Loops - Part 1

Monitor the MERA 145.47 repeater for awhile and you will not only notice
that antennas are a very popular topic of conversation, but that the most
talked about antenna is the loop the most versatile antenna ever designed.


What's a loop?

For starters, the loop is probably the most versatile antenna ever designed.
Most of the time that a loop is mentioned in these repeater conversations it
is in reference to a 1 wavelength long or "fullwave" loop antenna.
Geometrically, it is a circular, square, or triangular shaped (Fig. 1) piece
of wire that is 1 electrical wavelength long at the design frequency.
The physical length, of course, will vary based on this design frequency
(Fig. 2).

Unlike many other antennas, the loop can be easily made to be a local QSO
gatherer (with a high angle of radiation) or a DX QSO collector (with a
low angle of radiation) just by changing the location of the feed point
(Fig. 3).  The loop has two very popular shapes a square or "quad" and
a triangle (more commonly called a "delta loop") which is named after,
"Delta", the fourth letter of the Greek alphabet, and is the shape
the loop takes.

If you take the quad (square) loop and pull it so that 2 sides become
longer than the other 2 sides, you will eventually end up with a folded
dipole.  Actually, the fullwave loop was initially designed in the inverse
order of this by starting with a folded dipole and pulling it out to look
like a square.

There are advantages to each of the geometric shapes which will unfold as
the article goes on, but let's look at some of the operational advantages
of the 1 wavelength loop.


Loop Advantages

A loop will provide an increase in gain over a dipole or a vertical 
monopole and requires a similar amount of physical space.  Both 
horizontal or vertical polarization and therefore, high or low angle 
radiation, can be attained just by changing the coax or ladder line 
feed point location.  On receive, loops provide more signal capture 
area and are inherently a lower noise antenna than similarly polarized 
dipoles or monopoles.

Loops have a low Q factor which means broad banded operation and hence
a greater useable "SWR" bandwidth.  Loops often cover the CW and phone
portions of the band for which they are cut.  They are very easily
multibanded with the addition of a tuner, and the feedpoint impedance
excursions seen from harmonic operation are much less than the very large
impedance excursions of a center fed dipole operated on its harmonics.

Loops can be mounted vertically, horizontally, or sloping.  One delta loop
configuration requires only 1 tall support.  If necessary, they can be
loaded to be physically smaller if you are really pushed for space.
Sometimes they can be loaded without affecting the loop's performance
(see the Delta Loops article).  Multiple loops can be used parasitically,
or phased to attain more forward gain or directional characteristics.


DX Workhorse

Vertical plane full-wave loops are DX antennas even with being close to
the ground.  One main factor for this is the fact that with a horizontally
polarized quad loop, the horizontal elements are stacked on top of each
other, separated by ¬ wavelength and therefore automatically phased
(Fig. 4).  Another factor is due to the fact that the "closed" circuit
configuration provides no high impedance points to couple to noise and
the 1 wavelength piece of wire provides a larger capture area. 


Space Requirements

Loops use twice as much wire as a dipole, but surprisingly take up about
the same amount of space as a dipole since they are a folded, 3-D type of
antenna.  Loops will also utilize this space more efficiently from a
performance point of view.


Loop Books:

 The Quad Handbook by Bill Orr
 The Low Band DX'ing Handbook by ARRL


Why Do Antennas Work?

Ham transmitters generate a carrier frequency signal, modulate it with
information, amplify it and send the signal on its way down the coaxial
transmission line.  The signal is an alternating current or AC voltage
varying sinusoidally in amplitude at a "radio frequency within the
amateur bands."  At the other end of the transmission line is the antenna,
the "transducer" that converts this RF electrical energy to electromagnetic
wave energy.

What causes this electrical to electromagnetic energy mode conversion at
the antenna?  The key is one physical property of all AC (even 120 volt
house power).  AC generates an electromagnetic energy field that influences
the immediate area surrounding the wire which the AC current is flowing
through.  Higher frequency AC in the kHz and MHz radio frequency range,
produces an energy field of electromagnetic waves which travel away from 
the antenna wire and never return to the wire, like the waves from a pebble
tossed into a still pond.  These electromagnetic waves will travel through
the atmosphere until being captured at a receiving antenna wire and 
converted back into electrical energy and sent down the transmission line 
to a radio receiver in the shack of a fellow ham listening to our words of 
wisdom.

Next: Details on the "Delta" Loop and building your own loop.

                      [See figures]