April 1947 Radio News
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Folded dipole antennas, as the name suggests, are about half the length of a regular dipole, and work just as well for many applications. I have had one attached to my FM radio receiver for many years and it does a great job pulling in stations from as far away as Toronto, Canada, and Detroit, Michigan (I'm located in Erie, PA). Receiver sensitivity and oscillator stability has been able to obviate the need in most cases for super performance antennas in modern receivers, as evidenced by ear bud wires and even conformal patch antennas in smartphones sufficing in lieu of a 'real' antenna. It is a real tribute to the brilliance of engineers that cellphones work so well on multiple bands that accommodate frequencies ranging from 88 MHz (FM), through 900 MHz (GSM), 1.5 GHz (GPS), 2 GHz (UMTS), and WiFi (2.4 GHz) - all in one compact device with no external antenna!
Folded Dipole FM and Television Antenna
New 300-ohm transmission line makes possible low-priced dipole antenna systems for TV and FM.
Fig. 1 - Electrical representation of the FM and television folded dipole antenna.
Fig. 2 - Mechanical details show how the 300·ohm transmission line may be used.
This simple, low-cost 300·ohm line receiving antenna can be conveniently placed under the rug.
A simple, low cost FM and television receiving antenna which is superior in performance and easier to install than many of the more elaborate and expensive types can now be constructed using the new 300 ohm lead-in wire K-1046, manufactured by Federal Telephone and Radio Corporation. Requiring only between 5 and 10 feet (depending on the frequency) of this inexpensive, highly flexible twin conductor transmission line, this antenna is very efficient and provides a perfectly matched folded dipole and lead-in for FM and television.
As indicated on Fig. 1 this antenna, a "T" match type, consists of a 300 ohm cable which is a half wavelength long, is shorted at both ends and has a one conductor cut in the center as the input or lead-in point. The entire construction takes between fifteen and twenty minutes and requires only three stripping and soldering operations.
First the cable is shorted at both ends by stripping the insulation for a short distance and twisting the two conductors together. Then they are soldered and an insulating lacquer spread over them to weatherproof the connection. These shorted ends also provide a means of supporting the antenna without affecting the characteristics of the transmission line.
Then the lead-in is connected to the midpoint of one conductor. This operation requires a little more caution since the width of the cut must exactly equal the conductor spacing of the lead-in. The cut is made just clear of the inside of one conductor exactly at the midpoint. Sufficient insulation is removed from the two ends of the conductors thus provided so as to enable the lead-in to be connected to them. The connections are soldered and lacquered and the assembly is now ready for mounting.
The antenna is mounted by simply suspending it on an insulating material, such as wood, using the two exposed short circuited ends as means of support. For maximum signal pick-up the antenna should be as high off the ground as possible. However when an outdoor antenna is either impractical (due to climatic conditions), or unnecessary, the antenna - due to the flat ribbon construction of the 300 ohm line - can conveniently be placed under the rug or behind a piece of furniture.
The antenna shown in Fig. 2 was designed for FM reception and is therefore 58 inches long. Since, as any folded dipole, it has an impedance of 300 ohms when removed from ground, and since the conventional input impedance of a FM or television set is 300 ohms, it provides a perfectly matched system.
The K-1046 is polyethylene insulated which makes it low loss, weather resisting, and extremely flexible even in sub-zero climate. This cable retains an attractive appearance since polyethylene is water repellent and therefore dries quickly and remains clean.
Posted September 1, 2016