September 1973 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
"Does a choice transmitting antenna
always constitute the best receiving antenna?" That question alone, by author Carl Eller,
piques my interest and gives confidence that this article is the type I have long said is
lacking in the amateur antenna system design realm. Most treatises, it seems, address only
the transmit half of the operation. Just as with the old aviation adage that with enough power
you can make a brick fly, in the non-professional world the attitude is often that a really
crappy base antenna / transmission line / power amplifier mismatch is fine as long a tuner
is capable of brining the VSWR into an acceptable range (2:1 or better is preferred). That
translates into a lot of loss from the sky to the equipment. All loss (attenuation), whether
resistive or reactive, adds directly to the noise figure of a receiver, and therefore to the
reduction of minimum discernible signal (MDS) level. Radiating peak envelope power (PEP)
from your antenna so that someone on the other side of the world can hear you does not do
much good if your antenna-to-receiver loss is so great that the response signal gets buried
in the noise. Part 2 appeared in the October issue, which I do not have yet.
Antennas for CB'ers and Hams
Clearing away half truths and superstitions
By Carl C. Drum Eller, W5JJ
Fig. 1 - Parameters for a normal mode helix. You can calculate various
dimensions for the 27-MHz CB using 36.3 feet for lambda. Antenna is used for long-range communications
at low frequencies. It is easier to resonate by external tuning than a monopole antenna of
the equivalent height.
If radio amateurs and CB'ers devoted as much thought, time, and money to their (largely)
nondirectional high-frequency antennas as they do to their directional beams, there might
be a much wider variety of both single- and multi-band antennas. Most of us, though are content
to use purely conventional horizontal dipoles, inverted V's, or ground-plane, quarter-wave
verticals. These, as a rule, give satisfying results - perhaps because we've been conditioned
to think of such antennas as standards of comparison.
Once in a while, however, some adventurous soul chooses a rhombic or a long-wire V - some
even try disc-cones or fat monopoles. Such departures from the norm often are quite rewarding.
Their users discover the convenience of having an antenna that's truly effective over a wide
band of frequencies, one that achieves this advantage without tuned feeders or other auxiliaries.
Then there are those who, by force of circumstance, are driven to the use of restricted-space
antennas. These antennas are approached with trepidation since both theory and popular supposition
hold them to be nearly useless because of low efficiency or narrow bandwidth.
Portable and emergency operations present another set of problems. Just how much like the
"ideal" antenna must a radiator be to put out an effective signal? Is there danger of damage
to one's transmitter from using unconventional antennas?
Questions. What are the facts? Is it true that there is no radiating system
that combines good efficiency, wide bandwidth, and reasonable space? How vital to acceptable
operation are such factors as careful design, meticulous attention to the impedance match
between feeder and antenna, and supporting the antenna as high as possible? Does a choice
transmitting antenna always constitute the best receiving antenna? Is the buried wire antenna
a myth or a reality? What about loop antennas for receiving and for transmitting? How do the
small ones compare with the large ones? How important is capture space? Does a wire have to
be stretched out, or can it be coiled and still retain good radiation characteristics? Just
how vital is a good ground to the performance of a Hertz antenna? A Fuchs? A Marconi?
What about feedline losses in h-f antenna systems? How does one use a counterpoise? Does
it have to be under the antenna? Does one use a single wire or many? How high above ground
should it be? Should a ground connection be used in conjunction with a counterpoise? If so,
how should it be connected? And what do you do when you're caught in a situation where you
can't use one of the conventional antennas? Just what can you use as a substitute? Can you
expect any truly useful radiation from unconventional radiators?
Well, we are not going to provide full answers to all of these questions. But we will review
the opinions of some of the better authorities on the subject and hopefully, we will stimulate
your thinking about some ideas that deviate from the humdrum and timeworn traditions.
Let's think first about that ideal combination: good efficiency, wide bandwidth, and small
size. No really good solution exists to the simultaneous achievement of all three of these
goals. However there are some ways of approaching the ideal.
Fig. 2 - Cage type of multi-wire dipole. Though a two-wire feeder is shown,
this antenna can be adapted to conventional 52-ohm coaxial feedline.
