A max. performance 40m - (60m) - 80m transmitting loop antenna.
imple and perfect wideband matching with remotely switched transformer pair.
Just make it, and it is ready for use. No tuner needed.
pa0nhc Updated version dd. 20170716. New transformer set.

Many thanks to G0CWT for the eye opening and proven results from his empirical research.

         Special :
This low-Q, wide band 10m circumference loop with maximum efficiency (-0.5dB) at 40m, and only a few dB less at 80m. Installed at low height it is an Near Vertical Indent Sky wave antenna, optimal for regional QSOs. Installed at 7-10m height it has real DX possibilities. Tested with 100W SSB, and perfectly wideband matched at 40m and 80m, using two remotely switched ring core transformers. No tuner needed.

Both transformers, the air tuning capacitor, the little 12V DC motor, a simple to construct 1 : 13.500 mechanical reduction system, and an anti-condensation heather are all installed in a large standard ABS installation cabinet at the open end of the loop radiator. Easy accessible as it is at the lowest point of the antenna.

Tuning range : from 2.53 MHz to 9.4MHz.

Matching ranges for VSWR <= 1 : 1.5, from 3.4 MHz to 5.8 MHz, and from 5.6 MHz to 9.4 MHz. 

Useable on 60m, with VSWR 1:1.5 . A tuner is needed.

Bandwidth for VSWR <= 1 : 1.5, read from the VSWR meter on a TS570D transceiver, abt. 16m coax feeder :
        7.1 MHz   : 95 kHz (!)
        3.65 MHz : 20 kHz.
More than enough for easy tuning and distortion-free SSB, AM and digital modes, even on 80m.

-3dB magnetic fieldstrength bandwidth measured with MagFieldIndicator and 5W RF ::
      7.1 MHz : 287 kHz. (Q=25).
 5,355 MHz :    91 kHz. (Q=59).
   3,65 MHz :    53 kHz. (Q=69).

Schematic of a tuned 1/4 lambda, 10m circumference loop with 40m wideband  transformer matching, developed by G0CWT.
1. The 40m matching transformer is a low capacitance version.
2. The loop is capacitively balanced to ground by an 4kV / 5pF capacitor, and grounded rotor shaft of the split stator tuning capacitor

For use on 80m an extra transformer is switched in.

Square loop, circumference 10m
Made from painted copper water tubing 35 / 28 / 22mm.
Copper knees are soldered with electronics tin and reinforced by bolting-through.
Insulator plates and cabinet are made from Trespa.
Stainless steel hardware.
Mast and crossbars : painted wood
(better use composite garden materials).
Base: zinc plated satellite dish foot with 4x 8kg concrete tile.
Top guy wires : pre-stretched polyester rope (flags line).
Side guy wires are also needed to prevent vibrating torque to the mast and the radiator.

Construction. Gearing and heather. New 40m+80m transformer set.
Capacitively balancing.
Installation. Other interesting loop
Sense and nonsense
about magloops
Properties of a 1/4 lambda
circumference loop
aluminum radiator ?
Circle shaped
or square radiator ?
Matching a 1/4lambda loop.

        Further properties of this loop :
- Small, only 2.5m square
- Omni-directional radiator for 40 (60) 80m.
- No radials nor ground needed.
- Installed at low height (loop center at 2.5m up) it radiates predominantly vertical (NVIS), optimal for regional traffic.
- If installed at 7m height => also very useable for DX (see radiation patterns below)
- Transformer coupling. Immediately perfect match on two bands.
- Closed, balanced system.
        pure magnetic radiation
        While receiving:        insensitive for E-filed man-made-noises
        While transmitting: Minimal chances for BCI/TVI
- No ATU  needed. Tune the loop to wanted frequency  => ready for transmitting with perfect match.

VSWR sweep at 80m. Results beside the VSWR 1:1 frequency depend on the length of the coax feeder.

Radiation patterns, OMNI DIRECTIONAL.
Calculated with MMANA-GAL_basic for specified height of loop center (The tuningC is 1.25m lower).

                   NVIS properties at low height on 80m band.             NVIS and DX properties on 40m band when at elevated height.

Download  "loopcalc.exe" with which the properties of small loops (circumference < 1/20 lambda) can be calculated.
Download "MMANA-GAL" antenna modeling software.

The grounding and denoising of my antenna system