by Archive-name: radio/monitoring/am-fm-dxing
[Last revised: Oct 3 1993]
AM/FM DXing
By Scott Fybush and Earl Higgins
One of the easiest parts of the radio spectrum to explore are the
broadcast bands. This posting will attempt to offer some hints to make
your exploration of the mediumwave and VHF-FM bands more enjoyable.
I. WHAT ARE THE AM/FM BROADCAST BANDS?
The mediumwave (commonly referred to as AM) broadcast band currently
extends from 525 to 1605 kilohertz. Channels are spaced in even 10 kHz
increments; i.e.: 530, 540, 550, ... , 1600 kHz in the United States and
Canada. Elsewhere, channels are spaced in 9 kHz increments, i.e.: 531,
540, 549, etc.
In the United States, the band is being expanded to 1700 kHz. Stations
which are currently experiencing high levels of interference will begin
appearing on the 1610-1700 kHz frequencies sometime in late 1993 or
1994. They will simulcast the new frequencies with the old for a period
of a few years, eventually dropping the `old' frequency. This opening up
of new channels presents some once-in a lifetime opportunities for the
alert mediumwave DXer.
The VHF-FM broadcast band in the United States extends from 88 to 108
megahertz. Channels are assigned at 200 kHz increments; i.e.: 88.1,
88.3, 88.5, ... , 107.9. The channels from 88.1 to 91.9 are reserved
for noncommercial educational stations. Outside the United States and
Canada, the boundaries and channel spacing vary. In Japan, the band
starts at 76 MHz. In Western Europe, the band generally runs from
88-108 MHz, but channels can be irregularly spaced, i.e.: 101.25 MHz.
II. SIGNAL PROPAGATION
The distant stations you are able to receive will depend largely upon
signal propagation. This varies depending upon the time of day, the
season, and other factors. For mediumwave, the single most important
factor for good DX is the time of day. Mediumwave signals almost always
get absorbed by the D Layer of the ionosphere during daylight hours. As
a result, all mediumwave signals received during midday hours will
arrive by ground wave, making reception of signals over a few hundred
km/miles away unusual in daylight. At night, however, the ionosphere
reflects mediumwave signals, making it possible for signals to be heard
at much greater distances, up to a few thousand km/miles, via `skywave'.
To a lesser extent, the period up to two hours after local sunrise, and
two hours before local sunset, called "Critical Hours", have varying
levels of skywave, and also can provide some very unusual reception
opportunities for the mediumwave DXer. Reception also tends to be better
in winter than in summer, due to lower levels of atmospheric noise and
longer hours of darkness. In the United States, due to the large number
of stations, many smaller mediumwave stations are required to sign off
or reduce power sharply at sunset so as to reduce interference to
distant stations.
Whereas the mediumwave band can be counted on to provide distant
reception with much dependability, this is not the case at all on the
VHF-FM band. Under `normal' conditions, VHF-FM signals generally carry
no more than 150-250 km (100-150 miles), or `line of sight', since the
ionosphere generally does not reflect VHF-FM signals. VHF-FM
transmitting antennas are thus usually located as high as possible. Tall
towers, high buildings, and mountaintops are common VHF-FM transmitter
sites.
However, under certain rare conditions, the atmosphere will even reflect
VHF-FM signals, thus making it possible to receive these stations at
quite long distances. There are two major forms this distant reception
can take; the most common is Tropospheric Ducting, or tropo for short.
Typically, this occurs when a warm air mass forms on top of a cooler
mass closer to the ground. The area between these masses acts like a
pipe, `bending' the signals back to the earth well beyond the horizon.
This reception is most common in local late spring and summer months, in
the post-sunrise hours. It will enable the alert VHF-FM DXer to log
stations up to 800 km (500 miles) away in optimum conditions.
The other relatively widespread form of VHF-FM DX is called Sporadic E,
or E-skip, because it is the E Layer of the ionosphere which reflects
the signals. Like the name implies, this form of propogation is very
sporadic, yet very intense. When it's in, it is VERY strong. Stations
from a relatively limited geographic area 1300-2000 km (800 to 1200
miles) away will suddenly boom in, strong, often in stereo but quite
fadey, even overpowering semilocals in many cases. It will start at the
bottom of the VHF-FM Band (actually TV channels 2-6 first) and work its
way up in frequency. The highest frequency at which signals are
reflected by the ionosphere is called the Maximum Usable Freqiuency
(MUF), just as it is in shortwave, and it can occassionally surpass the
top of the VHF-FM dial in an unusually good opening.
