Low Noise Receiving Antennas and Arrays

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Noise and common mode noise.

Power line and other noise sources.

Pre-amplifiers.

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.

Comparison or Ranking of Receiving Arrays or Antennas      

The 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.

RDF (directivity) will be an almost perfect indicator of what you can expect from your antenna as long as:

• Noise is not from the same general direction as the desired signal
• Noise field strength is not greater than the ratio of peak antenna response to depth of the pattern in the direction of noise
• Noise is not coming from within the antenna's nearfield or Fresnel zone

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: 

1. Direction and polarization of arriving signals and  noise constantly vary, and so the relative relationship of each to any individual antenna's response will vary.
2. Through various unavoidable errors or omissions, antennas in the real-world may not work precisely as predicted.

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 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.

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.

Some of this is explained in my precipitation static page, and also touched on in the quad antenna and loop antenna pages.

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