Last month I described how to use various types of RF filters to combine the outputs of two FM antennas on the roof of a studio facility (as an example). That's not all you can do with them, though.
Again, let's say that you have a cable system running around your offices (GM, PD, etc.) and you're not particularly interested in running other cables. (Work smarter, not harder, right?) We saw last month how to combine multiple FM antenna outputs on that cable. But what if you added say, an AM station? And perhaps reception is a little shaky after dark? Or how about this: You want to add video to a TV channel on that same cable. There must be a way to do that, right? The answer is yes on both accounts.
A very handy device to have in the first case is a simple FM modulator used in CATV systems. There are multiple sources, but Blonder-Tongue and Pico Macom are two common options. Either of these is 1RU, with simple analog left and right inputs on the back (-10dBv level).
Of course before using one of these, you'll need to know about the spectrum already on your cable system. Find a hole in that spectrum, make a note of it, and then order up one of the modulators for that channel.
Now again we'll use simple two-way splitters that can also be turned around and used to add signals together, mainly because they have good port-to-port isolation. They also have attenuation in the path. For example, the two-way divider generally will have about 4dB of attenuation from input to either output. Likewise, if you run signals into the outputs you'll find the same attenuation as measured on the sum (or input) side. See Figure 1. One on side, we take the CATV system in its original form (with whatever is already on it) and then, using the two-way splitter (turned around) we add our FM modulator, through an attenuator. You'll want to have the level from that device (as measured on the output side of the combiner) to be in the ballpark with other signals already there. There's no need to have it be very much stronger, and you don't want it re-radiating out on the antenna anyway.
Once you have that done, run the new audio in to the modulator. Magically it will appear on all the radios throughout the offices. Don't forget to tell everyone where to listen.
Now let's say you have a cable system running around that has the FM band as well as cable TV channels on it, and you want to add your own video channel (or channels) to it. (Reasons for doing so are many - for example, an IP camera at the transmitter site, or front door.) First thing you need to do is to find a hole in the TV spectrum, to find a spot for your NTSC modulator. A spectrum analyzer is your best way to do this. Then, refer to this chart that I use for this kind of thing.
Scroll down to CATV channels because you'll match up the spectrum hole with the closest CATV channel. When you do that, you'll be able to order an NTSC modulator that will slip in to the available spectrum easily.
You can obtain an NTSC modulator easily enough - again, Blonder-Tongue and Pico Macom are sources.
See Figure 2 for the best way to combine these new sources. Note that I've combined the new sources and then taken that sum, and combined that with the cable system using the two-way splitter. While it may seem easier to simply use a three- or four-way combiner, remember that the throughput attenuation goes up when you do that. Doing it the way shown in Figure 2 will provide a higher level (or should I say less-attenuated) version of the combined spectrum coming in from the original sources, as measured on the output side of the combiner.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at firstname.lastname@example.org.