I promised last time to talk about weird wire stories. So here goes!
Back in 1985, I was working for a small FM here in the San Francisco Bay Area. Management had finally bitten the bullet and approved a new transmitter and antenna.
I really got lucky with the transmitter. Another local station was also putting in a new transmitter. In their case, it was a 5 kW Harris. They were concerned that, as they switched from their old rig to the new rig, something would go wrong, so they ordered a “temporary backup” transmitter, a Harris FM2.5K.
Well, something did happen; their old rig died before they could finish putting in the new rig, so the little “emergency transmitter” kept them on the air one afternoon drive time.
I heard about this, and a 2.5 kW rig was exactly what I was looking for. So I convinced the local Harris rep that it would be silly for him to send back this “used” transmitter (six hours used) to the warehouse, when he had a willing customer standing in front of him.
Of course, all he had to do was to give me the right price. It wasn’t hard. I saved my station $7,000. And he probably saved about as much by unloading it. I had a crew pick it up and deliver it to my site. All Harris had to do was send out their install tech to check everything out.
75 vs. 50
In the process of installing the rig, I found a very strange thing. The coax cable running from the exciter to the IPA was wrong. Of course, all of these little cables are supposed to be RG-58 (50 ohm). In this case, someone had installed a piece of RG-59 (75 ohm). Still the rig checked out just fine. So I waited for the expert from Harris.
When I told him about the 75-ohm coax, he said “Wrong, wrong, wrong.” So I dutifully opened the rig, took out the 75-ohm cable, and put in a piece of 50-ohm cable. We closed it up and turned it on, into our dummy load.
The rig went haywire. The tuning was all off, and we couldn’t get it to stabilize. After screwing around for an hour or so, we decided to put the other cable back in, which we did. When we brought the transmitter back up, it was right on the money. Every spec was perfect.
Of course, we couldn’t look at the VSWR on that one cable, but the VSWR on the output of the exciter was very low, and the output of the entire rig was 1.05:1, pretty nice.
So we left it as it stood. As far as I know, the rig is still there, humming along with a piece of 75-ohm coax in its innards. Can anyone explain to me how this darn thing worked? And why the 50-ohm didn’t?
Now, even more amazing is the fact that, after I had written the paragraphs above, I opened a back issue of this very newspaper to John Bisset’s Workbench column. He says, “I found an exciter that was wired with 75-ohm video cable.” So maybe my discovery was not as unique as I thought.
The long way home
In that same issue, John asked for some interesting stories regarding RPUs. Well, in terms of wire and cable, this isn’t a story so much as an observation.
I remember doing a remote one day, and having this sudden realization of all the technology that was involved in what I was doing. The actual thought was formed while looking at the talent. He was on a wireless mic, and listening to himself on-air with a Walkman-style receiver. From his mic to his earphones, here was a list of the wire and cable (and technology) used.
The wireless mic went across the room to the wireless receiver where (1) a short microphone cable attached it to a Shure M267 mixer. Then (2) a multipair snake, only one channel used, went up to the roof of the building where the 455 MHz RPU transmitter was. Of course, I could have run a long piece of RG-8 or half-inch Heliax to the roof, but you get more power and less loss if the transmitter is close to the antenna.
The output of the RPU went through a short piece of (3) Belden 9913 RG-8 to the antenna. The signal was picked up at our transmitter site and fed to an RPU receiver with (4) Andrew 1/2-inch Heliax. The output of that receiver was fed to another RPU transmitter in the 450 MHz band. This was fed to another antenna through (5) another piece of 1/2-inch Heliax and aimed at our studio location. There, a final RPU antenna picked it up and another piece of (6) Belden 9913 fed it to an RPU receiver.
This receiver could switch between the 450 MHz and 455 MHz assigned frequencies. So we could either go direct to the studio for close or line-of-sight remotes, or we could power up the second hop transmitter and the studio receiver could switch to that frequency. This one was, of course, a two-hop project.
The audio from the RPU studio receiver was fed to a distribution amp through (7) single-pair audio cable. And the output of the amp was fed though (8) a multipair snake cable to the on-air studio console. The output of the console was fed through two pairs of another (9) snake cable to another distribution amp, and from there (10), a pair of audio cables fed our on-air processing (11, 12) and stereo generator.
Our studios, not being line of sight to the transmitter, required that we use a Modulation Sciences composite line driver. This required (13) an RG-58 coax from the stereo generator to the composite driver, and some twinax (14) to run 1,000 feet to a building that did have line of sight.
There the twinax was converted back to (15) RG-58 coax, which fed a pair of TFT 950 MHz transmitters. (These days, I’d just use a T-1!). From the microwave transmitter, it was (16) a run of 1-inch Heliax because of the high frequencies and cable loss at 950 MHz, up to the roof of the building and to a Scala full-size dish.
This signal went to our FM transmitter site, to an identical dish, and (17) through 1/2-inch Heliax to a TFT receiver. The composite output fed some (18) RG-58 that attached to the exciter in the Harris transmitter.
Then we have the strange case of the (19) RG-59 mentioned above. The transmitter output was (20) a 1-5/8-inch Heliax, through a switcher, and (22) another piece of 1-5/8-inch Heliax to a two-bay ERI antenna. The output of that antenna was picked up by the antenna wire (23) in the talent’s Walkman, and the audio fed up through (24) the wires to his headphone.
Twenty-four cables, of 12 different types, working in five different parts of the spectrum.
And all this happened instantaneously! Truly a miracle of technology!