What Caused His Erratic VSWR?
Here’s a little troubleshooting quiz for you to
Let’s imagine that the weather has been hot, with record
high temperatures; afternoon storms are the rule, not the exception. After one
such storm you’re called to a newly built FM site, where the transmitter is
showing high VSWR.
Fig. 1: Wallwarts lined
up like soldiers — and identified.
You suspect the
transmitter but there are no alarms; and readings are close to normal except for
reflected power. You can’t verify what the transmitter will do using a dummy
load because the site doesn’t have one.
You can’t see the antenna bays because they are
covered with radomes. You check the transmission line, figuring it has lost
pressure, but then you remember that the line is foam; there is no nitrogen
Fortunately, the transmitter is not folding back … yet.
The next day, VSWR is a little lower but still
there’s nothing obvious. The transmitter is staying on the air, albeit with the
high reading, though you’re worried how long it will last. Yet by the end of
the week, the VSWR is nearly back to its normal low reading.
Then another storm hits. And the problem repeats.
Do you have ideas of how to fix this? No, I’m not going
to make you wait until the next column to reveal the answer. We’ll disclose the
answer at the end of the column, so stay awake!
Curt Yengst handles
IT for Star 99.1, WAWZ, in Zarephath, N.J. Curt has found another way to put his
Brother P-Touch Labeler to good use.
When you are
plugging dozens of devices into power strips in the back of an equipment rack, label
your plugs and “wallwart” power supplies. This way you can tell at a glance
what each plug or supply is powering, thus eliminating guesswork. The extra
moments it takes to label things also will eliminate the worry of whether you might
take the station off the air if you unplug something. Labeling also ends tedious
fumbling in the rack while trying to trace power cords. And it helps too when
you should need to relocate the station (as Curt did recently; see radioworld.com,
keyword Irene). It’s no fun rummaging through a box of wallwarts trying to
figure out which goes to what piece of gear.
Figs. 1 and 2 demonstrate Curt’s efforts.
Curt also suggests including a few extra outlets —
or even a short power strip — for plugging in a soldering iron, trouble light or
the odd piece of test gear. Adding the extra outlets would prevent anyone from
needing to unplug anything in the first place.
Fig. 2: Label plugs at the outlet strip to speed troubleshooting and identification.
(Curt’s suggestion has helped him toward recertification.
Snap a few pictures of something you’ve done around your station to make your
life easier and email them to me at email@example.com.
Remember, published Workbench submissions qualify for SBE
Back to our VSWR problem.
The giveaway was that the problem recurred with a
subsequent storm. You may have guessed lightning; however, rarely does lightning
damage to an antenna repair itself.
A tower rigger was dispatched to the top of the
tower. When he got there, he saw no weep holes in the radomes — until he looked
at the top of the radome clamshell. Yes, the tower crew had installed the
radomes upside down.
He drilled a few holes in the bottom radome,
unleashing a torrent of water — and curing the problem.
I remember a similar problem at a higher-power
installation where the rigger could hear gurgling as he approached the bottom bay.
The water actually was boiling inside the bottom half of the radome.
In addition to the VSWR issue, the weight of water
inside a radome can generate unwanted “beam-tilt” and possible bend the bay or
interbay line. This is serious stuff.
Even if your radomes are installed properly, debris
from birds or insects can clog the weep holes. Make sure your rigger checks the
holes when inspecting your antenna. The lesson here is to supervise
installations at your site regardless of what they are. Know what your
contractor is doing and don’t be afraid to speak up.
Workbench. You’ll help your fellow engineers and qualify for SBE
recertification credit. Send Workbench tips to firstname.lastname@example.org. Fax to (603) 472-4944.
Bisset has spent 43 years in the broadcasting industry
and is still learning. He is SBE certified and is a past recipient of the SBE’s
Educator of the Year Award. He works for Elenos USA, an FM transmitter company
based in Miami.