Practical_Antenna_Handbook_0071639586

520 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
Shortwave and AM BCB “Skip” RDF
Radio direction finding is most accurate over relatively short distances. If you have the
choice, it’s better to use the ground wave (which is what you use during daylight hours
for nearly all AM BCB stations). Skip rolls in on the AM BCB after local sundown, so
you can hear all manner of stations up and down the dial. However, with a little practice,
you can even RDF distant stations.
Unfortunately, there are some practical difficulties associated with skip RDFing.
When we look at propagation drawings of skip in textbooks we usually see what’s happening
in a single plane at a time. Typically, a single imagined “pencil beam” radio
signal is shown traveling at some angle up the ionosphere, where it is “reflected” (actually,
it’s a refraction phenomenon but looks like reflection to an observer on the surface)
back to earth. We can tell from the drawing that the angle of incidence equals the angle
of reflection, just like they told us in high school science classes. Unfortunately, the real
world is not so neat and crisp.
In the real world the skywave signal we hear is an amalgam of many such “pencil
beams”, coming in from a near-infinite number of directions and elevation angles. The
strongest component of such a signal aggregate is always the path that has the least
total loss from start to end, but for skip distances that path is always shifting because
the ionosphere that makes skip reception possible is a continuously shifting medium. A
radio wave refracting through the ionosphere must feel like you and I do when taking
a ride through the house of warped mirrors at an amusement park. It’s entirely possible,
therefore, that at any given instant the preferred path to your antenna for a signal
source located to the northwest of you might be a so-called skew path, coming in from
as far around the compass rose as the southwest!
Terrestrial reflections also cause problems, especially when RDFing a station in the
high end of the HF band or the VHF/UHF bands. Radio waves will reflect from geological
features such as mountains and from man-made structures (e.g., buildings). If
the reflection is strong enough, it might appear to be the real signal and cause a severe
error in RDFing.
Bottom line: Be wary of RDF results on HF when the “skip is in”. Similarly, don’t
let the canyons of New York City or the mountains of upstate fool you on VHF and
UHF.
Adcock Antennas
The Adcock antenna (Fig. 23.9) has been around since 1919, when it was patented by
British Royal Engineer and Lieutenant Frank Adcock as a way of overcoming the tendency
of RDF loops to lose directional accuracy at nighttime. Adcock surmised that the
horizontally polarized component of incoming skywave signals induced loop output
voltages indicating certain spurious directions when the output voltage due to the vertically
polarized ground wave was near zero. To minimize this error, Adcock designed
a number of different configurations based on the use of vertical elements, both monopole
and dipole.
The Adcock antenna shown here once found use in the United States by the FCC
at their old monitoring sites. It consists of two center-fed, nonresonant (but identical)
vertical radiators. Each side of each element is at least 0.1l long, but need not be reso-