I love Ham Radio and the Hams in it. What could have been
received as a frivolous question, is input resistance the
same as radiation resistance, was answered by Hams from
Italy, Germany, and across the U.S. I will try to summarize
comments.
Craig, W1JCC, uses a unipole with a driven element (12"
triangular wire cage) the same size as the grounded element
(Rohn 25); it works well for him.
Kevin Olson said: "Hint: the folded section is just a
transformer."
Guido, IK2BCP: The current returning to the ground is the
sum of all the currents flowing thru the wires. So, the
current flowing to ground is almost the same if you use 1 or
100 wires. Nothing is free."
Steve Miller, N8SM: When it comes to antennas, there is no
such thing as a free lunch. The N wires transform the Rin by
N squared, but the Rr and ground/loss resistance is also
transformed N squared compared to a single wire; the antenna
efficiency remains the same.
Mauri, I4JMY: He defined the difference between radiation
and feed point impedance. He used the example of a dipole in
free space having a radiation resistance of 73 ohms, and
pointed out it would have the same rad. res. regardless of
how many folded wires were used. He likened the folded
dipole to a thicker dipole having lower resistive losses,
but overall leading to the same efficiency, Rr, and other
losses.
Willi, DJ7RJ: He uses a folded unipole when on DXpeditions,
homebrewed, 12m high, and top loaded, with two radials. He
believes it is a good system to multiple the input
resistance of the antenna!
Dave Bowker, K1FK: States radiation resistance IS NOT the
same as input resistance in a mult wire vertical. The
multiwire vertical serves as an impedance transformer, and
does nothing to alter the radiation resistance of the basic
element. He states this has been a long time misunderstood
conception, and many of the handbooks further promote this
misunderstanding.
Pete, N4ZR: The multi-folded wire vertical transforms the
feedpoint impedance whose resistive term includes all the
real world loss resistance components. and efficiency
remains unchanged.
Tom Rauch, W8JI: Tom went into great detail on the subject,
and an article could be written on what he said. I will
paraphrase his comments, which were very helpful to me. The
myth of Rr = Ri lives on, but is slowly dying. The poor
definition of Rr has been at fault. Rr = Ri when the Rr is
normalized to the spot where the date is measured. When you
fold the antenna, both Rr and ground loss resistances are
multiplied by the same amount. (That statement creeps in a
lot) If you use the IRE definition of Rr, rather than
feedpoint resistance, there is no change in radiation
esistance by "splitting" currents into parallel wires; the
Rr of a folded dipole is still 72 ohms. His final comment is
the consensus of the group: "If you multiply ground losses
by the same factor as Rr, the answer will be correct."
Conclusion: there is no increase in a vertical antenna's
efficiency, by using multi-folded wires.
Many thanks for your comments and vy 73,
Henry G. Elwell, Jr., N4UH
--
FAQ on WWW: http://www.contesting.com/FAQ/topband
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|