Passive Antennas

* The dipole
* The random wire
* Other passive antennas

For the shortwave listener, passive antennas -- no electronics -- typically are thought of as wire antennas. These include but are not limited to the dipole, a random wire, loop, vertical or Beverage.

 

The dipole

Experimentation with antennas has existed since radio began. One of the basic antenna forms is a dipole, whose length is half that of the wavelength it is to receive. The center of the dipole is cut so that a coaxial cable can be connected. At this length the antenna impedance best matches that of the connecting cable, namely 50 ohms, and there is a maximum transfer of energy.

The dipole may be mounted horizontally, as an inverted-vee (with the center higher then the ends), or as a sloper (one end is mounted higher than the other).

Dipole

A dipole performs best on the wavelength for which it was cut. On other frequencies, reception will also be possible, but the impedance is then much higher than 50 ohms, e.g. 1000 ohms, or much lower, e.g. 5 ohms. This leads to much poorer energy transfer.

Three solutions to the energy transfer issue may be found in the G5RV antenna (so named for R.L. Varney, an amateur radio operator in Sussex, England), in the fan dipole and in the T2FD antenna. The G5RV antenna is a variant of the dipole and uses a specific length of 300- or 450-ohm twinlead and 50-ohm coaxial cable as the feedline to achieve a reasonable impedance match across the 3 to 30 MHz spectrum. The fan dipole is simply multiple ?-wavelength elements, one pair for each frequency range desired , connected to a common feedline; the wire elements are spread out in opposing directions as a woman's fan. The T2FD (Terminated Tilted Folded Dipole) antenna is a sloped two-wire antenna that achieves a wide 1:5 frequency coverage; the formula to calculate its length in meters at the lowest design frequency is 100/MHz. A 20-meter long T2FD designed for 5 MHz will provide coverage to 25 MHz. A 10:1 balun is required to match the antenna to the coaxial feed line.

There are many solutions to help reduce this problem of mismatch. A longwire, 5 times the length of the desired wavelength, has a more constant but higher impedance. Some communication receivers even have a 600 ohm antenna connection to suit this type of antenna. But because coaxial cable cannot be used as the antenna lead-in wire, such a longwire antenna is more susceptible to local interference sources (fluorescent lights, thermostats, etc.).

Antenna tuners are often advertised as a way to match the antenna to the receiver. But they work best when they're mounted as close as possible to, or even in the antenna. There is little point putting one at the end of a 50 ohm coaxial lead-in wire as the mismatch has already taken place.

 

The random wire

Also known as a longwire (a misnomer) or an end-fed wire, a random wire is simply a length of insulated or uninsulated wire with one end connected to the receiver. To be sure, the antenna will pick up signals. If the antenna is brought into the near field noise environment of the house, the unshielded wire will pick up these noises. The impedance of the antenna approaches a nominal 50 ohms on those frequencies where the antenna length is approximately at the odd multiples of one-quarter wavelength. See the following table for measurements.

The wire may be installed in the form of an inverted-L or bent as necessary to fit it into your property. The end-fed wire can be an effective antenna so long as the appropriate impedance matching network is applied to the juncture of the wire and the coaxial feedline.

 

Other passive antennas

The loop antenna can take the shape of a square or a triangle. For best long distance reception and sensitivity to the low-angle signals, these antennas should be mounted vertically and broadside to the area of the world one wants to listen to.

Loop


The loop antenna is a full wavelength long. As the loop antenna exhibits a radiation resistance of approximately 100 ohms, a balun or matching section of cable is required. While more difficult to put up, the loop does exhibit gain over a dipole.

The vertical antenna has the advantage of taking requiring relatively little space. The vertical is a quarter-wavelength of wire for the intended frequency. Radials and a ground connection are required. You'll have to build your own antenna(s), as we note only commercial products intended for the radio amateur. The task is not difficult, nor is it expensive. Recommended reading is Edward Noll's Easy-Up Antennas for Radio Listeners and Hams.

The Beverage antenna is one of the most intriguing antennas we have used, but it requires a lot of space. It is a longwire one or more wavelengths long mounted near to the ground and oriented in the direction of the desired reception. A nominal 6:1 balun is required at the juncture of the wire and the coaxial feedline. The far end is terminated with a nominal 600-ohm resistance. The antenna is highly directional, responsive to low-angle signals, has little noise pick-up, and produces excellent signal to noise ratios. Our tests show it is most practical up to 7 MHz or so.

beverage

The antenna was invented in the early 1920s by Harold H. Beverage. It was first discussed in a paper titled 'The Wave Antenna - A New Type of Highly Directive Antenna' written by Beverage, Chester W. Rice and and Edward W. Kellogg for the journal of the American Institute of Electrical Engineers (Volume 42 , 1923, page 215 ff.). The paper discusses longwave (between 25,000 and 7,000 meters -- 12-43 kHz) tests of antennas 11 kilometers (7 miles) long at Riverhead, Long Island, NY, and mentions 'shortwave' tests around 450 meters (665 kHz) as a practical upper limit in subsequent experiments. While others have since written about the antenna, if you can find a reprint of this original work in a research library, you'll find the paper is fascinating reading. In 1938, the Radio Institute of America presented Beverage with its Armstrong Medal for his work in the development of aerial systems. The Beverage antenna, the citation said, was 'the precursor of wave antennas of all types.' Harold Henry Beverage, Stony Brook, NY, USA, passed away on January 27 , 1993, at the age of 99.