Small 'Magnetic' Transmitting Loop Antennas (STLs)

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K4PP's small transmitting loopAn 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 on STLs.


Links, References, and Models

EZNEC Loop models



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:

Loop spreadsheet screen capture

  Octagonal loop above uses 20' of 1" rigid copper water pipe (1.25" O.D.) and 45° copper elbows; 10 to 100 pf  tuning capacitor.
The data in the table below is from the above loop calculator using these pipe dimensions.

Frequency, MHz
1.8
3.5
7
10.1
14
18
21
24
28
Efficiency, %
(Signal decrease)
1.514
(-18 db)
13.6
(-8.7 dB)
64
(-1.9 dB)
86.5
(-.63 dB)
95.3
(-.2 dB)
97.98
(-.09 dB)
98.8
(-.05 dB)
99.3
(-.03 dB)
99.5
(-02 dB)
3 dB Bandwidth, KHz
1.47
2.34
7.95
25.5
85.5
227
417
709
1309
Tuning capacitor, pf
1489
394
98
47
25
15
11
8.4
6
Tuning capacitor voltage
@1500 watts
10,441
16,101
17,464
14,068
10,648
8,398
7,229
6,339
5,442

* 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!

EZNEC 6ft loop 5ft high 7MHz.gif EZNEC 6ft loop 5ft high 7MHz Elev.gif





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 NVIS radiation.

EZNEC 6ft loop 33ft high 7MHz.gif EZNEC 6ft loop 33ft high 7MHz Elev.gif




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.
EZNEC 67ft dipole 5ft high 7MHz Elev
Fig. 6a - 67 ft. Dipole 5 ft. high @ 7 MHz

66' dipole 25' high @ 7 MHz
Fig. 6b - 66' Dipole 25' high @ 7 MHz


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 due to earth losses. Verticals come into their own near saltwater, but not over lossy ground.

33' ground-mounted vertical @ 7 MHz
Fig. 7a - 33' Ground-mounted Vertical @ 7 MHz

40 meter ground plane 10' high
Fig. 7b - W0BTU 40 meter Ground Plane 10' high






Fig. 4 - 6 ft. square STL loop 66 ft. high @ 7MHz. Not recommended. :-)

EZNEC 6ft loop 66ft high 7MHz 3D.gif




 
Fig. 5 - Octagonal loop polarization vs. direction.

EZNEC oct loop 7MHz AZ.gif




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http://www.w0btu.com/magnetic_loops.html - Created August 24, 2011 - Last Edited July 21, 2014

 

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