-- the "baby" beverage antenna for everybody --
by Cyril Dufault, VE4QRP
The antenna consists of wire, a transformer T, and a resistor
R. Length and height should be as large as possible. The transformer
is a simple impedance transformer, while the resistor is a standard
carbon resistor of 1/2 watt power. The best location is outdoor,
but in a tight situation run it along the walls and ceiling
of any room.
last few years, I have been using a variation on the theme
of terminated transmission line antennas, for reception of a range
of shortwave frequencies. The antenna is shaped like a rectangular
loop antenna, is broadband and provides better reception than
the built-in whip antenna on my Sony 2010. The antenna described
here is versatile, can be used in a variety of locations, and
is simple to construct. Perhaps this antenna will help improve
shortwave reception for you, especially in the lower frequency
Principle of Operation
The antenna can be viewed as a lossy transmission line, lossy
in the sense that signals will not be contained between the wires,
but will radiate (or signals be received) to a certain extent.
With a termination resistor, the response of the transmission
line is flattened out compared to an open or short circuit. It
is the flattening of the response away from resonance that helps
to broadband the antenna. With a flat response however, we do
not get the gain or efficiency of a resonant antenna, but we hopefully
get an improvement in bandwidth.
This type of antenna is not new, and I have seen it used for low
frequency work in some industrial situations. To some, it may
look like a "baby" beverage antenna, especially a terminated beverage.
The resistor attempts to provide a matched load to the line, and
the transformer provides impedance matching to the coax for the
receiver. The similarity ends there, however, because this antenna
is much more square than long and rectangular, and its size is
very much less than several wavelengths.
Since the main use of the antenna is to try and improve reception
over a simple whip or random wire, its dimensions are not specified,
and neither is the attempt to match it exactly as a transmission
line since its length is too short. Length and height can be anything
convenient, but should be as large as possible, for good low frequency
response. I once setup a temporary one in the basement of my house,
along a wall of about 16 by 6 feet. It provided some improvement
in signal (about 3 red LEDS on the Sony 2010) compared to the
whip, at 7 Mhz (40 meter band). Considering it was almost underground,
and surrounded by heating ducts, it performed reasonably well.
The best location for this type of antenna would be in the attic
of a house, or outside in the backyard. In a tight situation,
it could be run along the walls and ceiling of a bedroom, near
the listening station. Don't hesitate to make a bend in the antenna,
should that be required to make it as long as possible. Just make
the upper and lower wires of the antenna follow one above the
In regards to directivity of the antenna, it should work best
for signals arriving in the direction from the resistor to the
transformer, so if you have the available space you may want to
consider this. Of course, if there is a major bend in the antenna,
then some omnidirectional coverage should be possible.
The range of frequencies in which the antenna seems to perform
well is related to the biggest size you can manage. On the west
coast I had an outdoor one of 130 feet for the upper wire, from
transformer to resistor (it had a major 70 degree bend due to
location of the highest tree in the yard). The ground return wire
was run along the base of the fence. Reception was very well with
it, down to 1.8 Mhz (160 meter band). It didn't work as well on
the AM broadcast band, however, since the built-in Sony ferrite
rod had much better efficiency. The practical upper frequency
limit was around 14 Mhz (20 meter band), and is due mainly to
the impedance transformer.
Description and Construction
The antenna consists of wire, a transformer T, and a resistor
R, as shown in the figure. The antenna wire itself can be anything
convenient, such as #14 AWG stranded copper, which is easy to
work with and will hold up well in outdoor weather. For an outside
installation, I have put ground rods connected to the ground return
wire (the lower wire of the antenna). Two ground rods were used,
one at the resistor, the other at the transformer. The ground
rods are for lightning protection. A lightning arrestor was also
connected on the upper wire (in parallel with the resistor) and
connected to the ground rods. Note that in this arrangement, the
resistor and transformer will bleed off any static electricity
Safety note: - never, ever, run any antenna wires
near, or over, any hydro power lines, for your safety and the
safety of others!
The load (shown as R in the figure) is a standard carbon resistor
of 1/2 watt power (do not use a wire wound resistor because it
will act more like an inductor rather than a resistor at radio
frequencies). Connect one end of the resistor to the upper antenna
wire, the other end to the ground return wire. See the following
transformer notes for the value of resistance.
The transformer (shown as T in the figure) is made from a ferrite
rod from an old discarded BC band radio. Any size of rod will
do. The coax for the receiver is connected to the primary side
of the transformer. The secondary of the transformer is connected
to the antenna wires, which are connected at the far end to resistor.
The transformer is a simple impedance transformer, with the secondary
wound first, and the primary wound directly over top.
The impedance ratio of the transformer depends on the load resistor
to be used. For a resistor of 1200 ohms, a transformer turns ratio
of 5 to 1 is used, to provide a match to 50 ohm coax cable. A
25 turns secondary and 5 turns primary will provide a 5 to 1 turns
ratio, for an impedance transformation of 1250 ohms to 50 ohms.
Of course, you can experiment with these values for the impedance
ratio and the resistance to use. The range of values for the load
resistor should be from 300 to 1200 ohms. If you change the resistor
value, then change the transformer turns ratio also. It should
also be mentioned the resistor value will always be an approximation
to a flat response, so that no one value will be best for all
the SWL bands you may listen to.
Number 20 AWG solid insulated copper wire is used to make the
transformer. Wind the secondary first, directly over the ferrite
rod, in the center of the rod. Ensure the windings are wound close
together. You may cover the windings with electrical tape, or
masking tape, to hold them in position, but this should not be
necessary if you are using solid copper wire.
Wind the primary directly over the secondary winding, but offset
from the center of the rod, toward one end, in the same winding
sense. To clarify - "winding sense" is the clockwise (or counterclockwise
as the case may be) turns of wire on the rod. Looking down one
end of the rod, if the secondary is wound clockwise away from
you, then the primary must be wound in the same clockwise direction.
At this point, there will be 4 wires to be connected. There should
be 2 wires, one from each coil, close to each other at one end
of the rod (since the primary was offset from the secondary winding,
one end of the primary will be close to one end of the secondary).
These can be called the "ground" connection wires of the coils.
The primary coil "ground wire" is connected to the shield of the
coax. The remaining primary wire is connected to the inner conductor
of the coax cable. The secondary coil "ground wire" is connected
to the antenna's lower ground wire. The other secondary wire is
connected to the upper antenna wire. Solder all connections and
cover with electrical tape. (Note that you may wish to install
connectors to the transformer instead of doing the hard wiring
that was just described.)
If the antenna is installed in the attic, then in order to prevent
the coax shield from becoming part of the antenna, wind about
10 turns of the coax into a loop of about 6 inches diameter (about
15 cm). At the receiver end of the coax, the coax should be grounded
to a lightning rod. Also install a connector appropriate for the
In the shortwave listener world, a good antenna is difficult to
find. With the above experimental type of antenna, an improvement
of low frequency reception may be possible. Although efficiency
suffers, the improvement over a short random wire, or built-in
whip antenna should be evident.
As with most antennas, installation location can have a very dramatic
effect on performance. Try and keep the antenna away from metal
structures, house wiring, and put it up as high as possible. Drop
me an e-mail if you take the time to built this antenna, and if
you find it useful!
Cyril Dufault, VE4QRP