Low Noise Receiving Antennas and Arrays
Other related pages:
Noise and common mode noise.
Power line and other noise sources.
Note: The top of this page has links to various receiving antennas such as Beverage, "magnetic" loop, and vertical low-noise DX receiving antennas.
How Low-noise Receiving Antennas really work
This area deals primarily with low noise antennas, and discusses effect of antenna directivity on weak-signal reception.
Directivity Comparison of Receiving Arrays or Antennas
The table below rates receiving antennas in order of increasing performance. It uses directivity, with results 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 propagated distant 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 determines how well a receiving antenna works. In virtually all installations without clearly dominant direction or directions of noise arrival, RDF (receiving directivity factor) accurately predicts receiving antenna performance.
RDF (directivity) will be an almost perfect indicator of what you can expect from your antenna as long as:
If antennas are within two dB of each other in directivity (RDF), a lesser ranked antenna may outperform a better ranked antenna. This is because:
In a majority of cases, the following RDF (directivity) table shows relative performance of antennas in ascending order:
Gain vs. Directivity Myth
One 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 in the table above. In the aqua colored areas, we can follow comparisons between a single 1.75λ Beverage and various spacing pairs of 1.75λ phased Beverages. In a case where spacing is .2λ, the single Beverage has a gain of -6.5dB. A pair of Beverages spaced .2λ has a gain of -3.51dB. This is a gain increase of about 3 dB. Despite the gain increase, antenna directivity and pattern do not change a noticeable amount. RDF (directivity) only increases 0.2dB, an undetectable difference. Pattern remains essentially the same, so reception remains essentially the same. Significant new nulls, or deeper nulls, are not created at close spacing.
Here is the same table showing only 1.75λ Beverages:
Gain of any spaced pair is around 3dB more than a single Beverage, but reception improves and antenna pattern changes only with relatively wide spacings. Spacing must be at least be 1/2λ or more for phased Beverages to add a reliable improvement in reception quality. Wider spacing improves null depth off the sides, and narrows front lobe beamwidth. At 3/4λ spacing directivity improvement for evenly distributed noise and QRM falls short of 3dB, although side suppression of signals improves greatly!
Of nearly equal importance, many 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.
Horizontal vs. Vertical
One popular claim is that vertically polarized antennas are noisy, and horizontally polarized antenna are quiet. Another myth related to that claim is that noise sources are predominantly vertically polarized.
There is some truth to the claim that a horizontally polarized antenna can be quieter, but this requires a special condition where the majority of noise is local extended groundwave noise. For example my dominant daytime noise on 160 meters comes from Barnesville, GA and Forsyth, GA. These towns are about 7 miles from me. If I use a tall vertical antenna on 160, the noise from these towns is almost 20 dB stronger than the noise from my 160-meter dipole that is at 300 feet.
This is because the vertical responds better to extended groundwave than the horizontally polarized dipole.
At night time, when the band opens and the dominant noise source is propagated in by skywave, there is absolutely no signal to noise advantage to the dipole!!!
This is explained in some detail on my NOISE page.
DC Grounded vs. Open Antennas
Another myth is that dc grounded antennas are quieter, filtering noise by shunting it to ground. This would require the antenna to short a 1.8 MHz noise, while NOT shorting a 1.8 MHz signal!!! That would be pure magic.
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