'Magnetic' Transmitting Loop Antennas (STLs)
An STL does not have to be a
compromise antenna. If the losses are kept low, the RF
will be radiated efficiently, even if the antenna is small.
The RF energy applied to the loop has
to go somewhere; it
will not 'cancel' and simply disappear into thin air. It will either be
dissipated as heat or be radiated into space.
To the right is K4PP's small transmitting loop. Note the vacuum
variable tuning capacitor and the large diameter copper tubing.
Bandwidth plots and photos are here.
Most loops like this are mounted about 8 feet off the ground, and
perform amazingly well despite their small size and low height. (See
the EZNEC pattern plots below)
Below are links to some of the best material I've found on the Internet
EZNEC Loop models
Overview of the Underestimated Magnetic Loop Antenna by VK5KLT (PDF)
- The ARRL Antenna Book,
19th edition (first printing), pp. 5-10 par. 1 — 5-19 par. 4
- QST, June
1986 p. 33-36,
"Small, High-Efficiency Loop Antennas" W5QJR
- QST, July
1985 p. 42, "The Loop
Transmitting Antenna" (Technical Correspondence) - W5QJR
- QST, February
1985 p. 44-47,
"Six Winners Emerge from the ARRL Antenna Competition" - KU7G
- QST, November
2011 p. 35-37, "An Antenna Idea for Antenna Restricted Communities"
Fig. 1 - Screenshot of AA5TB's loop calculator*, showing a 6.4 foot
diameter STL (Small Transmitting Loop) made from 1" type M copper pipe:
Octagonal loop above uses 20' of 1" rigid copper water pipe
(1.25" O.D.) and 45° copper elbows; 10 to 100 pf tuning
The data in the table below is from the above loop calculator using
these pipe dimensions.
|3 dB Bandwidth, KHz
* Note: See Owen Duffy's (ex-VK1OD) review of this loop calculator (link above).
How does our low STL compare with a Dipole or a Vertical?
Fig. 2 - 6 ft. square loop 5 ft. high @
7 MHz. Not bad for an antenna only
five feet off the ground!
Fig. 3 - 6 ft. square STL loop 33 ft. high
@ 7 MHz.
Notice that raising the loop from 5
to 33 feet does not increase the gain at low angles, and decreases the
Below are two dipoles for comparison.
Take a good look at the Gain figure at
higher angles for the above low dipoles. Great for close-in
communications, but the gain is still fairly high even at 45 degrees.
Fig. 6a - 67 ft. Dipole 5 ft.
high @ 7 MHz
Fig. 6b - 66' Dipole 25' high @
Below are two verticals for comparison. Take a good look at the
gain figures, and compare them to the low STL and the dipole above. The results might surprise you.
At first glance, it might look like we've improved things
at low angles. However, notice that even though we have greatly
decreased the high-angle radiation, the low angle gain is much lower
earth losses. Verticals come into their own near saltwater, but not
over lossy ground.
Fig. 7a - 33' Ground-mounted
Vertical @ 7 MHz
Fig. 7b - W0BTU 40 meter Ground
Plane 10' high
Fig. 4 - 6 ft. square STL loop 66 ft. high @ 7MHz. Not recommended.
Fig. 5 - Octagonal loop polarization
© 2011-2014 Comtech Research LLC. All Rights
Created August 24, 2011 - Last Edited July 21, 2014