Uncle Steve’s Weird Wire Stories
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.
Modern miracles
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!
Steve Lampen is technology specialist, multimedia
products for Belden Electronics Division in San Francisco. His
book "Wire, Cable, and Fiber Optics for Video and Audio Engineers"
is published by McGraw-Hill. Reach him at shlampen@aol.com
