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Those Pesky Laws of Physics:
Fans of the
original Star Trek series will remember Chief Engineer Scott telling Captain
Kirk "Ya canna change the Laws o' Physics, Cap'n." Scotty would no doubt be
shaking his head over some of the portable HF antennas on the market today.
We'll explore just a very little bit of how antennas work. If you'd like to
know more on the subject, the ARRL Antenna Book is a great overall resource,
tutorial, and
reference. Antennas are not magic. They obey the laws of physics, despite the
claims of some designers and manufacturers.
Transmitters are designed to operate into a specific load -- typically 50 ohms.
When they are connected to a 50-ohm load, they deliver their rated power
efficiently. For this reason, most feed lines used with today's rigs are also
designed with a 50-ohm impedance. When the feed line is connected to a 50-ohm
load, all of the power going into the feed line is delivered to the antenna
(minus a small amount of loss in the feed line). If the feed line and load
impedances are not the same, some of that power is reflected back down the feed
line. This creates a "standing wave" of voltage and current peaks and nulls on
the feed line and increases its losses. The ratio between the voltage or
current maximum and minimum points is known as the standing-wave ratio (SWR). If
the line is matched to the load, there are no peaks or nulls, so the SWR of that
line is 1:1. Older transmitters with vacuum tube final amplifiers had a series
of controls to match their output circuit to whatever impedance was presented by
the antenna system. Modern rigs have solid state final stages, designed to
operate into a 50-ohm impedance. Operating into a serious mismatch can cause
heat to build up in the final transistors, possibly damaging or destroying
them. For this reason, most modern transmitters employ a "foldback" circuit
that will (hopefully) reduce output power to a safe level before damage occurs.
When an antenna presents something other than a 50-ohm load, therefore, some
type of matching network is necessary to keep the transmitter "happy". This is
accomplished by putting some
combination of inductance and capacitance between the source and the load. To
further complicate matters, the impedance of a given antenna varies as the
frequency changes. Enter the antenna tuner, sometimes called a transmatch or
matchbox. Tuners provide a wide range of combinations of inductance and
capacitance, allowing your transmitter to see a 50-ohm impedance with an antenna
that may be quite a bit higher or lower.
The MinuteMan 20™
allows you to adjust the antenna itself for the best match at your operating
frequency -- in seconds. Changing the whip length and coil tap positions will
allow a wide range of adjustment. Rather than using a matching circuit in the
transmitter, or an external tuner, you're able to adjust your antenna for best
efficiency.
Designing a load
that presents a 50-ohm impedance to your transmitter over great excursions in
frequency is actually easy. Several companies make exactly that. They're
called dummy loads, and are useful for many tasks in the shack. But of course,
they make poor antennas. Simply put, nearly all of your power is dissipated as
heat by the load and almost none is radiated as RF. Sure, the SWR is great, by
definition. But now you should understand that a low SWR, by itself, is not
necessarily indicative of good performance. You must also consider an antenna's
efficiency.
Some so-called portable antennas on the market are little more than portable,
nicely manufactured dummy loads.
In simple terms,
antenna efficiency is the ratio of the power actually radiated as RF by the
antenna divided by the total power applied to it. If you apply 100 Watts of RF
energy to an antenna and 95 Watts is radiated as signal then 5 Watts is
dissipated as heat in either the antenna itself or in ground loss. The
efficiency is then 95%. According to those pesky Laws of Physics, there's just
no other place for that energy to go. As heat loss increases, though,
efficiency goes down, until the antenna becomes, in effect, a dummy load.
At Radio
Frequencies, loading coils act in part like resistors, converting some part of
your transmitted power to heat -- power that, of course, adds nothing to your
signal. When designing compact antennas, some type of loading is necessary.
Therefore the properties of the loading coil are of paramount importance. Since
any loading coil presents at least some degree of loss, we designed the
MinuteMan 20™ to use as little coil as possible. And we designed our coil to
maximize its efficiency.
The efficiency of
a coil increases greatly as its diameter increases with respect to length.
Ideally, a coil should be "oversquare" -- that is, greater in diameter than it
is in length. The MinuteMan 20™ coil is a full 2 inches in diameter, and only
about an inch in length. This 2:1 "aspect ratio" is one of the factors that
makes our antenna so much more
efficient
than other designs. Long thin coils, on the other hand, degrade efficiency.
Your transmitter will still see an acceptable impedance (low SWR) but a lot more
of your signal will be lost as heat rather than being heard by that rare DX
station.
So do you need a
tuner with the MinuteMan 20™? In most cases, you’ll be able to get better than a
2:1 SWR on the 20, 17, 15, 12, and 10 meter bands using combinations of whip
length and coil tap positions. In some instances, especially when there are
nearby metal objects, you may wish to use a small tuner. These are available
from several manufacturers. A tuner may also allow you to operate on lower HF
bands. Since each installation is different, we won't guarantee it -- but give
30, 40, and even 80 meters a try. With the right conditions and tuner, you may
have many enjoyable QSOs there -- just don't expect the High Performance that you
get on the other bands with your MinuteMan 20™.
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