[BC] Franklin MW Antenna Ground Unneeded?

[Richard Fry]

As a follow-up on the subject of Franklin antennas, the link below leads to 
a clip from a paper published in the January, 1936 issue of the Proceedings 
of the I.R.E. titled A Critical Study of the Characteristics of Broadcast 
Antennas as Affected by Antenna Current Distribution, by George H. Brown of 
RCA Labs.  A short section of the paper discusses Franklin antennas.

Figure 22 in this clip shows the groundwave field intensity at 1 mile for 1 
watt radiated by a 100% efficient Franklin antenna system over a perfect 
ground plane.

For a 2-section Franklin (180/180) that field for 1 kW would be about 329 
mV/m, compared to a 228 mV/m field for a single 180-deg, non-sectional 
monopole.  The 2-section Franklin produces a groundwave field equivalent to 
about 2 kW radiated by a 180-deg, non-sectional monopole.

But Brown's Figure 21 in the clip shows that a 2-section Franklin with equal 
power and phase in each section also radiates a sidelobe with a peak 
relative field of about 18% at an elevation angle of about 44 degrees, which 
could produce nighttime self-interference to the groundwave under the right 
conditions.

Later "Franklins" used various combinations of heights, powers and phases 
for the two sections with the goal of producing an anti-fading radiation 
pattern in the vertical plane.  But in doing so it is likely that the 
groundwave field is reduced from the values in this George Brown paper.

Probably in the later iterations the performance of a 2-section Franklin was 
not much different than a 195-deg, non-sectional monopole.  That, and the 
cost of installing and maintaining a Franklin may be the reasons that not 
too many of them were/are used for AM broadcast in the U.S.

http://i62.photobucket.com/albums/h85/rfry-100/FranklinAntennas_G_Brown.gif

RF