In one of my columns
several years ago, I
reported on wet baluns
and loose F connectors
as causes of poor
DTV reception. I was
pleased and amazed
at how many readers
wrote me on these topics.
One reader kindly
sent me a copy of the SCTE Standard 160-2010 concerning F connectors while another
wrote that there is a torque wrench
for F connectors. Now that I know how
much torque to apply to F connectors, I’ve
ordered the wrench from tecratools.com
My colleague Linley Gumm, recently
encountered another new reception problem
in receiving a Ch. 10 signal. His
trouble started suddenly one evening
when he discovered interference across
the high VHF band that he had never had
before. Using a spectrum analyzer, he was
able to discern that the interference was
so strong that Ch. 10 was lost, although
he lives 10 miles from the tower
and has a rooftop antenna.
|Fig. 1: Spectrum of Chs. 8, 10, 12 off-air with FMI, second
harmonics of FM radio signals generated in the antenna system,
by passive intermodulation.
Fig. 1 shows the spectrum with interference.
These spikes are the second harmonics
of local FM stations. He measured
the received power of the FM radio signals
from his rooftop antenna to find that the
total power (88–108 MHz) was about –5
dBm. But these signals had never generated
second harmonics jamming Ch.
So he investigated further. His antenna
feeds a four-way signal splitter in his attic
so he re-tightened the F connectors of this
signal splitter. Voila, next evening, the interference
was gone, (see Fig. 2).
He realized that there must have been a
poor connection which he had disturbed
up in the attic. So how can a poor connection
produce second harmonics?
This is known as “passive intermodulation
interference,”or PIM. Some of you may
remember copper oxide rectifiers, which
were used by early radio experimenters
as detectors back in the
days of crystal sets. Others
will recall “foxhole
radios” discovered by
U.S. soldiers who used
rusty razor blades as the
detector of a crude “foxhole” crystal radio in
Italy. PIM is a significant
source of interference
in base stations where
there are multiple transmitters
and a number of
very sensitive receivers
clustered atop a tower.
|Fig. 2: The same spectrum after the FMI was
eliminated by lightening all F connectors in the
receiving antenna signal distribution system.
Well, now we have
a documented case
of PIM with FM radio
signals being rectified
and actually jamming a
high-band VHF station’s
reception. I’d like to
hear from readers who
have experienced PIM.
THE LED BULB
Another reader experienced
form of DTV interference
when he reported
suddenly losing reception
on all the DTV receivers
in his home. Prior to the reception
failure, he had replaced an incandescent
40-Watt bulb with a new and shiny 8-Watt
LED bulb. When he switched that light
off, reception returned. His experience intrigued
me enough to buy the same brand
of 8-Watt LED as his, and I got the same
result. I too have a spectrum analyzer, so
Linley and I looked for noise near this LED
bulb and we found it mostly between 110
MHz and 250 MHz.
I borrowed three brands of LED bulbs
of varying power from a local electrical
supply dealer and am happy to report
that these don’t generate noise—but the
8.5-Watt bulbs both my reader and I had
obtained from the same manufacturer,
sure do. Those noisy bulbs are claimed to
comply with FCC Rules for Class B digital
With most of this noise between 110
MHz and 250 MHz, both aviation radio
and DTV on Chs. 7–13 may be affected.
Today, there are probably only a
few such bulbs in use, but as the cost of
these extremely efficient, long-life bulbs
decreases, they may become commonplace.
I can imagine that radios on aircraft
in flight might pick up such noise
from perhaps a million LED bulbs as it
flies over a major city. Obviously the jamming
of these radio receivers would be
of short duration, but that will only make
tracking the problem more difficult. Remember
aircraft radio use AM, not FM, so
they are more sensitive to jamming, but
not all LED bulbs generate noise so this
problem can be solved. Perhaps the FAA
might look into this.
Meanwhile, if you have experienced
an interesting DTV reception problem,
I’d like to hear from you.
Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via
e-mail at email@example.com.