III. RECEIVERS
Almost any radio is capable of some broadcast-band DXing, especially
long-distance mediumwave reception. However, most recent radios, even
those designed for quality shortwave reception, do not have outstanding
broadcast band reception. One exception is the General Electric
Superadio III (Model 7-2887.) The SR III is designed for optimum AM/FM
broadcast performance, incorporating:
* RF amplifiers on both bands
* Ceramic filters and Automatic Frequency Control on FM
* No PLLs or digital displays for less electronic noise
* A 2-way speaker system with 1 watt of audio power
The SR III is a bulky (4" x 10" x 12") portable radio which can be run
off 120V AC or 6 "D" batteries, providing over 400 hours of battery
life. This radio has become popular among the DX community for its
exceptional performance.
It costs between thirty and sixty dollars in the US, and may be found at
many discount outlets. It can be obtained from Bennett Brothers (Order
#R3116) at 1-800-621-2626 or 1-800-631-3838, or from Best Products
(Order # 140457) at 1-800-950-2398.
If you don't have a Superadio, some important things to seek out in a
receiver are:
* External antenna connections. These make it easier to use a better
antenna than the one supplied with the radio.
* High selectivity. This refers to the receiver's ability to reject
strong signals on adjacent frequencies, and is more important to good
reception than is sensitivity, since a good antenna will provide
more-than-adequate signal strengths.
* Digital frequency display. While the circuitry involved can add to
the level of internal electronic noise in the radio, digital display
makes it possible to more easily determine what station is being heard.
IV. ANTENNAS
For mediumwave reception, most receivers have a short internal ferrite
rod. This will provide acceptable signals for high-powered distant
stations. Ferrite rods are quite directional, and the radio can thus be
turned to null out strong interfering signals, or to improve reception
of the desired signal.
For more advanced DXing, external antennas offer certain advantages. The
most common external antenna is a simple random wire, 15m (50 feet) or
more run out the window and then as high as possible (up in a tree, for
example). The wire can be connected to the external antenna terminal.
If none exists, you can open up the radio and wrap the wire a few turns
around the ferrite rod inside. It is also possible, although less
desirable, to simply wrap the wire around the entire radio. If the
radio has a terminal marked "ground" or "GND," another wire can be run
from this terminal to a copper rod driven a meter/a few feet into the
earth.
One problem with a random wire antenna for mediumwave work is it's
inability to reject strong local signals. Most receivers today lack the
dynamic range to effectively deal with the extremely strong signals from
a local mediumwave broadcaster as picked up by a random wire antenna.
Thus, some sort of tuned antenna is best for all but the most isolated,
rural locations.
The most popular antenna for mediumwave DX today is called a `loop'
antenna, and can be either of two types: ferrite rod or air-core wound
wire loop. These antennas are small, 25-100 cm (1-3 feet) in diameter,
and sit on the DXers desk or shack table where they can be easily turned
by hand for optimum peak or null of a signal. Each design works with a
tuned circuit before feeding the signal into your receiver, and usually
this circuit includes a small powered amplifier. Generally speaking, the
longer the ferrite rod, or the larger the diameter of the aircore loop,
(to a point), the sharper the null of the antenna. 45 to 55 cm (18 to
22 inches) would be optimum for a ferrite rod antenna.
Air-core loops need to be made by hand, as there are none on the market.
Ferrite loops, however, are available commercially from at least two
manufacturers; Palomar Engineers and Radio West. Unfortunately, these
two antennas do not have very long ferrite elements; and reviews of
their performance in the mediumwave press tends to be mixed. Ideally,
one would build their own antenna, or try to find either a used, older
Radio West loop or Space Magnet antenna, both pre-1980. Plans for
building all sorts of mediumwave loop antennas are available through
National Radio Club publications. The address is found later in this
FAQ.
A more advanced antenna is the "beverage" antenna. This is a length of
wire 300 m (1000 feet) or more, with extremely high gain and narrow-beam
directional characteristics. It is usually, but not always, terminated
at the far end with a 450 ohm resistor connected to a metal stake driven
into the ground. It should be pointed in the direction of the desired
station. The beverage antenna can, under good conditions, be used for
transatlantic and transpacific DX.
For VHF-FM, the important factor is height. The higher one can place an
antenna, the better reception will be. A multielement Yagi antenna,
which can be found in Radio Shack
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