This month I want
to report an unusual
problem that I
I did not write this up
until I learned from another
engineer that he could
not receive one of our local DTV signals
with a rooftop antenna. He asked me about
probable causes and I told him of my experience.
He reported that by replacing his
balun he now receives all local stations.
With a score of two out of two, perhaps
others may have this problem, too, and
have not even suspected the cause. Whoever
thinks about baluns?
All rooftop antennas were designed for
300-ohm twin lead. As everyone now uses
75 ohm coaxial cable for their downlead,
most outdoor antennas come with a balun,
which has an input impedance of 300
ohms (matching the output impedance of
the rooftop antenna). The balun transforms
from 300 ohms to 75 ohms by means of a
balanced-to-unbalanced transformer. It is
called a “balun to unbalanced” device.
This matches the 75 ohm impedance of
RG 59 and RG6-U coax cables. The insertion
loss of these baluns in less than 1 dB, but
mine measured over 7 dB. It was full of water!
I suggested that perhaps the engineer’s
problem might be a “water-logged” balun.
He has just informed me that his balun
was indeed defective and, upon replacing
it, he now receives all our local stations. He
sent the defective unit to me and I discovered
an open winding, which I suspect was
due to corrosion, again due to rain water.
When you examine one of these baluns it
appears to be water-tight, but with exposure
to both rain and sun over time, water may
get inside and ruin it.
I suspect water-logged baluns will be just
as lossy at high VHF frequencies as we have
found them to be for UHF channels. I also
expect that there are many old baluns out
there that should be replaced.
As shown in Fig. 1, when I replaced my
wet balun, I installed the new one in a PVC pipe. The 1-1/4-inch PVC pipe has a PVC
pipe cap cemented in place and the balun
is now out of the weather and out of the
My friend did the same thing, so neither
of us will ever have to do it again.
While on this subject of improving DTV
reception, a colleague and I have found that
in the daily checking of our RF test bed,
which feeds DTV signals to 26 DTV converter
boxes, one or more of them exhibits
abnormally low sensitivity.
|Fig. 1: Rain-proof enclosure of PVC pipe for the new balun mounted on the antenna mast. The white cable is RG6-U to the receiver. The dark flat ribbon is the 300-ohm twin lead from the antenna to the balun inside the new enclosure.
The problem is solved either by tightening
the F-connector or by loosening the Fconnector,
rotating the coax cable back and
forth a few times and then re-tightening the
F-connector. Just finger tight is not satisfactory,
we’ve found. We use a small wrench.
I would like to hear from my readers
about their own experiences with F-connectors
and baluns. Many folks out there have
a marginal signal on one or more channels.
We’ve seen a change in sensitivity of up to 3
dB when we attack the F-connectors as outlined
In our testing regimen, we start with the
desired signal below threshold and increment
the D power in small steps to find the
threshold power of each unit every time
we run experiments. That is how we detect
these F-connector problems.
I suspect many of your viewers are experiencing
intermittent reception due either
to wet baluns or poor connections between
the coax and the receiver, or both. I’d like to
pass your experiences along to other readers.
Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via email