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.

My local wintertime 350Hz BW noise, compared to a sample of signals, (on one night) was:

Noise -127dBm miles
9H1BM -122dBm 5400 miles
OM0WR

 -95dBm

5100 miles
DF2PY -88dBm 4600 miles
WA8OLN  -78dBm 650 miles
W3GH -60dBm 650 miles
W1AW -53dBm 900 miles
W4ZV -32dBm 400 miles

The above signal levels may not be typical of every night, but they show the large signal level variations between weak DX and strong one-hop signals.  This chart also shows why a direct distance-corrected multiplier does not work! Every hop adds significant attenuation, as does a signal travelling near the earth's magnetic poles.

The signal level difference between noise floor and W4ZV was 95dB. W1AW and W4ZV, both in similar directions and both with similar power, have a difference of 21 dB. This is over 100 times difference in power levels at my receiver. This illustrates how important the combination of antennas, location, and propagation (W4ZV is one sharp hop away) are, rather than power, location, distance, or antennas alone. Differences between signals from the same area can be quite pronounced.

Before talking about receiving antennas for lower frequencies, it is important to understand a few basics. We all understand the primary reason we use special receiving antenna systems is to improve signal-to-noise ratio. On the surface this sounds like the same reason we use directional transmitting antennas, but there are some very important differences between transmitting and receiving applications.

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:

  • 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 (directivity) table shows relative performance of antennas in ascending order:

Antenna Type

RDF  (dB)

20-degree forward gain (dBi)

Average Gain (dBi)

1/2λ Beverage

4.52

-20.28

-24.8

Vertical Omni, 60 1/4λ radials

5.05

1.9

-3.15

(Ewe Flag) Pennant

7.39

-36.16

-43.55

K9AY

7.7

-26.23

-33.93

1/2λ end-fire Beverages

7.94

-20.5

-28.44

1λ Beverage

8.64

-14.31

-22.95

two verts optimum phasing 1/8 λ spacing

9.14

-22.46

-31.6

two 1λ Beverages Echelon 1/8 λ stagger

10.21

-15.45

-25.66

Small 4-square 1/4 λ per side (optimum phase)

10.70

-15.79

-26.49

1-1/2 λ Beverage

10.84

-10.88

-21.72

Small 4-square 1/8λ per side (opt. phase)

10.97

-30.28

-41.52

Single 1.75λ  Beverage

11.16

-6.50

-17.66

2 Broadside 1.75λ Beverages .2λ spacing

11.36

-3.51

-14.87

2 Broadside 1.75λ Beverages .4λ spacing

11.91

-3.50

-15.41

.625λ x .125λ spaced BS/EF vertical array 

12.5

-19.5

-32.0

2 Broadside 1.75λ Beverages 5/8λ spacing

12.98

-3.50

-16.48

2 Broadside 1.75λ Beverages .75λ spacing 

13.48

-3.49

-16.97

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:

 

Antenna RDF (dB) Gain  Directivity Change dB Gain Change dB  

Single 1.75λ  Beverage

11.16

-6.50

0

0

2 Broadside 1.75λ Beverages .2λ spacing

11.36

-3.51

+.2

+2.99

2 Broadside 1.75λ Beverages .4λ spacing

11.91

-3.50

+.75

+3.00

2 Broadside 1.75λ Beverages .625λ spacing

12.98

-3.50

+1.82

+3.00

2 Broadside 1.75λ Beverages .75 spacing 

13.48

-3.49

+2.32

+3.01

 

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.

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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