Parameters Of the Base Element

The double-loaded loop element described in the previous section has several parameters that need to be selected.

Load Impedance

The input impedance of the receivers connected to the loop element affects sensitivity, frequency response and receiving pattern of the antenna. I have created a NEC-2  model of the double-loaded element to study its characteristics as af function of load impedance. The size of the element in this model was 1 m per side, and the element was made of #14 wire. The results of the modeling are shown on Fig.1. The value plotted on the chart is signal power, in dBm, that the antenna delivers to the input of the receiver when the external field strength is 1 V/m. The chart data are available in a grf file which you can open using the Graph software [300].





Fig.1. Sensitivity of the double-loaded loop



Several publications [301] [204] state that a small wideband loop must be loaded with a low-impedance amplifier, so the first value I tried was 3Ω as suggested in [204]. The chart demonstrates that the frequency response of the element in the loop mode (Sig = RX1 - RX2) loaded with 3Ω (solid blue line) is extremely flat, which is probably the reason why low impedance loading is recommended for small receiving loops. Unfortunately, the sensitivity of the loop with such loading is very low. In the dipole mode (Sig = RX1 + RX2), the frequency responce is not flat at all but still very low (the dotted blue line).

For a short dipole receiving antenna, a high input impedance amplifier is recommended [302]. The plot for high impedance (200 kΩ) loading (the green lines on the chart) is flat in the dipole mode and varies widely in the loop mode. The sensitivity is as poor as with low impedance loading.

The sensitivity peaks at a load impedance around 650Ω. Also, at this loading the element has about the same gain in the dipole and loop modes (the pink lines on the chart). This is almost the same load impedance as used in the W2PM Mini Diamond Flag [206].

We choose the input impedance of 650Ω for the receivers because this results in the the best sensitivity and good balance between the loop and dipole modes.



Feedline Parameters

The short runs of open-wire feedline that connect the receivers to the loop element must have the same characteristic impedance as the input impedance of the receivers.

We choose the distance of 5 cm between the wires of the feedline.

This results in the characteristic impedance of 500Ω, which is close enough to the ideal value of 560Ω and results in a compact size of the structure.



Loop Size

The size of the loop is a compromise between the conflicting requirements of high sensitivity and small dimensions. I have created the NEC-2  models of several popular receiving antennas to see how the proposed element compares to other solutions. The sensitivity charts are shown on Fig.2., they include only some of the modeled antennas to avoid cluttering. All charts are available in the grf file.




Fig.2. Sensitivity of some HF antennas


Some familiar relationships appear on this chart. The sensitivity of a dipole at its resonant frequency is 2 dB better than that of an isotropic antenna (2 dBi gain), and a 3-element Yagi has a gain of 8 dBi. A tuned 1x1 m loop is better than the wideband W2PM Diamond Flag, and a 1x1 m wideband crossed loop with low impedance loading is the least sensitive of all modeled antennas.

The 1x1 m double loaded element is in the middle of the chart. Its sensitivity is not as good as that of the full size or tuned antennas, but it is more sensitive than a loop of the same size loaded with 3Ω, by up to 20 dB at high frequencies.

The shadowed areas represent the minimum sensitivity that is required to make the power of the external noise picked up by the antenna equal to the power of the internal noise of the receiver, assuming the receiver's noise figure of 10 dB. The external noise was modeled as man-made noise specified in the ITU Recommendation [303]. Other noise types, such as atmospheric and galactic noise, when taken into account, may relax the requirements to the receiver sensitivity. It follows from the chart that the proposed receiving element has sufficient sensitivity on the frequencies above 3 MHz. The sensitivity may be increased by increasing the size of the loop. With a size of 1.5 m per side, the element has the same sensitivity as W2PM Diamond Flag.

We choose the size of 1x1 m for the double loaded element

because this results in the adequate antenna sensitivity. The size may be increased to 1.5x1.5 m if more sensitivity is required.