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Noise and common mode noise. Power line and other noise sources. Pre-amplifiers.
This page has top links to other receiving antennas such as Beverage, loop, and vertical antennas. This area deals primarily with low noise antennas, and discusses effect of antenna directivity on weak-signal reception. Noise Floor and signal Levels at my location:My local wintertime 350Hz BW noise (after amplifier) compared to a sample of signals on one night was: Noise -127dBm 9H1BM -122dBm OM0WR -95dBm DF2PY -88dBm WA8OLN -78dBm W3GH -60dBm W4ZV -32dBm The dynamic range between noise and W4ZV was 95dB! It also illustrates how important antennas, location, and propagation are rather than power. The dB difference between signals from the same area can be profound. Many DX signals run at or near noise floor, while others can be 40dB out of noise floor. The data above is certainly not typical of every night, but it shows how large the signal level variations between weak DX and strong signals can be. Over a period of time I've found 85 to 90 dB dynamic range about the most that is every needed. In a simple installation with a single Beverage, 80 to 85dB IM3 and blocking DR is probably enough. Receivers with less than 80dB IM3 and BDR probably compromise a reasonably good station's capabilities.
Comparison or Ranking of Receiving Arrays or AntennasThe following is a ranking of receiving antennas based on noise being evenly distributed in all directions. These rankings are most accurate in the frequency range of AM broadcast, 160 or 80 meter bands when: 1.) The receiving location shows a nighttime increase in noise level. In other words the system is not limited by local or internally generated noise, instead being limited by skywave or distant propagated noise. 2.) Thunderstorms or other local noise such as power line noise from specific directions does not dominate the receive system noise floor. There will be occasional exceptions, but as a general rule the ratio of peak response in the direction of the signal to average response in all directions is the dominant factor in determining how good an antenna works for receiving. In virtually all installations without clearly dominant direction or directions of noise arrival, RDF (receiving directivity factor) very accurately predicts receiving antenna performance.RDF (directivity) will be an almost perfect indicator of what you can expect from your antenna as long as:
In the vast majority of systems, the following RDF table indicates relative performance of antennas:
If antennas are within two dB of each other in RDF, a lesser ranked antenna may occasionally outperform slightly higher RDF antennas. This is because:
Gain vs. Directivity MythOne common rumor or myth is that higher antenna gain results in improved reception. Gain is an unreliable way to predict receiving ability on frequencies below upper UHF! A clear example is illustrated above by comparing gain of the single 1.75wl Beverage to the pair of 1.75wl phased Beverages that are spaced .2wl apart. In this case, the single Beverage has a gain of -6.5dB. The pair of Beverages has a gain of -3.51dB, a gain of about 3 dB. Despite the gain change, antenna directivity and pattern do not change a noticeable amount. RDF only increases 0.2dB, and undetectable difference. Pattern remains essentially the same, so reception remains the same. Spacing must be at least be 1/2 wl or more for phased Beverages to add noticeable improvement to reception. Even at 3/4 wavelength spacing the directivity improvement falls short of 3dB! Gain of any spaced pair is about 3dB more than a single Beverage, but reception improves and antenna pattern changes only with relatively wide spacings. Of nearly equal importance, end-fire arrays actually work better with closer spacing. For an example, compare the 1/8th wl four-square RDF with the 1/4-wl four-square array. How well does the above hold true?Over the years, I have had virtually all of the above systems. I always have multiple phase-locked receivers on multiple antennas listening in stereo or a very fast way to "A-B" antennas. When an antenna sits unused most of the time, I replace it with a more useful antenna. My single Beverages are now virtually all eliminated, my last phased loops were in the 80's (when I had four end-fire diamond terminated loops). Even on 80 meters, my large arrays with 300-350 foot spacing almost always beat my single long Beverages. I've migrated towards the bottom end of the chart with all my antennas because they actually do receive better. If you ask operators who visit for contests, everyone prefers the large vertical or wide-spaced Beverage arrays. Guest operators, given a choice, almost never not use single Beverages or close-spaced Beverages. You can listen to directivity examples on my DX Sound files page. |
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