Low Noise Receiving Antennas and Arrays

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Other related pages:

Noise and common mode noise.

Power line and other noise sources.

Pre-amplifiers.

Coaxial Cable Leakage

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 meter, 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.

RDF (receiving directivity) will be an almost perfect indicator of what you can expect from your antenna so 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

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: 

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

In a majority of cases, the following RDF (receive 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, while horizontally polarized antennas are quiet. Another myth tied to the claim verticals are noisy is noise sources are predominantly vertically polarized.

There is some truth to the claim that a horizontally polarized antenna can be quieter than a vertically polarized antenna, but this requires a special condition or bondary. The special condition occurs when the predominant system noise is local extended groundwave noise. For example my dominant daytime noise on 160 meters comes from Barnesville, GA and Forsyth, GA. Both towns are about seven miles from my location. If I use a tall vertical antenna on 160 meters, the daytime noise from these towns and other groundwave sources is almost 20 dB stronger than noise from my 300-ft high 160-meter dipole. 

The vertical has more noise for comparable antenna gains because the vertical responds better to extended groundwave than the horizontally polarized dipole. The horizontally polarized dipole has virtually no groundwave or zero elevation angle response at all.

At night time, when the band opens and the dominant noise propagates via skywave, there is absolutely no signal-to-noise advantage in using 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.

Loop vs. Open End Antennas

Loops are commonly rumored to have less noise than other antennas. There is a thread of truth to this, but that truth is not applicable to anything except very specific cases.

First, in inclement weather the earth and sky can develop a more concentrated voltage gradient. That voltage gradient causes corona to leak off earth objects. We call that corona P-static.
The loop is a relatively blunt-ended antenna. The "blunt" high voltage areas mute the electric field right where the antenna is most sensitive to such discharges. An open ended antenna, especially an antenna without a large hat or other end termination, has a very strong electric field response at the open end. It couples better to the areas where high impedance corona fields cluster. This is a poor weather issue.

Second, a loop typically has a very high common mode impedance. The high common mode impedance does not drive common mode feeder currents very well, nor does the loop with its high common mode impedance couple to the low impedance common mode feeder noise as well as antennas like dipoles or verticals. This is a  poor feedpoint design or implementation issue, we get away with a "sloppier" installation with a loop. 

Both of these loop advantages are easy to nullify.

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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