Like mushrooms after a heavy rain, unwanted reradiators
sometimes appear almost overnight in the near field of AM antennas.
Legally and technically speaking, this should not happen. All
construction permit applicants are required to identify AM antennas
located within a certain distance from the proposed tower, and a
condition that requires detuning is made a part of the subsequent
construction permit. Unfortunately, this is not always done, and
field strength measurements are made only after the construction.
Verifying RF field distortion is not possible without
Although cooperation and pre-construction measurements are the
responsibility of the new tower owner, it is frequently a source of
acrimony and litigation when the proper measurements are not
performed prior to construction. For this reason, it behooves chief
engineers to maintain a close watch on FCC releases and new local
construction so that ignorant or uncaring operators can be
contacted in time to ensure that proper procedures are followed,
and the owner of the disturbing tower pays his due part of the
Review on reradiation
Parasitic reradiation was discussed in the January 2001 RF
Engineering column on parasitics, and it was noted that the
longer the reradiator, the greater the reradiation. Visual
inspection will often pinpoint the possible culprit by its location
with reference to the antenna system. If in the major lobe,
reradiators located farther than one mile away can sometimes cause
directional pattern problems such as out-of-tolerance monitor
points. When in doubt, the field strength meter is invaluable in
identifying such sources. In my opinion, when problems of
unexpected RF levels emerge, it is best to suspect any and all
protrusions above the ground until the culprit is found.
Quite often, an out-of-tolerance monitor point is the first
intimation that an engineer has of the presence of such an
offending reradiator. To identify the offending reradiator, orient
the field intensity meter (FIM) at a right angle to the transmitter
while facing the suspected reradiator. Look for minimum direct line
station pickup and maximum parasitic reradiated signal. Approach
the object, and watch the meter reading. Place the vertical edge of
the lid adjacent to it. A large increase in signal strength is a
good indication of parasitic reradiation.
The method of detuning a reradiator depends on the electrical
length of the reradiator and its location. Let's examine coaxial
line theory and see how it is applied to tower detuning.
Although the electrical effects of coaxial lines were well known
prior to WWII, it took radar's development to illuminate and
exhibit the rather unusual properties of quarter-wavelength coaxial
If one end of a quarter-wavelength coaxial transmission line is
shorted, the other end will present an open circuit. Consider
Figure 1a. Expand this idea into a quarter-wavelength antenna that
has a skirt of wires hanging down from the top, as in Figure 1b.
This really forms a section of coaxial cable and will resemble a
folded unipole AM antenna. But instead of driving this device
through the bottom of the skirt, a tuning circuit to ground
provides fine adjustment to tune the array for maximum impedance at
the bottom of the skirt.
This high impedance at the operating frequency places an RF
electrical open circuit at the base of the tower, thus floating it
at the operating frequency. Operating as an ungrounded tower, the
parasite will not develop any significant voltage and will not
reradiate the station's signal to any great extent. However, it
must be remembered that at frequencies other than the desired one,
the tower will reradiate as before; although, any off-frequency
radiation will not affect the fundamental signal.
One theoretical point to consider: assume a badly tuned
directional array with a very large 10kHz sideband impedance rise.
This sideband frequency may be reradiated by a tower detuned for
the fundamental frequency. Audio distortion can result as the
off-fundamental frequency impedance increases with a very steep
It is customary to use three or more dropwires to form the
skirt. These are suspended from cross arms placed at the top of the
tower. Stand-off insulators are placed as needed down the tower to
prevent shorting to it in high winds. At the lower ends, a loop of
wire connects them together and is insulated from the tower and
securely anchored to the ground. It is unusual for a tower to be
perfectly detuned in this manner, and a parallel tuned circuit is
connected to ground from the loop and is adjusted to produce very
high capacitive or inductive reactance. The system is thus
antiresonated, and reradiation is reduced to a minimum.
Although three dropwires are usually needed, it is sometimes
possible to detune a parisitic radiator with a single dropwire. I
recall a problem about thirty years ago where a police tower went
up, unknown to the station, some distance away in the major lobe of
a four-tower array on 1280kHz. Two monitor points were put out of
limits, and eventually the new tower was suspected and
We dropped a single wire from the top of the tower and insulated
it. We then tuned this wire for maximum current with the method
described above. The two monitor points came back in. Although it
worked that time, it is usually best to plan on using a minimum of
three wires to be certain of adequate suppression.
Guy wires must always be handled with care. Usually, the
standard breaking up of guy sections to suitable electrical lengths
will be sufficient. However, sometimes it becomes necessary to use
Phillystran or similar non-metallic lines for guys. Such
circumstances are rare in normal installations, but it is a good
idea to be aware of all possibilities when erecting antennas or
Sometimes, on large self-supporting towers, individual wires
placed down each side corner leg, insulated and spaced away from
the tower leg down to the splayed-base supports, will work well.
Although, in the case of tall towers (much more than a
half-wavelength), it is usually best to build the tower in sections
insulated from each other.
A tall multi-section tower would have tuning networks inserted
between several insulated sections to isolate them electrically and
make the tower non-reradiating.
Most engineers are familiar with the use of an RF choke in the
ground line of a wooden electric supply pole to detune the vertical
ground wire. We can compare this to the use and effect of a
quarter-wave line. In each case, the detuning component effectively
opens the RF circuit to ground and floats the tower. It is
important to remember that ground wire continuity on wooden power
line poles must never be broken.
The quarter-wavelength choke effect is not limited to the base
end of a tower. Many times it is practical to use a quarter-wave
section down from the top of a tower to detune a section, as shown
in Figure 2.
As a matter of interest, ungrounded half-wave (quarter-wave
plus) towers can be detuned by constructing a quarter-wave section
that is grounded to the tower at the bottom. The top end of the
wire skirt is left open. This effectively produces a high impedance
up the tower at the open skirt and suppresses reradiation.