Topband: Monopole Radiation Patterns, takeoff angles etc

[Herb Schoenbohm]

In 2006 Tom Rauch, W8JI mentioned the disappointment with 3/8 wave 
vertical antennas and Carl mention today abut how "BCB stations migrated 
from 5/8 wave and 1/2 wave antennas."  I added to Tom's rejoinder that 
several AM stations spent considerable amounts of money with the 
Franklyn design which was claimmed to lay more radiation at lower 
angles.  This is possible if the two is insulated and a phasing device 
is place between the upper and lower tower sections.  Presumably it can 
be accomplished even with reduced height or a squashed design of the 
true Franklyn.  Admittedly I have yet to hear of any TB'er to use this.  
However a 3db signal enhancement at low angles in all directions may be 
something to consider.   I would also wonder if putting to much RF below 
the critical angle (since DX-ers) are not particularly interest in 
ground wave coverage and need sky wave instead) would be detrimental.  
There are times when a higher angle take off is the difference between 
being heard or not especially, I think, during SR/SS Grey line 
enhancements, and maybe on some skews and spotlights.  I post the 
Franklyn information just the same for those who may have missed the 
original post.


Herb Schoenbohm, KV4FZ

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 balloon lengths has increased my curiosity in learning what principles are
working here. Theoretically, very low angle radiation could be obtained by a
balloon supported long wire with "controlled current distribution".  (ARRL
Antenna 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 noticeably better.

I once worked for KUOM which shared a tall tower with KSTP 1500 kHZ in
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)  Some claimed
this was due to sky wave and ground wave out of phase arrivals in which case the
Franklyn actaully redued the sky-wave component, at least in theory.

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 Angles)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 apparently 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 the Franklin design and if
the shortened Franklin (ala KNBC) has any  merit for consideration on 160
meters as a shortened 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. (The 3/8 wave if converted to an Inverted L was popular in the 60's
as it provided a 50 to 60 ohm feed point with just some inductive reactance to tuned out
to actually resonate the wire as a 1/4 wave.  In an inverted L configuration there is
radiation in both the horizontal and vertical portion.  I mention this since this would
be a totally different antenna then a bottom feed 3/8 wave vertical tower.)



  Herb Schoenbohm, KV4FZ









On 5/6/2012 12:31 PM, ZR wrote:
> The BCB stations migrated from 1/2 and 5/8 wave antennas, diamond shaped
> towers, and mountain tops by the early to mid 30's as they started to
> understand how things worked...or didnt.
>
> Carl
> KM1H
>
>
>
>