No one uses an untuned mobile whip, who desires to get the most power to the
antenna and radiated. As in any non resonant antenna system, once you use
the tuner, you have matched (canceled the antenna reactance) and you are
resonant everywhere in the system. This is covered well in Maxwell's
original QST papers and in his revisions, "Reflections II" which is
available thru "World Radio" bookstore.
The losses in a good tuner and short transmission line in the mobile case,
are a pretty small part of efficiency. It is still possible to get near
resonant dipole efficiency, if your other losses are minimized and you
complete the mobile whip with a good radial system. The lack of a balanced
element for the whip as a counterpoise or ground radial is the major mobile
whip loss factor.
In the mobile case, it is the ground losses, and the missing half of the
antenna system, as compared to a dipole, (doublet) that lowers the results.
Also, the mobile whip is too close to the ground, as compared to the normal
dipole. It is blocked by parts of the vehicle from optimum radiation. It
would not be a valid comparison to a normal dipole.
When you compare antennas, it is better to compare a shortened doublet to a
full size dipole at the same height. Going across polarizations, such as
mobile vertical whip to a dipole, gets into other sources of performance
differences; too much so to make a meaningful comparison. Even a tuned
shortened dipole up high in the clear should out perform an 8 foot mobile
whip on the typical vehicle. L. B. Cebik, W4RNL, has applied modern
antenna modeling and re calculated well known rules for how much you can
shorten an antenna and still retain high efficiency. Certainly a doublet
2/3 of normal length retains high efficiency. Exact results can be seen on
the web site: www.cebik.com These have been validated by experimental
construction as well by various authors. You can even vary the diameter of
the conductors and retain most of the "normal" dipole efficiency, and Cebik
has investigated this as well. Of course, as the element grows smaller,
your bandwidth for low SWR becomes more narrow, as compared to "normal" no.
12 wire doublets. Remember, antenna action consists of currents mostly
traveling in the shallow skin depth of the conductors and launching
electromagnetic waves, thus what very large diameter conductors brings you
are simply wider bandwidths for a low SWR,and allows a shortening of the
doublet, if the conductor is sufficiently large.
We presently are developing miniature antennas for the forces that are HF
types and are only 14 inches high for 13 to 30 MHz, but are 90 per cent
efficient. It can be done!
But, you have to match the antenna with a transmatch to a 50 ohm
transmitter. The antenna, to perform that well at 14 inches off a ground
plane, has to contain the elements in a volumetrically efficient form to
accomplish bringing the feedpoint to near 50 ohms. This is done with four
elements in the form of transmission line transformers in parallel. They
are folded dipole half elements, and a feature of folding an element is
raising the feedpoint impedance when in free space. With elements in close
proximity, it was found multiple folded elements longer than normal resonant
quarter wave elements gave the needed matching and efficiency. To perform
the needed impedance transformations, one element is made of no. 8 wire and
the other of no. 12 at a given parallel line spacing. By altering the
slant height, height,and wire gauges, you can accomplish the same
transformations with other conductor diameters. These look like conical
springs, but make very useful antennas for vehicles.
Those having access to a College of Engineering library can look up a paper
on this FLEX antenna by Dr. Robert L. Rogers, of the Univ. of Tx. in the
IEEE San Antonio Antennas Symposium transactions, June 2002. In antennas
there is no free lunch.
To accomplish this volumetric saving, Dr. Rogers has to use four times the
element conductor length of a dipole, plus 20 to 30 per cent. If you fold
and spiral elements, you can still get efficient radiation, and occupy a
lower profile than conventional doublets. In the present forms a vertical
spiral conical antenna of this type has vertical polarization, and requires
radials or ground plane. However, research is continuing with balanced
biconical FLEX radiators, as well as nested FLEX antennas to cover multiple
bands.
-Stuart
K5KVH
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