Normal Mode Helix. One approach, which lies within the constructional
abilities of the average amateur, is the "normal mode helix" (from Sylvania Antenna Reference
Index, Sylvania Electronic Systems). Used as a vertical radiator and worked against a ground
plane or a good ground, it has a nondirectional horizontal radiation pattern, and the vertical
pattern is of a reasonably low angle. By varying the dimensions, the polarization can be made
either elliptical or circular. As compared with an isotropic radiator, the gain is 4.76 dB.
Bear in mind that a conventional dipole has only 2.15 dB gain. The bandwidth of 5% is not
too impressive, but the helix is easily resonated by external tuning units. This gives promise
of extending the bandwidth substantially without too much drop in efficiency.
There are whispers to the effect that the use of a ferrite core in the helix performs wonders
in retaining efficiency while making possible drastic reductions in overall dimensions. Other
whispers say that getting a type of ferrite usable for high power and high frequency poses
limitations. In each case, the probable definitions of high power and high frequency are such
that a kilowatt of dc input power and a frequency range of 1.8 MHz to 30 MHz are wholly feasible.
Why do we say whispers? There is reason to believe that the findings of research in this field
are classified - after initial and excited announcements, all reference to the subject disappeared
from the technical literature.
Figure 1 shows the form and dimensions of the normal mode helix. The wire or tubing used
to make the helix should be heavy enough to hold its form with a minimum of support. Of course,
it should also have a low skin resistance. Polished copper protected from corrosion by some
type of weather-proof varnish is quite acceptable.
Broadband Dipole. Although they take a lot of space, variants of the prosaic
half-wave dipole offer exciting possibilities. They can be really broadband if you get rid
of the concept of a thin-wire radiator. Figure 2 shows one broadbanding possibility. The use
of the multi-wire radiator transforms the dipole from a narrowband device to one that is good
for at least two adjacent bands plus everything in between.
The multi-wire flat-top, flared from a common feedpoint, is easy to build but a bit awkward
to keep in position in order to maintain a constant capacitance to ground. This may require
as many as five supports - one at the center and one at each end of the two spreaders. Research
indicates that, for the greatest bandwidth, the wires of the flat-top should be of unequal
length, approaching a quarter-wave (per half of the dipole) for the lowest and the highest
frequency included in the design. In all cases, the wires must terminate in an insulator;
do not interconnect them at the far end.
Although it is more trouble to construct, the cage antenna, another old but sadly neglected
design, presents no problem in maintaining a constant capacitance to ground. Usually, it is
built with conductive circular spreaders every few feet of its length, with the middle (at
the feedpoint) and the far ends tapered to a point for ease in attaching to an insulator.
There is no verified opinion as to whether better performance would be obtained by using nonconductive
spreaders and terminating each wire with an insulator. Such a practice, however, is employed
in the "fat" monopoles used extensively by the military (Fig. 3). This type of monopole has
a diameter at its broadest point that is an appreciable fraction of its height. In this configuration,
the bandwidth is slightly over three to one! Centered at 7 MHz, it would be quite effective
at 3.5 MHz and 14 MHz, as well as every frequency between.
In such applications, using a "fat" conductor (usually multi-wire) has an effect on the
antenna's radiation resistance, reactance, length for a given frequency, and ohmic resistance.
Each of these effects is a desirable one! The total effect on the feedpoint impedance is not
so drastic as to interdict the use of the popular 52-ohm coaxial cable as a feeder; it really
more nearly approaches a true match (not that this is of any great importance). The amount
of reduction, from the formula 492/f (where f is in megahertz) for computing the length of
a dipole, depends on so many factors that it is best arrived at by experimentation. Fortunately,
the antenna is so broadband that a near-miss approximation probably will suffice. In the matter
of reduction of ohmic resistance, a small improvement in overall efficiency will be obtained.
Other than in exceptional instances, the ohmic resistance of an antenna is not of paramount
Fig. 3 - Shown here as a vertical, rotating the dipole to horizontal plane
swaps the radiation patterns and changes polarization to linear horizontal. Structure need
not be solid as a cage retains same characteristics. Note small change in radiation resistance
for large change in diameter.
Impedance Matching. Impedance matching, that interest-provoking interface
between feeder and antenna feed point, is so hedged in by what is half truth and half superstition
that one hesitates to discuss it for fear of affronting the pietistic convictions of some
devotee of a particular school of belief.
