcoats) 5 years to track me down and hit me with a $1000 fine. Of which, the community shelled out the funds in contributions. Nice area that I live in!
 

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Posted by Tim Reed (Member # 137) on 01-01-2004, 09:43 PM:
 

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Tim, Seems like there is a FCC 50 mW AM rule that says you can run 50 mW AM carrier to an antenna,

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However, with both mediums you CAN radiate a certain amount of energy legally.

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I'm happy with a couple of nice 100 watt tube amplifiers running a field full of speakers!

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Sure, Ken. Too bad your customers don't all have 4" speakers in their car stereos!

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Gee whiz Bill, 1000 watts.....

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Yah... but we've missed asking him about his antenna. For all we know, he might be buried in a valley somewhere, running through a tube bored out of a flagpole for "neighborhood aesthetic reasons"
 

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Posted by Phil Hill (Member # 371) on 01-02-2004, 02:55 PM:
 
Antenna? Antenna? We don't need no stinkin' antenna! We want badgers! And Mushroom, mushrooms! Oh oh, a snake, a snaaake!

>>> Phil
 

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Posted by John A. Parker Jr. (Member # 1719) on 01-03-2004, 07:37 PM:
 
If you have the am transmitter, do you have the field matching
unit? The am sound systems used the old field wiring as an
antenna. This required a pi-matching network to match the
output of the transmitter to whatever you were using as an
antenna (field wiring, barbwire fence etc).
The hum even with the input disconnected could be caused by a
very bad output match. This will cause the output section to go
nuts. Used to have one at the lumberjack drive-in years ago.
I hope this helps.
John A. Parker Jr.
 

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Posted by John Anastasio (Member # 607) on 01-03-2004, 09:58 PM:
 
I suspect that this is the real culprit, John. There's no matching network on the output of this transmitter. It's a homebrew job that just has a barrier strip connected to the output coil. I got a better match when I tested it and that's why it didn't hum on my bench. I've worked with LPB AM transmitters in carrier current installations and they have nifty output matching systems that are tuneable. Bernie needs to get the right size coil to mate up with whatever he's using for an antenna. I think you hit it on the nose.
 

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