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.

Over the 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 ranges.

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 other.

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 to ground.

Safety note: - never, ever, run any antenna wires near, or over, any hydro power lines, for your safety and the safety of others!

Resistor

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.

Transformer

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 receiver.

Conclusions

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
Winnipeg, Canada
email Blues56@my-Deja.com