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Suggestions for receiver with composite / baseband output

R

RadioMan203

I'm working with a high school that wants to have an on-campus terrestrial FM signal to go along with their net stream. I was planning on wiring the entire building with LPB's radiating coax connected to the campus-limited modulator but the cost to wire the entire building is out of reach for the budget (if only the powers-at-be listened to me while the building was being rennovated). Running the wiring in the building by the outside contractor proved to be too costly.

My idea was to put an FM modulator on the building's internal RF distribution system and place smaller, lower powered modulators in the key areas they want to cover. The only hitch is I wanted to demodulate the FM cable signal to baseband and feed it into each FM transmitter instead of demoding down to left and right and having to get additional stereo generators for each satellite FM transmitter. What I need is a recommendation of some sort of radio where I can pick off the baseband and pipe it back into the transmitter.

I'm not concerned about the interference between the transmitters since the separation between locations on campus is great that a single FM transmitter will have output to cover.
 
The Innovonics works well and has a baseband output. If it's out of reach, any radio which works in the specified location on which you can get a schematic and which has discrete components around the detector/deemphasis/stereo input section, so you can take a 10Uf NP and bag the composite in front of the stereo decoder, where it hasn't been deemphasized.

DOS Tip: If you've sufficient gain, feed the composite into one winding on an old REP 11 repeat coil. Each winding can be a seperate source. The REP-111 has response beyond 65KHz.

You might also keep an eye on eBay for used consumer grade tuners... most of which either have an MPX output, or have the place in the circuit to get it.
 
I've read before that paging contractors have used radio-shack rg-8x as radiating cable, as opposed to the considerably more expensive "radiax". I guess the sheilding is way less than the spec'd 95%. Supposedly field strength measurements between the two yield negligible differences, and given that you can run as much TPO as you want as long as you stay within pt. 15 radiation limits, this might be way cheaper (and simpler) than buying all those receivers and modulators.

One problem that I could see with this scenario is that in order to get a useable signal at the end of the cable, you'd be radiating too much at the transmit end of the system. You could plan ahead for this scenario, and use a higher quality cable (just make sure it isn't 100% shielded) for, say, the first half of the run. Meaningful places to take your FS measurements would be 3m from the transmitter and 3m from the dissimilar cable junction. Adjust power for a reading under 250mV/m at both points, and you're finished.
 
Dayton Industrial http://www.daytonindustrial.com/ makes some decent fixed frequency rack mount tuners that have base band (composite) outputs. They are fairly tamper-proof which would be good in a school environment.

As for leaky coax, the poor mans solution is to use some RG-58, but at the point you want the cable to radiate, reverse the shield with the hot lead, allowing the signal to radiate from the shield. The easy way to reverse the shield and hot wire is to use a plastic mini-box with a couple of connectors on it, and swap polarity within the box. Connect your feed cable to one jack and the "leaky" radiating coax to the other jack. At the end of the leaky run, put a 50 ohm (or whatever is close - 47 ohm is a lot more common) terminating resistor between the shield and center connector. You're done, except for measuring to make sure you are not radiating past FCC specs.

This works very well for temporary Part 15 set ups, like inside sports arenas, along Christmas light displays, etc. The signal only needs to radiate a few feet from the cable to be useful, and it can do that for however long the radiating cable happens to be.
 
Thanks for all your suggestions, not only for the receivers but as far as the leaky coax goes. I think I'm going to go with a couple of economy FM tuners to get what I need to feed the each of the transmitters on campus.

The good (or maybe "bad" thing) is this building has tons of metal in it. Thanks to the physical campus being so large even with a fairly large power output at the transmitter location (approx 1.5 watts with the test generator) it still doesn't leave the building (at least from the initial tests we haven't picked it up on the FIM). I could certainly use standard shielded coax to feed the radiating system and that would probably allow to drop my power more, since the transmitter is located in a utility closet somewhat close to the desired coverage area. There may be some spotty coverage within the building but that will be deemed acceptable by the school staff, as these will be areas not meant for prime coverage areas.
 
Bill DeFelice said:
Thanks for all your suggestions, not only for the receivers but as far as the leaky coax goes. I think I'm going to go with a Thanks to the physical campus being so large even with a fairly large power output at the transmitter location (approx 1.5 watts with the test generator) it still doesn't leave the building (at least from the initial tests we haven't picked it up on the FIM).
Click to expand...

Since the signal has a hard time getting through the outside walls of the building, you can more or less do whatever you want inside the building without exceeding radiation limits of 250 mv/meter on the outside of the building.

Since it is a school, it is my impression that the FCC is a little more forgiving about where they measure signal strength. I know at one time, they would "let it slide" and take the measurements from the edge of the campus, rather than from the radiating element. I don't know if that exception is written in stone or not, but many colleges and universities run campus limited systems under that premise.

Of course, the best thing to do is engineer a system that generates no problems or complaints, so you never need to have this discussion with your local FCC inspector.
 
I've been working closely with the folks at LPB and they say that they have come to terms with the FCC regarding this. They state that you can measure the signal strength 3 feet from the exterior of the building and as long as it's not above the stated rules it would be fine. If you consider how sensitive a typical Walkman is I would think you really wouldn't need a ton of signal to get an acceptable signal at the receive point.
 
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