WSPR Evaluation of Horizontal Receiving Loop

Low-noise capability was not found at my location!

Brian Beezley, K6STI, proposed a horizontal loop design for low-noise reception on the 160 and 80-meter ham bands. This was described in September 1995 QST, pages 33-36, including arguments why this could reject local noise. An accompaning article by Edwin Andress, W6KUT, gives the details for constructing ne of these loops. This same Andress information is available on the Internet.

I contructed a loop per the Andress article, with some differences in the dimensions. Mine was 12-ft on-a-side all made with #14 house wire. This was on sloping ground, about 65 ft from the south edge of the house. To make the loop horizontal, the loop was about 18-feet on the uphill side and 22-ft on the downhill side. A single 17-ft tapered 2x6 board, supported by trees on each end, runs across a diagonal and holds the feedlines. The other corners are held by ropess going to trees. I attempted to photograph the antenna, but being in a clearing in the woods made a decent picture out of my capability.

The feed system is identical in topology to that described by Andress, except that my loop, being smaller, had an inductance of about 11 uH, requiring around 160 pF to achieve series resonance. The feedpoint resistance was the smaller, as well, so a 3 to 10 turn transformer was used. The measured Q, loaded by the radiation resistance was about 70, producing an 3 dB bandwidth of around 50 kHz. About 100 feet of RG-8/U coax led off to the radio.

Next came the experiment to measure the relative performance using WSPR reception. The loop was connected to a receiver set up for WSPR. As a reference, a second separate WSPR receiver was connected to an 80-m dipole that was at a height of about 50-feet above the back of the house. This dipole favors north and south directions, with the ends at 105 and 285 degrees true. Both receivers were operated under WSPR reception from the start of 19 September 2013, UTC, until about 1530 UTC on 20 September. There were several interuptions in the reception, primarily caused by a PC that chose to reset twice during the early morning hours. A total of about 30 hours of operation provided sufficient data.

Both WSPR setups were connected to the WSPR reporter on the Internet with calls W7PUA for the loop and W7PUA1 for the dipole. This made data collection simple, as the WSPR data base includes information on the signal-to-noise ratio (S/N) being observed, as well as the location of the transmitting station. Eleven different stations provided signals for measurement with the result that 478 data events occurred for which there were simultaneous measurements from both antennas (956 total data points). For each of the eleven stations, the difference in the dB levels of S/N for the two antennas was taken. These were then averaged for the comparison. Note that averaging of dB values does not average powers, but rather finds the geometric mean of powers, converted back to dB. For large dB values, this can be questioned, but here the dB differences are typically only 5 dB. We are only trying to look for significant performance differences, and this will meet that goal.

Data Summary The chart on the left summarizes the observations. The important column is the "Average Loop-Dipole dB" which is positive if the loop out-performs the dipole, and negative if the dipole performs best. The loop should be omni-directional, but the dipole has poor reception off the ends of the wire. For this reason the azimuth angle of the transmitting station is the important independent parameter, and is shown in the second column. The "Std Dev Loop-Dipole dB" is the dB standard deviation of the unaveraged differences, that gives some feel as to the spread in multiple data points. Low values would suggest greater accuracy to the data. Also important for data accuracy is the number of points, as the final measurement standard deviation is roughly the value shown, divided by the square root of the number of points.

Data Summary Now, on the left we have a graph of the dB differences plotted against the azimuth angle of arrival. True north is zero degrees. Each blue diamond corresponds to one of the eleven stations in the chart. The red letters "N" at the top of the graph correspond to the angles where the dipole tends to null. Similarly, the red letters "P" at the bottom correspond to angles where there are peaks in the dipole pattern. The P and N symbols are not data points.

The graph strongly suggests that in the directions broadside to the dipole, the loop falls perhaps 5 dB behind. Near the end-point directions of the dipole, the loop is able to pull even. In other words, the loop does not provide the S/N of the dipole except in directions where the dipole performance is poor.

This all needs to be taken as applying to my location, and not as a general conclusion. I generally have few problems from local man-made noise. In particular, my noise problems are almost all associated with computer or monitor related sources. I do not have line noise issues. A noisier location might have a different result. Likewise, if my dipole was lower, and therefore closer to the house, it might pick up more noise and the loop could look better.

In order to better understand the details of noise rejection, I tuned a receiver carefully around the 80-m band, comparing the man-made sounding noises using both antennas. This was backed up by spectrum analyzer measurements. It just did not appear that the loop was quieter. This activity was basically trying to compare the man made artifacts against the atmospheric noise. The level of these noise artifacts did not seem lower on the loop. The atmospheric noise on the loop was about 20 dB below that from the dipole (as expected). But, in both cases, the atmospheric noise easily covered the receiver noise.

One further test was to retune the loop to the phone band and listen to two local SSB stations, KC7WW and W7SZ. Both of these stations are located in directions favored by the dipole. They reduced power to behave like weak stations and subjective measurements again clearly favored the dipole.

The broad sweeping conclusion is that this horizontal loop is not beneficial for me.

Many thanks to all the stations that provided signals. Some got on 80-m WSPR to aid this experiment, and other run on a regular basis, helping all the time. All are appreciated, and again WSPR has proven to be a most useful tool for antenna evealuation. --Bob, W7PUA

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This page was last updated 2 October 2013, W7PUA

Please email comments or corrections to boblark 'the at sign' proaxis dot com