Practical Amateur Aerial Design
July 1934 Radio News and the Short-Wave
of the nice things about antenna design articles is that regardless
of when they were written, all that is needed to make them entirely
contemporary is to substitute a transistor schematic symbol for a vacuum
tube and substitute the units "kHz" and "MHz" for "kc" and "mc," respectively.
If the article delves into detailed circuit design, a substitution of
"μF" for "mf" might also be required and depending on the frequency
range, "pF" for "mmf" or "μμF." Some readers might feel the urge to
replace SAE units with metric units, but even a hundred years ago there
were people who needed to do that. "Aerial" and "antenna" are still
interchangeable in modern radio parlance. With that in mind, please
enjoy this 1934 article on basic antenna design.
July 1934 Radio News & Short-Wave
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Practical Amateur Aerial Design
The one big job for an antenna is to "soak up juice" from the transmitter
and radiate as much energy as possible into space. An antenna must be
resonant and should be properly coupled to the power source
A. H. Lynch (W2DKS)
E. Glaser (W2BRB)
The type of antenna system to be used for amateur short-wave work is
of secondary importance and depends largely upon the individual location
and the frequency bands to be covered in regular operation. Some types
are easily adapted for multi-band work while others are suited to only
a single frequency band. In general, the latter types are somewhat more
efficient but not obviously so. The object of this article is to bring
together some practical information to help in choosing the most suitable
antenna system for particular cases.
Better efficiency usually
results in using a Hertz antenna (1/2-wave wire in free space) than
when using the 1/4-wave Marconi type which must have a good ground system
or a large counterpoise. (In this latter type every inch of wire in
the system radiates energy, so that a rather small percentage is radiated
by the uppermost parts of the antenna which do the business.) The horizontal
Hertz, on the other hand, is entirely up-in-the-air so it becomes obvious
why the efficiency is better. And, of course, this applies equally well
to signals coming in. Antenna systems for single-waveband operation
will be first considered.
This simple half-wave antenna diagrammed
in Figure 1 is a split doublet, each side being one quarter wavelength
long. The length of each half equals 0.78 times the wavelength or 468
/ (2 x frequency [mc.]).
It is ideal for portable work at any
frequency and does a first class job for both transmitting and receiving.
The transmission line used to couple this doublet to the transmitter
or receiver need not be tuned and can be of any length provided its
impedance is correct. Forms of rubber-covered, twisted lamp cord provide
a suitable impedance match.
"Giant Killer" feeder wire is intended
especially for this purpose. Its impedance is slightly higher than that
at the center of the antenna (about 75 ohms) so that it should be fanned
somewhat when connecting to either side of the center insulator to be
theoretically correct. This type of antenna is suited only to the waveband
(fundamental) for which it is cut or any odd harmonic thereof. On even
harmonics a voltage (and impedance) maximum appears at the center and
cannot be matched to (is virtually short-circuited by) a 75-ohm line.
When used for receiving, the line will have no pickup and, therefore,
a maximum signal-to-noise ratio will result. The line is magnetically
coupled to the receiver by means of a small primary coil; to the transmitter
across a few turns of the last plate-tank coil or coupled same as receiver.
Bear in mind the necessity for sufficient insulation in the twisted
line when used for transmitting.
Another method of feeding a
half-wave Hertz is to use a spaced transmission line fanned at the antenna
so that the impedance of the line equals that of a part of the antenna
across which the line is connected. See Figure 2. The length of this
antenna and the distance between the wires are quite critical. The length
equals 463 / (frequency [mc.])
The distance (S) equals one-quarter
the antenna length for a 600-ohm line, and decreases as the line impedance
goes down. See table in Figure 2. A transposed line may be used and
is particularly desirable for reception. The line should be perpendicular
to the antenna for at least 1/4 wavelength and should be terminated
in such a manner at the transmitter so that it is balanced to ground.
Transposition blocks make an easy and efficient job of the feeder. In
putting up this type of antenna, the wire should be several percent
too long and should be cut about one percent at a time, observing the
performance after each cut. Fixed coupling to the transmitter (or an
oscillator) should be used so that the loading effect of the antenna
might be seen on the plate (or grid) milliammeter. When the doublet
length is correct, it will have a maximum load effect.
type of antenna for single-band operation is the single-wire feeder
affair shown in Figure 3. The length of the antenna is roughly the same
as the previous type but had better be determined by experiment, as
previously explained. The feeder should be connected one-seventh the
length off the center and must run at right angles for proper operation.