Really, though, it is a simple matter. Basically, any generator delivers maximum power
to its load when that load matches the internal impedance of the generator. If they are not
matched initially, we make them match by some means of impedance transformation. Take the
audio output of a receiver as an example. If the amplifier is a vacuum tube, it may need a
load of 4000 ohms. The speaker offers only eight ohms. So, we use a transformer to effect
the impedance transformation. There is no sacrosanct law that says the matching transformer
has to be mounted on the speaker; it may be in the receiver. Now, have you ever heard of an
audiophile demanding an eight-ohm cable to feed the audio from the receiver to the speaker?
The very same situation and considerations are involved in feeding r-f energy from the
active device (vacuum tube, transistor, Gunn diode, or what-have-you) in a radio transmitter
to its load, the antenna. As it's highly improbable that there is a natural impedance match,
you use some form of impedance transformation. A bit of this is built into the transmitter.
Because of cost considerations, manufacturers make a sharp limitation in the flexibility of
this built-in feature; so you may have to supplement it with an external device.
For example, suppose that you have an antenna operating in the high-frequency range (3
MHz to 30 MHz) and you are feeding it with RG-8/U cable. The antenna is constructed so that
its feedpoint impedance is 12.8 ohms. This would result in a 4:1 VSWR on the feedline if no
corrective measures were taken. But corrective measures can be taken: such as, using an impedance
matching device similar to the Drake MN-4 or the Johnson Matchbox on the tower supporting
the feedpoint. The device can be adjusted to transform the 12.8-ohm impedance into a resistive
load of 52 ohms. Then the RG-8/U will have no standing waves along its length and the transmitter
will load into it quite readily.
But suppose you don't like the idea of climbing the tower every time you want to shift
operations to another band? You can connect the RG-8/U directly to the dipole (using a 1:1
balun if you like) and move the impedance-matching device down next to the transmitter. The
transmitter will not know the difference because it is still working into a 52-ohm load. But
what about that length of RG-8/U, which now has a 4:1 VSWR? This is no problem since the only
thing bad about it is that a very tiny portion of your r-f power will be dissipated in heating
parts of the coaxial cable. Since you're operating in the h-f band, these heated spots will
be at the current maximums. On the 3.5-MHz band, these would be only at 132-ft intervals;
and how long is your feeder? Even on the 29-MHz band, you'd probably have only two or three
spots where a few milliwatts would be wasted in heat.
It's very difficult to convince many people of these simple facts. They have been brainwashed
by those who have not made a careful study of the subject. Collins Radio Company, in its publication
"Engineering Compendium, High Frequency Antennas," (which we recommend) states, with regard
to reflected power, "It is important to appreciate the fact that reflected power does not
constitute a system loss." Similar statements appear in other well-edited and authoritative
publications, yet the myth of reflected power loss dies slowly.
To ascertain the nature of power in the reflected wave and its relation to power in the
incident wave in a mismatched transmission line, an experiment was conducted. A transmitter
operating on 7 MHz was connected through an r-f wattmeter and a length of RG-8/U cable to
a 52-ohm resistive load. The transmitter was adjusted to provide a measured output of 100
watts; its de power input was noted and logged. Then the transmitter was connected through
an r-f wattmeter to an impedance-matching network, through it to a forward-and-reflected-power
meter, and on through RG-8/U cable to a 12.8-ohm load made up of four 52-ohm resistive loads
The transmitter was loaded to 100 watts power output, indicated both by the r-f wattmeter
and by the same dc power input as before. As this loading was being done, the impedance-matching
device was adjusted to reflect a 52 ohm resistive load to the transmitter. Between the impedance-matching
device and the cable, the forward r-f power meter indicated 120 watts of forward power. This
would make it appear that a purely passive device was generating 20 watts of r-f power. Switching
the meter to read reflected power resulted in an indication of 20 watts, precisely the amount
of the extra forward power. If stronger proof were needed of the fact that reflected power
does not constitute a system loss, one could insert an r-f ammeter in series with the feedline
at the junction with the 12.8-ohm load. Its reading, squared and multiplied by 12.8, would
indicate a full 100 watts of r-f power delivered to the load. Such verification was made in
a prior experiment.
In the second and concluding part of this article we will discuss other antenna types and
Posted September 8, 2017