February 1968 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
Here is an excellent
example of how scientific evaluations performed by two independent subject
experts can result in significantly different conclusions. James Gupton, Jr.,
reports on what previously had been considered secret military technology - an
active, electrically small receiving antenna. This particular example, dubbed
the Mini−Tenna, is designed for use on the FM radio band. After building and
testing his antenna, Mr. Gupton offered it to two Radio-Electronics
magazine editors for investigation. Each used a carefully considered method in
what are generally similar environments - urban dwelling with strong signals and
opportunity for multi-path, connected to a commercial FM radio receiver. The
divergent results were not commented upon by the author. As a side note, I still
have a 1980s vintage active FM radio antenna from Radio Shack (Archer 15−1821).
It does as good of a job at pulling in stations as the half-wave dipole antenna
mounted in the attic. The dipole needs to be positioned fairly precisely to pick
up one station I listen to often, whereas the electronic antenna can be set just
Build a Mini−Tenna
Build a Mini−Tenna for FM Radio
By James A. Gupton, Jr.
Here's a construction project designed around one of the most controversial recent
developments in electronics - the Subminiature Integrated Antenna, or SIA. You can
build it in less than an hour, for only a few dollars. And you'll be learning something
about the latest state-of-the-art development in antennas.
Background of SIA
In the spring of 1967 a new military communications development was revealed.*
Researched and built by the US's Edwin M. Turner and Germany's Dr. Hans Meinke,
the SIA was branded with an astounding claim - only 1/50 wavelength long, it can
perform as well as a conventional quarter-wavelength model.
Normally, a small antenna (with respect to wavelength) has little capture area,
hence doesn't pick up much RF signal. Thus gain is low and the antenna is inefficient.
At frequencies above about 30 MHz, moreover, the signal-to-noise ratio of a small
antenna is poor. Generally, then, an antenna should be a quarter-wavelength or longer
in size to provide a clean signal.
Fig. 1 - Secret of SIA is that transistor becomes integral part
Fig. 2 - Equivalent circuit of SIA's shown in Figs. 1-a and 1-b, RF signal is picked up by all antenna elements.
In the SIA, transistors are connected directly to antenna elements. Thus the
transistor multiplies the RF current in the antenna element by a factor equal to
the gain of the transistor. Another way of looking at the situation: In a passive
(conventional) antenna, resonant frequency depends solely on element length (unless
electrically altered by inserting capacitance or inductance). But the SIA's transistors
lower the resonant frequency. They also make the frequency response quite broad,
so a few inches of wire will have good response.
Its developers claim that the SIA can operate over a wide frequency band-from
a ratio of 2:1 to perhaps 50:1. Because the bipolar transistor used is a low-impedance
device, an SIA usually needs no special matching circuit to drive coaxial cable
Some antenna engineers, however, question the value of SIA. The same results
could be obtained, they say, by using an ordinary passive antenna with a booster.
Transistor noise levels, they note, can also limit performance of the SIA. And no
study has been made of cross-modulation in the new device.
The controversy grew when some predicted a 2-inch-long SIA for home TV receivers
would obsolete rabbit ears and rooftop antennas. But it mustn't be overlooked that
the tiny antenna was developed for military use where large antennas simply can't
be used and where some inefficiency must be tolerated.
How It Works
The simplest form of an SIA is merely a transistor connected to antenna elements
in anyone of three basic forms (Fig. 1). The ground plane is connected to one side
of the lead-in, and a transistor lead to the other side.
In Fig. 2 you can see the equivalent circuit of antennas Band C of Fig. 1. Generator
symbols represent the RF signal picked up by the three antenna elements. Distributed
capacitance and inductance are shown, as well as the transistor and the lead-in
terminals. Note that antenna elements 1 and 2 form loops, and respond to the magnetic RF field. All three elements respond to the electric field.
There are also three ways to connect a transistor to the antenna elements (Fig.
3). The emitter-base lead-in connection (B) seems the most likely to match low-impedance
coaxial cable and receiver inputs. A practical model of this antenna might look
somewhat like Fig. 4.
Fig. 3 - Three transistor elements may be connected three ways
in the SIA.
Fig. 4 - Developmental version of SIA shows practical method
My choice for an experimental model of an SIA was what I call a Mini−Tenna, for
it stands only 4 1/22 inches high and is 3 1/2 inches in diameter.
You can see the simple construction in Fig. 5. A 3/4-inch-diameter fiber rod
forms the main support (you can also use Lucite, wood or any nonconductor here).
At each end of the rod is a 3-inch-diameter round piece of copper-clad print board
(a piece of sheet metal will do) - one forms the top hat, the other the ground plane.
