Veteran AM engineers have known since the beginning: An antenna needs to
be a continuous electrical conductor. Intermittent connections cause real
problems, especially with solid-state transmitters that need to see something
close to 50 ohms with zero reactance lest they go into VSWR protection mode.
One bad connection can
throw the impedance off a little or a lot depending on where the errant
connection is. This problem is not just on series-fed towers. A unipole is a
“feed system” for an AM tower, and relies on the tower being a continuous
Now is the right time to
pay careful attention.
A tower is the “radiating
element or antenna” for an AM signal, not a support structure as in FM. Towers typically
are constructed of stacked 20-foot sections. With tons of downward pull on guy
lines, you might think sections would be sufficiently connected to be thought
of as a single electrical conductor. Not the case. Towers stand in rain and
perhaps salt air. Crippling corrosion happens where each section joins the next.
Typical weld on a solid-leg tower.
I have seen bonding straps
that go around joints to remedy the problem. Others have run a heavy copper
wire from the top to bottom of a tower. Those work, but a better approach is to
weld the sections one to another. Usually this is done by a tower worker with
an arc welder.
Do you have all three tower
legs welded one to another? Only one good leg connection is required. I usually
have the tower climber do two legs; it is convenient to do the two closest ones
when he is belted in at each level of the tower. That second leg is insurance.
Welds do not need to be
deep, they just need to connect sections electrically and reliably. Standard
procedure is for the worker to spray cold galvanizing over the joint to protect
it against rust.
Does welding make the tower
incapable of being reused later? Not usually. Welds can be ground off before
sections are separated.
Does welding last forever? No,
tower vibration can crack the welds over time. Don’t be surprised if you must
weld again after 20 or 30 years.
FM stations are not immune
to intermittent tower connections. With lots of RF radiating into the tower
from an FM antenna, corroded connections can cause RF noise that plays havoc
with STL systems, especially digital ones. Tower section to section connections
are not the only players. Any intermittent cable grounding/bonding connection
on the tower can cause the same symptom.
I had an interesting
experience in which an AM station would not pass the annual NRSC occupied
bandwidth and RF harmonic measurement tests. The sticking point was RF
harmonics. Try as I may, I could not tune the transmitter or do anything to fix
a second and third RF harmonic radiation problem. It simply would not meet the
FCC specification of 73 dB below carrier for 1,000 watts of transmitter power. It
is 80 dB for 5,000 watts or more. Section-to-section welding was the answer. After
that, the station met specs just fine.
Caution: Best disconnect
the AM antenna coupling unit from the tower before welding. The high voltage
could damage or destroy a component in the coupling network or transmitter. You’re
going to be off the air anyway because of RF radiation danger to the tower
There is a story about a radio
station in a foreign land. The station programming irritated the government; so
the manager, program director and engineer all were sentenced to death by
guillotine. On the appointed day, the manager was placed in the machine and the
cord was pulled. The blade came down but stopped one inch from his neck — “divine
intervention,” according to witnesses. The manager was set free.
They tried to execute the
PD but the same thing happened.
Finally the engineer was
put in the machine. Just before they pulled the cord, he exclaimed, “Wait, I
see the problem!”
Persons, WØMH, is certified by
the Society of Broadcast Engineers as a Professional Broadcast Engineer and has
more than 30 years of experience. He has written numerous articles for industry
publications over the years. His website is www.mwpersons.com.