Topband: Series Fed vs Shunt Fed Towers

[Herb Schoenbohm]

Over the past week several interesting comments were made on the 
advantages of a shunt fed tower over a direct fed insulated tower.  I 
was convinced long ago after listening to the John Mulaney (Mulaney and 
Associates) that a folded monopole has significant advantages in several 
important areas for the topbander including; lightning protection, 
simplicity of matching, increased radiation efficiency, and increased 
bandwidth.  In 1996 Ron Rackley along with Bobby Cox, James Moser and 
Tom King of Kintronics presented a paper at the NAB convention entitled 
"An Efficiency Comparison: AM/Medium Wave Vs. Skirt Fed Radiators." 
(http://www.dlr.com/dlrweb/papers/nabpaper/nabpaper.htm) Most amateurs I 
have talk to about this use single wire shunt feeds, rather than a three 
or four wire cage, of  #10 wire or larger. Depending upon spacing 
considerations of that single wire, sometimes results are not as 
convincing and matching becomes difficult.

The writers of the paper also tested the difference between a 
120-radial, 30-radial and a single ground rod for a tower of 98.3 
degrees and 61.5 degrees electrical height. All experimental tests were 
conducted on 1680 Khz with 400 watts while modeling used the NEC-4.1 
developed by Dr. Gerald Burke of Lawrence Livermore National Laboratory.  

To summarize briefly their findings:

(1) No major differences in field strength between the folded unipole 
and series fed test cases.
(2) The folded unipole was not found to have a significantly better 
radiation efficiency than the series fed for a given tower and ground 
system.
(3) No major differences in bandwidth for either system except when a 
skirted series fed tower has the skirt connected at both top and bottom.

The paper is 11 pages long and is a must read especially for persons 
wanting to decide which way to proceed in tower construction.  Of course 
the shunt fed tower is a better choice for anyone wanting to mount beams 
and other antennas which seems to be the case in most of the amateur 
applications.

73

Herb Schoenbohm, KV4FZ