One concept of the Franklin antenna is that its configuration and
performance is similar to a center-fed, full-wave, vertical dipole.
But a careful reading of the patent for the Franklin antenna shows
that both current and phase along each of its co-linear sections
differ from that of center-fed dipoles (and long-wire antennas).
Dr. George H. Brown of RCA Labs also discussed Franklins in
his 1936 I.R.E. paper "A Critical Study of the Characteristics
of Broadcast Antennas as Affected by Current Distribution."
He describes the Franklin as "a number of half-wave antennas
placed end to end on a vertical line, and so fed that the currents
in each element are equal, and in phase." This configuration
is not the same as that of a center-fed dipole.
The elemental parts of sectionalized MW radiators such as
the 180/180 Franklin and its variations are fed in phase, and
often with different power radiated by the two sections. This
permits increasing the gain in the horizontal plane while
reducing skywave radiation above 45 degrees elevation or so,
compared to a 195 degree monopole. This can reduce the
nighttime self-interference area, and move it further from
the transmit site.
The belief that a Franklin antenna does not need a ground plane
such as needed for MW monopoles may be based on thinking of
the Franklin as some kind of a balanced radiator. Balanced
radiators don't need or use a ground plane. But the two
co-linear sections of a Franklin antenna are unbalanced
radiators, and need a good r-f ground for the same reason
that a single monopole needs one.
Charles Jeffers of WOAI published a paper in the Proceedings
of the IRE describing the experimental studies he and others
did in 1948 on a 180/120 Franklin. They were done in reduced
scale, and used a buried copper-mesh ground screen that
extended more than 1/2 of a free-space wavelength from the
base of the model. WOAI used a Franklin type antenna for
quite a few years.
There are reports that the two Franklins at KFBK don't have
a buried radial ground system, but that would be good to
confirm. I know from good sources that other Franklins
such as at WHO do have one.
MoM modeling is useful to show these Franklin elevation
patterns and gains for various phases and currents in each
section, and the affect of a ground plane on them.
performance is similar to a center-fed, full-wave, vertical dipole.
But a careful reading of the patent for the Franklin antenna shows
that both current and phase along each of its co-linear sections
differ from that of center-fed dipoles (and long-wire antennas).
Dr. George H. Brown of RCA Labs also discussed Franklins in
his 1936 I.R.E. paper "A Critical Study of the Characteristics
of Broadcast Antennas as Affected by Current Distribution."
He describes the Franklin as "a number of half-wave antennas
placed end to end on a vertical line, and so fed that the currents
in each element are equal, and in phase." This configuration
is not the same as that of a center-fed dipole.
The elemental parts of sectionalized MW radiators such as
the 180/180 Franklin and its variations are fed in phase, and
often with different power radiated by the two sections. This
permits increasing the gain in the horizontal plane while
reducing skywave radiation above 45 degrees elevation or so,
compared to a 195 degree monopole. This can reduce the
nighttime self-interference area, and move it further from
the transmit site.
The belief that a Franklin antenna does not need a ground plane
such as needed for MW monopoles may be based on thinking of
the Franklin as some kind of a balanced radiator. Balanced
radiators don't need or use a ground plane. But the two
co-linear sections of a Franklin antenna are unbalanced
radiators, and need a good r-f ground for the same reason
that a single monopole needs one.
Charles Jeffers of WOAI published a paper in the Proceedings
of the IRE describing the experimental studies he and others
did in 1948 on a 180/120 Franklin. They were done in reduced
scale, and used a buried copper-mesh ground screen that
extended more than 1/2 of a free-space wavelength from the
base of the model. WOAI used a Franklin type antenna for
quite a few years.
There are reports that the two Franklins at KFBK don't have
a buried radial ground system, but that would be good to
confirm. I know from good sources that other Franklins
such as at WHO do have one.
MoM modeling is useful to show these Franklin elevation
patterns and gains for various phases and currents in each
section, and the affect of a ground plane on them.
