Loop Antennas for 630 Meter Transmitting

The loop is roughly square, supported by pine trees in several spots.

The plane of the loop is on a 50/230 degree (true) bearing.

Dimensions: 63' x 63' (19.2m x 19.2m).

Perimeter = 252' (76.8m) => 0.122 wavelength.

Area = 3969 square feet (368.7 square meters).

Conductor = Very old RG-8 with tinned braid, shield is 0.27" diameter (6.9mm).

Lower conductor run is 8' (2.4m) above ground.

The feedpoint is off-center in the lower run. Feedpoint impedance measures 3.65 +j366 ohms at 475 kHz.

Impedance measurements of loop antennas are complicated by the need to have the test equipment isolated from ground. As the feedpoint and tuning network are at a tree, I put a temporary wooden shelf on the tree, and put my VNA2180 analyzer and a laptop computer on the shelf. Battery power was used for both.

An analysis of the above information in the old but excellent program "RJELOOP" http://www.zerobeat.net/G4FGQ/page3.html gives the following:

Inductance = 120.6uH.

Radiation Resistance = 0.0266 ohms.

Conductor RF Resistance = 0.639 ohms.

The program also gives a ground loss resistance, but since we have an actual feedpoint measurement, we are better off doing the math ourselves, since RJELOOP does not do a good job with our sandy, rocky New England soil. The actual ground (or environmental) loss would be: 3.65 - 0.0266 - 0.639 = 2.98 ohms. But the most important figure would be the expected efficiency, which would be (0.0266 / 3.65) = 0.73%. Thus for my 150 watt transmitter, the radiated power would be (150 * 0.0073) = 1.09 watts. Because a theoretical half-wave dipole in free space and a loop in free space produce the same far-field radiation, this 1.09 watt radiated power can be called ERP under FCC terminology. To determine EIRP, multiply by 1.28 and get 1.4 watts.

Coupling transformer T2 is pretty simple. A turns ratio of SQR( 50 / 3.65 ) = 3.7:1 is needed to get to 50 ohms. A transfomer with a 11:3 ratio is perfect. I used two FT-240-77 cores, taped together with Scotch 27 glass tape. The antenna side is 3 turns of #10 electrical wire, and the line side has 12 turns of #18 wire with taps at 10 and 11 turns. As expected, the desired tap was at 11 turns.

T1 is a current transformer with the loop conductor passing once through the center, for a single turn. The FT-240-77 core is wrapped with Scotch 27 tape, and then wound with 100 turns of #21 enamel wire. Another layer of tape goes over the winding. When terminated with a 50 ohm resistor, 1/100 of the antenna current will pass through the resistor. For 10A full-scale, 5V RMS will appear across the resistor. That is rectified by D1 (should be germanium), and filtered by C2. This should give a full-scale voltage of 7.07 - 0.3 = 6.77V DC across C2. R2 is then used with a 100uA meter for a full-scale reading. Calibration may be done by disconnecting T1, and applying 7.07VDC across R1.

These antennas are also excellent for receiving, though this one is a bit too close to AC power lines to be ideal. A fairly large loop such as this, with this tuning network, will probably not need a preamp on a decent LF/MF receiver. Of course, you can't rotate the loop, so the cautions about directional characteristics do apply.