WHY NOT
A LOOP ANTENNA?
As some of you may
know, I am an advocate for wire antennas, and have been experimenting with
horizontal loop antennas for HF work. I am currently running a 160-meter full
wave loop that is suspended about 30-40 feet off the ground. Although the
feedline for my loop is currently coax, in future installations I am considering
the use of balanced, or open wire feedline for reasons of low loss, which may be
the topic of a future article.
One caveat that I
should mention before I start. The majority of my operating, and hence my
observations, is on the lower bands, typically 160 through 40 meters. At these
frequencies, the use of a yagi antenna is often impractical due to size
constraints. If you do operate on 20 meters and up, you may find a yagi a better
choice than the loop for its gain and directivity. That being said, what holds
true for a loop at 160 meters, also holds true at 10 meters, so even those who
operate on the "high bands" should find something of interest here.
The loop antenna has a
long and distinguished history, but is often overlooked in light of the current
focus on dipoles. However, loops do have some rather significant advantages over
a dipole. Since the design of a loop is typically a circle or square form, the
need for a long straight run of wire used by a dipole is diminished. A loop is
quite forgiving, and perfect symmetry is not essential. It is necessary only to
hang it in the configuration providing the greatest enclosed area. As such a
loop can be strung up in unusual places and still perform well. Treetop
suspension is ideal, but you'll be surprised how well it works just lying on the
roof where nobody sees it. Unlike a dipole that must be center fed, the loop can
be feed at any point, allowing a most flexible feedline arrangement.
A
loop is an efficient broadband radiator, even when low to the ground. The
majority of the amateur bands are harmonically related, typically the 1st
harmonic. This is where the loop really shines, as a loop is easily tuned to
resonance on all even harmonics of its fundamental frequency. A dipole by
contrast is easily tuned to resonance only on its odd harmonics. A loop starts
out with 1.2 dB of gain over a dipole on its fundamental frequency, and gains
are even higher if the antenna is less than a quarter wave off the ground
because that is where the dipole efficiency plummets. For example, in my
location my loop is quite close to the ground, only about 30-40 feet up at the
highest point. Nevertheless, the antenna both tunes, and transmits just fine.
A loop's gain over a
resonant dipole increases with the increasing frequency of operation, so when
used on its harmonics, a loop's signal advantage over a dipole likewise
increases. For a horizontal loop, that's not the end of the good news, because
as frequency rises, radiation angle drops lower and lower, producing increasing
DX results that can rival a complex multi-element beam mounted on a 100-ft
tower.
The venerable loop is
easy for your tuner to match, even when fed with coax. The feedpoint impedance
of a loop never gets as high or low as with an antenna that has free ends. Even
a 40-meter loop can offer full 80- 10-meter coverage. My 160-meter loop allows
me to both receive and transmit from the 160-meter band to 10 meters. Signal
reception is also quite good with a loop for a number of reasons. As it is a
terminated antenna, it is much less susceptible to atmospheric and man-made
noise. As the majority of man-made noise is vertically polarized, the horizontal
polarized loop can reduce electrostatic noise as much as 26db when compared to
dipoles or verticals, so it is great for noisy RF areas.
The theoretical
feedpoint impedance for a full wave loop antenna is approximately 100 ohms, but
this does change, and is dependent upon antenna height above ground, near-by
structures, and ground conductivity. When used with a 2:1 balun this presents a
good match to the typical 50-ohm coax. Due to the low height above ground of my
loop, I used a 4:1 balun which seems to offer a wider tuning range (lower Q).
With the use of a transmatch (antenna tuner) I am now able to use my loop from
160 to 10 meters.
Loops can be either a
1/2 or full wavelength long. The formula for a full wave loop antenna is as
follows: Length (feet) = 1005/fMHz. For example, a loop for the frequency of
3.800 MHz would be calculated as follows: 1005/3.8 = 264 feet. You can now
divide 264 by 4 to obtain the length of each of the four legs of the loop. 264/4
= 66 feet each leg.
So next time you need
a new antenna installation, consider the time-honored loop. Simple and
inexpensive to homebrew, you can put the money you saved toward something really
important...like a trip to this years Dayton Hamvention!
-Bruce KG2IC