There must be no sharp bends in this feeder or there will be reflection
losses and line radiation. This type is less suitable for reception
than either of the foregoing systems because the line is not so free
In the foregoing antennas, properly designed and
constructed, there should be no radiation or pick-up on the feeder and
any length at all may be used. If no radiation or pick-up takes place
in the feeder, the half-wave antenna is doing all the work, which is
exactly what we want. And in this case there is a marked directionality,
best transmission or reception being at right angles to the line of
the antenna. In adjusting these antennas to the transmitter, the plate
meter must be used as the antenna current in the feeders is very low
and is not necessarily a true indicator of power taken by the antenna
proper. Start with a minimum of coupling and gradually bring it up,
always retuning the plate-tank condenser, until the tube is properly
loaded. It is possible to use any of these types as a Marconi antenna
for lower frequencies, tying the two feeders together. In a pinch this
means may also be used on odd harmonics of the Marconi quarter-wave
fundamental but, at best, is a makeshift job.
antenna of Figure 3 may be used for all amateur frequencies but does
not perform as well on harmonics as on the fundamental frequency. This
is because there is a mismatch between antenna and feeder on harmonics.
It is possible to compromise and improve harmonic operation but usually
at the expense of fundamental efficiency. The feeder is usually moved
further from the center of the antenna. The feeder radiates, standing
waves appear and there are losses all around. Nevertheless, many hams
like this method because of its utter simplicity and obtain good results
with it. It should be cut for the lowest frequency band to be used or,
if this demands too great a stretch. Marconi operation may be used on
the highest wavelength, the antenna being cut for one band lower.
A second all-band antenna may be procured by simply cutting
any single wire (plus lead) into a half-wavelength for the lowest frequency
band to be used. Operation with a ground may also be used as above.
This is really a voltage-feed type with a feeder of zero length. The
end of the antenna is plenty "hot" and should therefore be hung close
to the plate on the tank coil. A separate tank of low capacity (to reduce
losses) may be coupled to the plate tank and the antenna hung on to
that. A ground is sometimes used at the other end of this coupled circuit.
However, some losses are bound to appear as the antenna comes into the
shack and into the vicinity of other apparatus. Not only that but there
is a strong field around the antenna which may be a nuisance - affecting
neutralizing, paralyzing the receiver, etc. Of course, the entire length
of wire radiates so that a lot of energy may be wasted before it gets
to the high part of the system. The length should be adjusted experimentally
as with previous systems because the antenna proper, coming into the
shack, is subject to all kinds of influences which might affect the
fundamental, or natural frequency. This antenna is equally good on all
harmonics because the ends of an antenna are always maximum voltage
points (Figure 4).
A third type, permitting many-band operation
is the old, reliable "Zepp," (Zeppelin) which is about the easiest antenna
to resonate because a certain amount of tuning may be done right at
the transmitter or receiver. See Figure 5. A half-wave antenna is used
with quarter-wave feeders. This might be considered a full-wave affair
with half a wavelength "folded," so that a point of maximum current
comes right at the feeder end. This is a true current-fed antenna, the
losses appearing in the previous voltage-fed system being entirely absent
here. Although the feeders must be a quarter-wavelength long, electrically,
much leeway is possible by loading or cutting with parallel or series-tuning
condensers. When multi-band operation is desired, a compromise in length
must be made to accommodate tuning to the various harmonics, the most
reasonable length being a little under three-eights wavelength. This
is not at all critical, compared to the chopping of the flat tops for
this family of antennas, but should be quite close. This limitation
of feeder length which appears here for the first time in this series
of antenna systems, might be worse were it not for the fact that the
feeders may be bent or folded to add length. Again, the half-wave flat-top
might well be cut too long and chopped although this is less important
with the Zepp than with most of the other types. Most hams do not get
a proper balance of feeder currents so the feeders do some radiating.
Even so, the results obtained usually justify this old favorite. It
is because some leeway is possible that they don't take the necessary
pains to do n perfect job. This is really an unbalanced system, but
it does a lot toward improving the signal-to-noise ratio, especially
when transposed feeders are used. However, in our opinion, the following
antenna is really the ace of all wave receiving aerials.
6 shows another very flexible system. this time a balanced "split" arrangement
that resembles the first shown doublet, but which is really a one-and-a-half-wave
antenna with two half-wave parts, folded as feeders. Since a current
maximum exists in the center of the antenna we need half-wave feeders
this time, to get back to another current maximum for current feed.
Again, compromising in feeder length for the sake of the harmonic family,
the feeders should be the same length as in the Zepp. This will allow
tuning to all bands in a similar manner. In both this type and the Zepp,
the full voltage of the antenna appears on the feeders, so that the
spacers must be first-class insulators and of low dielectric constant.
When a transposed line is used this antenna is excellent for reception.
although tuning must be used! This is the most versatile aerial of the
whole bunch for multi-band use and will cover a large part of the short-wave
To correct a common fallacy, solid wire has not the
lowest high-frequency resistance but, rather, a sort of cable made of
insulated wires, tightly twisted together, presents a much lower resistance
than the equivalent solid wire.
Posted August 13, 2013
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