Topband: Franklin Antenna Design

[herbs@surfvi.com]

Quoting Tom Rauch <w8ji at contesting.com>:


> Some of the biggest failure antennas I have used were 5/8th
> wave verticals at broadcast stations. We loaded one AM tower
> that happened to be a 5/8th wave on 160, and it was poor
> compared to a short vertical.

The baloon lengths has increaeed by curiousity in learning what prinicples are
working here. Theoretically, very low angle radiation could be obtained by a
ballon supported long wire with "controlled current distribution".  (ARRL
Anternna Compendium Vol. 2 pp. 132-135)

As I mentioned before in my case the 5/8 vertical 308 foot insulated tower,
totally surrounded by sea water was a big disappointment on 160 meters. I
tried it for 5 years and the lower antennas were always noticiably better.

I once worked for KUOM which shared a tall tower with KSTP 1500 kHZ
Minneapolis. Stan Hubbard, owner of KSTP was convinced to erect a Franklin
antenna design which was supposed to modify the current distribution on tall
towers to lay out a stronger ground wave then the 1/4 wave or smaller AM
radiators.  All the theory, the engineer and construction cost, sort of like a
Ringo Ranger for the Broadcast Band were very disappointing. Years of A/B
testing driving across the Dakotas, WCCO (although lower in frequency) was the
king of signals from the Twin Cities by a significant margin.  Both were 50KW
clear channel stations. (KSTP bragged 100KW Effective Radiated Power)

The Franklin concept can be found in Jasik's First Edition Antenna Engineering
Handbook pp. 4-35 and 4-36.  A traditional Franklin was two half waves stacked
end to end and fed in phase.  KNBC (Los Angeles)built one in 1949 as a means
of lowering the angle of radiation, but used a 550 foot tower since at 680 Khz
a true Franklin would have been 1500 feet tall.  They were apparantly able to
design a much shorter structure since their top portion was top loaded with a
capacity hat and only 150 feet tall.  (Put "KNBC Franklin Antenna" in your
search engine for some awesome pictures of this antenna.) Did it actually
improve coverage for KNBC? Are they still using it today?

It would be interesting to learn if any AM stations still use Franklins and if
the shortened Franklin (ala KNBC) has any  merit for consideration on 160
meters as a shortenecd gain low angle DX antenna  As far as I have been able
to find out, collinear verticals below VHF are just not worth the effort, but
that is not what the books tell us.  Yet in practice a 1/4 to 3/8 wave appear
to be the best topband performers for all the reasons stated in 
previous posts.

Herb Schoenbohm, KV4FZ