Horizontal Loop Loading Methods

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Note: None of the problems below occur when
the loop is on a band where the actual antenna is a full wave or larger.

The primary radiating area of a half wave loop is about 1/8th wavelength. We are
essentially end-loading a 1/8th wave long dipole when we use a 40 meter full
wave loop on 80 meters.

On 40 meters a full wave loop is like
end-loading two 1/4 wave long dipoles, so there is a full half-wave of total
radiating area and efficiency is high. Folding the antenna into a square kills
80-meter efficiency because the square is small, the sides out-of-phase, and so
only one side (the feedpoint side) radiates. One little mistake by adding a
small amount of resistance and efficiency falls into single digits.

In a recent QRZ thread, it became apparent some transmission line
calculations in models are not accurate. This actually is true for many
transmission line calculators, not just those in modeling programs. The poor
transmission line loss calculators are reinforced by articles that contain bad
measurements, like a
2009
article on ladder line loss.

Let’s look at a few 40-meter horizontal loops with an open circuit loop as a
reference, and compare it to others loaded by a 1/4 wave transmission
line stub. We can also see how built-in transmission line calculators compare to a
known good feed line loss calculator. One of the best calculators I’ve found is at VK1OD

http://vk1od.net/calc/tl/tllc.php

What makes a smaller loop so difficult to tune correctly?

Radiation occurs from charge acceleration. This means we must
have current flowing over linear (in line) spatial distance without something
else nearby opposing that radiation with an opposing current. When a loop is
full size, it has two areas that can freely couple to space:

Section 1 is the feedpoint, and all areas of section 1 can
radiate. Sections 2 and 4 have opposing currents inside each section, and with
each other. They almost cannot radiate at all.

A full-wave loop looks like two 1/4 wave long dipoles fed in
phase (currents support each other in radiating) while the sides suppress most
radiation.

At half frequency, where the loop is 1/2 wave perimeter, all
sides fight against each other. This means the antenna needs more current to
radiate the same power, which also means radiation resistance has greatly
decreased. The antenna needs much more current and more voltage to radiate.

When we try to “load” this loop, we can only move the current
maximum around. We cannot change the phase relationship. Without adding sections
or protrusions that carry common mode currents, we can only change where the
maximum and minimum of current and voltage occurs.

With no loads the loop is resonant, but current maximum is in
the middle of the side diametrically opposite the feedpoint. Feedpoint
resistance is terribly high, many thousands of ohms. If we only have a
moderately high feedpoint resistance with a closed 1/2 wave loop, the antenna
has high loss.

If we open the loop exactly opposite the feedpoint, the loop
behaves like a bent dipole. There is very high voltage across the gap, and much
higher than normal currents at the feedpoint.

The feedpoint now has highest current and lowest voltage,
making it a low impedance. Most radiation comes from the short area in wire 1,
only 1/8th wavelength long. Sides 2 and 4 tend to cancel each other because they
have the same currents flowing opposing directions. Side 3 has very little
current and as such cannot usefully contribute to radiation.

We essentially have a 1/8th wave long dipole, and have no real
way to make it better without extending the middle of sides 2 and 4 further
apart without shortening sides 1 and 2! In other words to increase
radiation resistance, the loop has to be
made spatially larger.

If we have a loop like this with a high feedpoint resistance,
over 10-20 ohms, the antenna has very high loss.

Reference loop, 40-meter full wave loop 36-feet high over
medium soil, opened opposite feedpoint to make reasonable feed impedance on 80
meters.  The
problem with relay-switching the loop open is, with only 100 watts, relay
contact voltage is 5000 volts peak. High voltage and high
current is a direct result of having a small radiating area (folding the antenna
in a square).

Stub decoupled loop (phosphor bronze) using real open wire
line 3 inch spacing #18 AWG

stub is modeled as two parallel conductors

(Efficiency a few percent higher using copper)

SWR plot of stub-switched loop. Resonant frequency 3.510 MHz

3.5 MHz using built in-transmission line loss calculator with
values for Wireman 551 450-ohm line:

3.5 MHz Wireman 450-ohm ladder line using VK1OD calculator values
inserted as a load:

There is 8.1 dB difference between the built in transmission line calculator
and the external more accurate VK1OD calculator.

Look like not much problem on higher bands, except SWR bandwidth:

14 MHz with transmission line in EZnec

VK1OD RF Transmission Line Loss Calculator 14 MHz