The copper sides of the boards face inward. Four lengths of No. 10 copper wire form
the side elements, and another section of wire the emitter ring.
The electrical circuit, shown in Fig. 6, is just as simple. Not shown here are
the distributed capacitances and inductances in the metallic elements. Battery bypass
capacitor C1 is merely a gimmick with a value of a few picofarads. You can solder
a short piece of hookup wire to each battery terminal, then twist the insulated
ends together to form the gimmick.
Mini-Tenna's dimensions are cut to about 1/35 wavelength (at 100 MHz). I
tried a 1/50 wavelength model, but the present antenna performs better. Since the
antenna isn't sharply resonant, but is broadband, exact dimensions don't seem too
important. In fact, the larger the antenna, the more capture area, and the more
New experimental antenna blends solid-state elements with conventional skywire
in new approach.
Two Radio-Electronics editors borrowed author Gupton's small antenna to evaluate
its performance. Their reports follow.
I compared the Mini−Tenna pickup capability with that of a dipole. The reference
antenna was a 50-inch, 72-ohm open dipole, gamma-matched to 72-ohm coaxial cable.
I oriented the dipole horizontally, north south, and placed it 7 feet above the
floor of my ground-floor apartment in Manhattan, at a location where the field intensity
of the stronger local FM stations is approximately 250 to 500 mV/m.
The coax was fed through a 72-to300-ohm balun into my mono Eico HFT-90 FM tuner.
In the tuner, the AGC network was disabled and all stages run at constant gain.
The limiter grid voltage was monitored by a Heathkit IM-25 solid-state vom with
11 megohms input resistance. This monitored DC grid voltage was then essentially
proportional to received signal intensity.
First I made a control run, measuring limiter grid voltage for signals from 14
local FM stations. Later measurements were plotted against this reference run.
Next I physically removed the dipole from its mounting at the end of the
coax, and inserted a 4 1/2-inch stub of wire in the coax plug; I oriented the
wire vertically. The stub represented the Mini−Tenna reduced to the bare minimum. By re-measuring
the same 14 FM stations, and plotting the differences against the reference dipole
readings, I got curve (A) (see graph).
Then I mounted the Mini−Tenna in place of the wire stub - at the same point the
dipole had been. With a 22-volt battery and a bypass capacitor in place, the "active"
curve (B) was obtained. Next I removed the battery and placed a jumper across the
break in the coax shield; this produced "passive" curve (C). Finally, all transistors
were removed from their sockets and a short jumper was placed between the emitter
ring (at the point where the coax ties to it) and the top hat. This gave curve (D),
Fig. 5 - SIA construction uses capacitance of top and bottom
"hats" for wide frequency response.
Fig. 6 - Diagram shows DC and mechanical circuit for the SIA,
but not distributed capacitance and inductance.
As the graph indicates, there seems little difference in the pickup of the
wire stub, the shorted or passive Mini−Tenna. It appears that the mass of metal
in the Mini−Tenna, as well as the unpowered transistors themselves, contribute little
or nothing to signal pickup.
The active Mini−Tenna, however, seems to produce some significant improvement
over the other modes of operation. It hasn't the pickup of a dipole, but it did
produce fully quieted signals on local stations. The Mini−Tenna seems essentially
nondirectional, but this might or might not be an advantage in metropolitan receiving
locations. I did not evaluate this antenna for stereo.
- Thomas R. Haskett
Response curves of Mini−Tenna vs reference dipole.
I found the Mini−Tenna to have no discernible effect (good or bad) on stereo
FM reception at my apartment in Brooklyn. Presumably the broadband character of
the antenna makes it frequency-flat and phase-flat over the necessary range of frequencies,
so that stereo separation isn't affected. However, like all more-or-less omnidirectional
antennas, this one may intercept delayed reflections of signals just as well as
it intercepts the direct signal from the transmitter; hence it may give distorted
reception or poor separation on some FM stereo stations. Signal strength was higher
with the battery jumper open than with it closed (when the battery was not used).
The gimmick had no noticeable effect. A 0.001 μF disc ceramic bypass capacitor
connected in place of the jumper had the same (negative) effect as closing the jumper.
(The reactance of 0.001 μF at 100 MHz is approximately 1.5 ohms.) This suggests
that the gimmick has no significant bypassing effect. I estimate its reactance (100
MHz) at 1000-2000 ohms - vastly greater than the impedance of the battery or that
of any other component in the system. In short, the Mini−Tenna seems to perform
about as well as a small piece of wire.
- Peter E. Sutheim R -E
* See "Major Antenna Breakthrough?" in News Briefs, Radio-Electronics, July 1967.
Posted April 28, 2023