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Progress in Design
Of
Extremely Short Transmitting Antennas
Short and still efficient, how is that possible?
By Juergen Schaefer, DL7PE
dl7pe@hotmail.com
Other hams like me living in a high-rise condominium where antennas are not
welcome or where only very restricted space is available. The only solution is to employ an
antenna small enough which does not attract the unwanted attention of the neighbourhood.
T
Author
here may be some environments where the dreaded antenna policy is so strict that
you not dare put any short-wave antenna of usual dimensions outside.
Photo 1:
A 40m DL7PE-MicroVert Monopole of the length of
only 32 inches fixed on a balcony during vacation.
Shown here in almost horizontal position. It is more
than it appears to be!
On the way to the marvellous DL7PE-MicroVert
It took me many years to test the various common designs of short antennas such as
magnetic loops, mobile radiators, helical-wound wires on fibreglass tubes, wire simply
hanging on a fishing rod and tuned by a coupler or the dipole-shaped Isotron type with its
capacitive plates. Some types with new ideas were published in German Magazines [6-10].
During this phase many more typical and non-typical aerials where thoroughly tested. All
antennas managed to radiate, however did not deliver great signals at my location. Only
the Isotron came very near to my expectations and attracted my attention. So I started to
study the principles behind it and tried to improve this design. The attempt to find a more
efficient solution resulted finally in a different concept of a Monopole.
The DL7PE-MicroVert introduced here is an extremely short and hardly visible short-wave
antenna with outstanding radiation properties
Fig.1
shows the evolution from a closed resonant circuit to the concept described.
1
There are no wonder antennas!
…although it may seem so to some of those radio amateurs having tested this tiny little
radiator and were surprised by its efficiency. However, all antennas will have to follow the
laws of physics, as also does the DL7PE-MicroVert.
Its operational capability is based on theoretical
principles discovered by the German scientists Prof.
F.Landstorfer and Prof. H.H.Meinke
[1]
published as
early as 1973 in the communications magazine “NTZ”
(Nachrichtentechnische Zeitschrift). Unfortunately not
much attention has been given so far by anyone in the
ham radio community. A high radiation resistance of
about 30 Ohms was discovered at this time for small size
monopoles being the sole secrete of its efficiency.
Still there was a lot of effort necessary to convert the
theory into a practicable solution and matching certain
conditions for their use as a radiating antenna. In spite of
its very small dimensions the DL7PE-MicroVert has a
high overall efficiency, which will according to my
knowledge not be achieved by any other antenna of a
similar small size.
Werner, DL6NDJ, has written about the very interesting
discoveries of the Stuttgart University Professors to you
in a separate article in this month’s issue of
antenneX.
Fig.1:
Evolution from a closed
resonance circuit to the open
series L/C circuit of the
DL7PE-MicroVert
The Monopole at a first glance:
It was the aim to develop a radiator with the primary objective of high
efficiency at stealth size. The dimensions of the DL7PE-MicroVert (Photo1)
are extremely short in comparison to the wavelength, i.e., 0,02λ. No special
counterpoise will be required apart from the coaxial-feeder cable. The
simplest possible Circuit Diagram is depicted in
Fig. 2.
It is only the capacitor in form of an Aluminium tube that radiates in the near
field, an almost pure electrical field evenly distributed along the rod.
Capacitance in general does not lead to as much electrical losses as it is the
case with inductive components.
At resonance, the feeding-point resistance is real at around 50 Ohms. Thus,
no antenna tuner will be required. Shifting of frequency (QSY) within the
working range can be done on many bands without any adjustments.
Fig. 2:
Simplest
schematic
2
For whom is this antenna useful?
As mentioned above, this antenna is especially for those people with antenna restrictions.
Also for those on the road, on camping sites or for amateurs with a second QTH where it is
difficult to build something permanent.
Although it is not a mobile antenna for cars it has been used as mobile antenna on non-
metallic Mobile Homes/Caravans as well as on non-metallic Ships.
However, if you can install a full-size antenna you normally are better off.
The principle of radiation
The principle is based on an open L/C series resonance circuit.
