Topband: shunt fed tower - question?

[herbs@surfvi.com]

Quoting Donald Chester <k4kyv at hotmail.com>:

>> "The paper can be downloaded at"
> I can't get the link to work.  Tried two different browsers, and the link
> seems to be dead.

Indeed it is and I should have checked but many edirect eMails are asking to
look at this.

I have a printed copy of the paper which I will try to scan for anyone who
wants it.  The www.dlr.com is still there but the paper is not.  Nor is it on
the Kintronics site where they provide a link to this paper.  (Kintonics and
Cortana both market skirt feed kits for grounded towers. The NAM appears to
require a membership to read this paper.  I have not tried the IEEE where it
may also be available.

For the AM broadcaster, in addition to increased bandwidth, the skirt feed
allows for direct grounding of the tower (lightning protection) and multiple
use FM, Cell, and Broadband antennas without the use of expensive isocouplers
for each service.

Since the link to the NAB paper on this appears to have been removed I will
offer a short summary which I posted on this reflector in 2003 after coming
across the work by DLR in 1996.

  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.

(4) Early proponents of the skirt feed claimed better performance over a
poorer ground system.  No so according to the study.

The paper also objected to the term folded monopole which they stress a skirt
fed tower is not.  "The folded unipole is not simply half of a folded dipole
which the name implies. Unlike folded dipoles, which consist of parallel
conductors spaced side by side at an appreciable portion of a wave length,
folded unipoles consist of a tower surronded by close spaced wires.  Because
the conductors of a folded dipole are equally exposed to the outside worked
they each carry radiation mode current which contribute to the far field
radiation of the antenna, as well as transmission mode current."....."Because
the wire cage of a folded unipole tend to shield the tower from the outside
world, the radiation-mode current flows principally on the cage and the tower
primarilly carries transmission mode current."..."any modification of the
input impedence is primarilly due to the shunt effect of the reactance
produced across the feedpoint by what is effectively a coaxial transmission
line consistin as the outer conductor and the tower as the inner conductor, an
effect that could also be produced by placing a reactance in parallel at the
base of the a series fed tower."

Just read the post by W4ZV that he plans to use a 2 conductor shunt feed to
increase B/W.  I wonder if two conductors will obtain the "shielded from the
outside world" effect refered to above? Does it make any difference?

73

Herb Schoenbohm, KV4FZ