ALA 1530 - Figures and measurements in the field (III)
Here are some figures I recorded on Saturday November 22, 2003 in various conditions on the heights of Wépion, Belgium (50.3°N, 4.8°E), a spot theoretically away from QRM (300m ASL, rural, open fields, main city 7 km away). The first relay tower for RTBF is 7 km away, and the first AM pylon is located over 40 km away. None of these antennas creates intermodulation.
I installed in the garden a G5RV multi-band dipole purchased at WiMo, 31.1m (100') long, with 10m of ladder line and a 4:1 balun connected to 15m of RG-58 coaxial partly buried in the ground. At that time is was tight 5m high only, the remaining ladder line being placed 50 cm above ground covering in zigzag a surface as wide as possible. Its gain is 5 dBi at 56° of elevation with 4 main lobes in azimuth at 0°, 90°, 180° and 270°. Local and DX operations were accessible (DX up to midwest K, PY, 4S, UA9, 7X, etc).
The ALA-1530 magnetic loop was set down outdoors in the middle of the garden, 5m away from the shack. The loop was also installed in a near tree and attached to a window shutter; in each case results were similar. It captured maybe a little less QRM when it was placed 0.5m above ground, probably due to the decrease of ground effect but the difference was not important.
When QRM was strong, the orientation of the loop in the vertical plane was more important than its place in a room or another.
In ham bands from 160 to 10 meters, the signal was globally stronger on the ALA-1530 but it was also jammed by RFI (like the one generated by power supplies) and more generally by QRM than G5RV (even untuned). The difference is ranging from 1 to 4 S-points (6-24 dB) more QRM on the loop, mainly on bands from 15 to 10 meters.
Listening to weak signals (S-4 and below), the QRM was usually (not always) more important on the loop as well. I also listened some white noises where I found free frequencies without any broadcast. Even without signal, the loop picked up also more QRM (1 to 4 S-points stronger too) than G5RV, in both VLF and SW bands. Of course, with a strong signal (e.g. RS-59), the dipole and the loop are quasi no more sensitive to QRM, even if it is still present simply because the S/N ratio was higher, the Q signal covering the QRM.
Comparison between the ALA-1530 (ALA) and a G5RV (31m long, no tuned outside ham bands) in good conditions :
Comment : Take the first line of the next table. A signal received 54 with the G5RV and 56 on the ALA "including 2 S-points QRM on ALA" means that the received signal is perfectly readable R-5 with a strength of S-4 on the dipole including the QRM if there was. The RS-56 on the loop included 2 S-points or 12 dB for the QRM, the signal arriving S-4 too but it is covered with intermittent RFI or beating QRM. In other circumstances the loop displayed a signal 1 or 2 S-points (6-12 dB) lower than on the G5RV.
1. Compared to the signal received with the G5RV, ALA-1530 was very sensitive to QRM when my portable PC was connected to the main (RFI generated by the external transformer and its LED). The signal was at 80% flood in QRM on this frequency (only this frequency) !
2. Signal flood in QRM with G5RV and unreadable. Signal above QRM and readable on the loop.
Here are other figures I recorded at noon and at midnight too but from my appartment in Luxembourg, using a 35m sloper long wire, one half being tight indoor at ceiling level, the other one being tight outdoors, from the window of my 2d story to a fence located at the end of the garden. Of course 15m of this long wire running indoors, it captured much more RFI, to reach at some frequencies 59+. I used two magnetic loops, ALA-1530 and ALA-330 placed indoors, in front of a large window in the living room located street side then in the middle of the living room at ground level where sometimes it gave better results. The powerful emitter from RTL located about 20 km away didn't interfere with received signals.
In average and contrarily to the first table that was recorded in a spot away from RFI, in conditions of heavy QRM both loops were able to extract weak stations that were flooded in the QRM with the long wire. Sometimes using the loops the QRM dropped between 5 and 7 S-points, the equivalent of 30 to 42 dB !
Comparison with a 35m sloper long wire in worst conditions (indoors with QRM) :
°Loop was not rotated according to the signal intensity
* QRM w/... means the level of QRM measured with the RTX built-in S-meter