Primarily, my job as transmission systems supervisor for Clear Channel in New York is to keep all of our radio stations on the air no matter what. However, as the RF guy, I'm also tasked with all reception issues at our studio HQ downtown; this includes everything from off-air signals to CATV around the facility. Many of us in this business are hams and as such know a little something about transmitters; what is also true, though, is we learn a lot about reception and how to optimize signals going in to a receiver. Let's take a look at a couple of common reception issues.
First, let's say you have an antenna on the roof pointing at the transmitter site, ultimately driving a mod monitor or perhaps an in-house cable system that feeds offices (like the PD's office) and others around your facility. One day your company adds another radio station to the mix, but the trouble is that its transmitter site is way off; one antenna cannot be used for mod monitors for both stations because while it works great for one transmitter site, it's horrible for the other. How can you get around this?
The obvious thing to do is to put up yet another antenna on the roof, and to bring a coax down from it to your technical center. But how do you make it show up on the same cable system already running around? You can't tell your PD to swap cables on the back of his or her receiver when changing stations, right?
You can add the signals from two different antennas by using the appropriate filters and an inexpensive splitter/combiner. Station A comes in from antenna A and station B (the new one) comes in from your new antenna B. While it's tempting to just combine the two with the splitter/combiner, most likely you'll be making a horrible mess as far as the receivers are concerned -- terrible multipath noted on the cable because of two sources of the same RF. The key to making this work is to manage which signals get into the combiner. The most simple situation is one in which you have a single station on antenna A, and one on antenna B (Figure 1). In that case an easy way would be to just install a bandpass filter on antenna A to pass station A, and another bandpass filter on antenna B that passes only station B. You can combine the two outputs then to drive your in-house system, since each station is now represented by only one source.
Another scenario can be seen in Figure 2. Here, we have multiple stations coming down in from antenna A (because most of the stations transmit from that site), and just one station coming in from antenna B. Let's say on antenna A, you need to cover all of 88.1 to 107.9, but on antenna B you need 99.5 only (just as an example of course). In this case, you would install a notch filter for 99.5 in antenna A, and the bandpass filter in antenna B. You can successfully combine those two outputs in to one feed then.
These kinds of filters are available from Microwave Filter Corporation.
Next month, more tricks of the trade with respect to combiners.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at firstname.lastname@example.org.