For an antenna to function, it should be electrically resonant. The DL7PE-MicroVert consists
of 4 components
–
–
–
–
–
the capacitive radiator,
the reactance coil,
the resonant coaxial cable-counterpoise and
the electrical insulator (RF-Choke).
A proper match is automatically achieved with the right combination of those four
components.
Any antenna needs also a sufficient area to couple the radiation into the atmosphere. By
stretching the capacitive plates to a greater physical length, as well as by the
implementation of the feeding coaxial cable as a low-radiating counterpoise, this area has
thus been considerably enlarged and contributes to high efficiency.
The antenna emits in the near by field mainly an electrical field (E-field) which is >30dB
higher than the still existing small magnetic field which may be neglected. So we can
consider the DL7PE-MicroVert aerial as an electrical radiator. The principle is opposite of
that we know from the small magnetic loop with a predominantly magnetic field in the near
distance (H-field).
However, both antennas will form a homogeneous electromagnetic field at some distance.
Comparison with the small magnetic loop antenna
According to Kraus
[2],
the electrical area of a small magnetical-loop with its evenly
distributed current I is comparable with the equivalent to the area of a short Monopole.
Thus, a direct comparison between the two complementary systems is offered to us.
The efficiency of the
≈1
foot long 20m DL7PE-MicroVert
is also comparable with a
≈
5 foot
diameter small circular magnetic loop antenna.
Evolution toward the DL7PE-MicroVert
Often it turns out that ideas, which seem to be new are not so new at all. This is also true
with this antenna. Already during the 1980s Ralf Bilal
[3]
came out with his Isotron antenna
(Photo
2),
which is since commercially produced in the USA. Before that a similar design
was used by the CIA. Also, many other amateurs did create such types, e.g., Douglas E.
Person, W4DXV
[4]
with his variant of MicroVert. However, supporting literature was not
found.
3
Many experiments followed according to the principle of trial and error to improve the
efficiency and a great deal of developing and optimisation work was still necessary to
arrive at the DL7PE-MicroVert in its present stage. During the course of evolution, also the
appearance changed. The following steps were taken:
1. Converting the Top capacity plate from a flat sheet to a tubular capacitor of a greater
physical length. This gives the advantage of a slim, less visible shape as well as a
greater electrical ”area”.
2. By implementation of a counterpoise, the RF-“area” was again considerably
enlarged. But instead of using a second tubular capacitor, the available coaxial
feeder is used for this purpose. However, also dipoles have been
built with 2 radiator tubes, which shall not be discussed here.
According to Landsdorfer/Meinke there is an additional capacity C1
apart from the radiating capacity C2 that they called “dead-capacity”.
This capacity forms a closed rf-field to the near environment, similar to
that of a conventional capacitor.
C1 does not exist if the radiator is of ¼
λ
length, while its capacity
increases steadily with the decreasing of the radiator’s physical length.
Hence, the capacitance C1 becomes part of the antenna capacity and
results in a new radiating resistance R
r0
of almost constant 30 Ohms
regardless of the frequency used. This phenomenon actually is the key to
success (Fig.
3).
In the next paragraph we will discuss the four components used.
Photo 2:
The Isotron with the
vast capacity plates
Fig. 3:
Equivalent circuit including C1 (“dead”
Capacity). X
A
stands for the reactance
Coil, while R
l
is the summary of all losses.
4
Fig.4
Principle design of a DL7PE-MicroVert Monopole
Component 1: The radiator:
The dimensions of the radiator used have been designed in such a way and optimised that
with the shortest possible physical length a reasonable operational result is achieved. Any
further reduction of length would immediately reduce the efficiency dramatically as well as
reduce the bandwidth. With
Formula 1
this length can be calculated for any frequency.
4700
l
s
(mm) = ————
f (MHz)
Formula 1:
Radiator Length
The capacity of the radiator is dependent on the length of the tube as well as on the
diameter used. Should an attempt be made to reduce the length and increase its diameter
so that it would result in an equal capacity, the effectiveness would considerably suffer as
the following example shows.
Effects on changes observed:
Signal strength of a full-size Dipole:
Signal strength of a DL7PE-MicroVert:
Signal strength of 50% reduced Radiator:
S9 + 10 dB
S9
S7
5

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