12 July 2016
combination TV-FM antenna. Tune your receiver to an
unused channel and set the receiver to “Mono” which
usually enables audio output even if no signal is present.
If you can aim your antenna, point it at a city or
region 300 miles or so away with an FM station on that
channel — well past the radio horizon. Then, listen on a
day when the atmosphere is quiet and stable.
If tropo occurs, you might hear the station fade in
(perhaps surprisingly strong) for a few minutes or longer.
You’ve just become a tropo DXer! (“DX” stands for
Another way to predict when tropo might happen is
to use Hepburn maps. Created by William Hepburn from
weather data, these maps ( www.dxinfocentre.com/
tropo.html) show when and where atmospheric
conditions are right for tropo. You can also keep an eye
on the real time ham radio contact maps at www.dxmaps.
com/spots/ map.php to see if any activity is being
Spring and summer are the best seasons for tropo,
although it can occur anytime there is favorable weather.
The “real DX” comes
from propagation using an
entirely different — and
much higher — region of the
atmosphere. Starting at
about 30 miles above the
ground, the ionosphere can
bend radio signals back
towards the ground where
they are received thousands
of miles away. The
ionosphere is almost a
vacuum; in fact, the
The key is in the name. “Ion” refers to the effect of
solar ultra-violet (UV) radiation on the sparse collection of
gas molecules in the upper atmosphere. The UV is
ionizing radiation energetic enough to free an electron
from an atom, creating two free electrically-charged
particles: a negative electron and a positive atom. If
ARRL Antenna Book — An extensive collection of antenna
designs with a detailed chapter on propagation from MF through
ARRL RF Propagation — Links to articles and resources
dealing with propagation: www.arrl.org/propagation-of-rf-signals.
Don’t forget the ARRL Tech Portal: www.arrl.org/tech-portal.
Spaceweather — Daily news and articles about phenomena in
space, on the Sun, and here on Earth: www.spaceweather.com.
Spaceweather Prediction Center — Real time information on
shortwave propagation conditions, including solar and geomagnetic
But Why A Duct?
One variation of tropospheric propagation is referred to as
“ducting.” This occurs when more than one reflecting (remember,
it’s really refraction) layer is present. A signal can be trapped
between the layers and bounce between them until one layer
disappears and lets the signal out of the duct.
Figure 3 gives a typical example of how ducts are created by
a storm front. Whenever a strong front moves through your area,
Hams know to aim their antennas parallel to the front as it
moves through, hoping for a duct to form and carry their signals
far from home.
Does Propagation Really Matter?
To most short-range links like Bluetooth and Wi-Fi, line-of-sight and tropo don’t have a significant effect. Once you start
considering longer range point-to-point links or receivers on the
tops of buildings or hills, you should be aware of the potential for
tropo to disrupt communication. Hams may enjoy “working DX”
over long distances but it you’re just trying to shovel bits, a signal
from far away is not something you want to receive.
Interfering signals or distortion caused by multi-path from
ducts or inversion layer propagation can shut down a comm link
or cause loss of contact with a mobile or airborne platform.
Be aware of the possibilities and have a “plan B” in mind —
either on a different frequency band or by aiming your antenna in
a different direction.
FIGURE 4. Using portable dish antennas, these hams are attempting to launch 10 GHz signals
along the inversion layer that can be clearly seen at the bottom of clouds over the Mt.
Washington, NH region.