March 1948 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Have you heard of an Alford Loop antenna? I hadn't until reading
this article. It is a four-sided structure consisting of identical
folded ½-wave dipoles on each side, with a common feed. Opposing
sides have their elements 180 degrees out of phase with respect
to each other. The intention is to provide nearly omnidirectional
FM radio reception across the entire 88-108 MHz band. A little
research on the Alford Loop reveals that
Mr. Andrew Alford developed this configuration to enable
simultaneous, co-located transmissions of FM radio stations. In
that case the four antennas are individually fed by transmitters
on different frequencies. Alford is credited with inventing antenna
systems for the VFH Omnidirectional Range (VOR)
and Instrument Landing System (ILS) navigational aids.
All-Way FM Antenna
By Julian T. Dixon
This antenna has been found to give optimum reception of each
of several FM broadcast stations located in different directions
from the receiver. Its response is substantially independent of
direction. Its gain is the same as that of a properly oriented dipole.
It is an Alford loop comprising 4 folded-dipole elements arranged
in a square, half-wave length on a side. Transmitting antennas of
this basic type have been described previously and are in use at
many FM broadcast stations.
antenna components are made of readily available and inexpensive
sections of twin-lead transmission line. The 4 folded-dipole elements
A are made of 300-ohm line and have a terminal impedance of about
300 ohms. The terminals of the dipoles are connected to a junction
in the center of the antenna through equal lengths B of 300-ohm
line. The B sections may be made long enough to provide sufficient
slack to prevent pulling the dipoles out of shape.
Since the B sections are effectively in parallel at their junction,
the impedance there is one-fourth of 300 ohms, or 75 ohms. The standard
receiver input impedance is 300 ohms; consequently a quarter-wave
length section C of 150-ohm line is used to step up the 75-ohm impedance
at the junction to the 300-ohm impedance of the line D to the receiver.
The D section may be of any length sufficient to reach the receiver.
It was found that an antenna constructed according to the dimensions
shown worked well over the entire FM broadcast band, 88 to 108 megacycles,
although it was cut specifically for 100 megacycles. S-meter readings
indicated an improvement of as much as 17 decibels over the dipole
previously used, for signals from stations which were not in the
broadside direction of the dipole. For signals arriving from the
dipole broadside direction, the loop gave an equal input to the
Less fading due to reflection of the signal from airplanes overhead
was noted with this loop, as compared with the dipole. This improvement
is attributed partly to the fact that the loop has some directivity
in the vertical plane, giving maximum response at zero degrees elevation
and decreasing to zero response at 90 degrees. Signals reflected
from airplanes above the horizon are thus rejected to some extent.
Signals arriving from the dipole minimum direction had been especially
subject to flutter fading due to the reflected signal being received
from airplanes which were broadside to the dipole. This condition
was remedied by the omnidirectional response of the loop in addition
to its vertical plane directivity.
The loop must be mounted in the horizontal plane and the C and
D sections should drop away vertically from the junction of the
B sections for a distance, preferably, of 5 feet or more. The dipole
elements may be suspended conveniently from the ends of 2 light
diagonal wooden supports. For an attic installation, four nails
can be driven into the rafters as supports.
Care should be taken to connect the components exactly as shown.
The 180-degree twist in 2 of the B sections as shown provides the
required 180-degree phase relationship of opposite dipoles while
maintaining an in-phase condition around the perimeter of the loop.
The dimensions of the antenna components in wave lengths are
given below for the convenience of those who may wish to construct
similar antennas for use on other frequency bands.
0.45 wave length
0.25 wave length (or more)
0.193 wave length (see note)
Note: Length of C section is 0.25 wave length multiplied by 0.77,
the velocity constant of the 150-ohm line.
A Teleran experimental installation is being made near Washington,
D. C., for operational tests of the new navigational aid. Teleran
is a name coined from the words Television-Radar-Air-Navigation.
The unique system of air navigation and traffic control combines
ground search-radar and television to furnish the pilot a constant
"aerial roadmap" on a screen on his instrument panel. This composite
pictorial presentation of route, terrain, traffic, and weather data
clearly identifies all mountains and other obstacles to aviation,
and is expected eventually to make all-weather flying a practical
Posted December 29, 2014