Thursday, July 21, 2011

Why Vertical Antennas?

Why Vertical Antennas?

The selection of using vertical antennas was not a natural choice. The "conventional wisdom" for contest-expeditions and Dxpeditions was to use Yagi antennas when ever possible, regardless of the height. "Conventional wisdom" would tell you that a 3 ele Yagi at 25' to 30' high on 10m, 15m, or 20m would be the correct (if not only) choice.

When first researching our antenna selection, most of the major contesters in the USA said that the Caribbean stations ALWAYS came in at a high angle. Again, more "conventional wisdom" indicating that a low Yagi would be the best choice from Jamaica.

When we were running our propagation programs, the team noticed that the take off angle to the USA was actually a 1-hop LOW angle. Not the high angle that most people indicated. Which was right: conventional wisdom or the computer models?

In our first trip to Jamaica during the ARRL DX CW contest, the group used a blend of antennas:

  • Verticals for 40m-160m
  • Both a horizontal Yagi and vertical array for 20m
  • Yagis for 10m and 15m.

The use of Yagis on the high bands seemed appropriate, as we would elevate them between 0.5 and 1.0 wavelength high. The computer models indicated that the Yagi should perform OK.

While in Jamaica before the contest, the team ran extensive tests with the 20m 2 ele Yagi (45' high) and the 2 ele parasitic vertical array. The results were astonishing: of the hundreds of tests, there were only a handful of times the horizontal Yagi was better than the vertical array (and only by a small margin).

Most of the time the vertical array was 2 S-units stronger than the horizontal Yagi! In fact, there were many occasions when a signal was an honest S-9 on the verticals (on the FT1000-MP) and S-0 and almost unreadable on the horizontal Yagi. This extreme difference can only be attributed to a signal arriving at a VERY low angle, where the Yagi had little gain.

These results, amidst many additional months of research, convinced the team that verticals by the ocean were the only logical choice for competitive contest-expeditions.

The following graphs were created by N6BV of the ARRL:

The first chart (below) shows an analysis of the take off angles from 6Y2A to Japan on 80m compared to various antenna choices.

First, let's look at the vertical bars (light blue). This is a statistical grouping of all take off angles between 6Y2A and Japan over all times of the sunspot cycle, over all months of the year, and over all conditions. The data is displayed in the percentage of times a signal will arrive at a given angle: right X-axis is the % of time, and the Y-axis is the take off angle.

Thus by looking at the first bar on the left, we can see that on average, signals will be arriving at 1 degree take off angle nearly 17% of the time. In fact, you can see that ALL signals will arrive at or below 13 degrees on all occasions. Those are very low angles!!

Overlaid on the take off angles are the elevation-plane analysis of standard 80m antennas. Lets look at the 80m dipole at 100' (pink with inverted triangles). By most standards, this is a very good antenna. But you can clearly see there is little gain at the low angles.

Next, look at the 1x2 verticals over good soil (2 ele parasitic vertical array). This antenna provides you with a large improvement over the dipole.

Now lets jump to the "ultimate" 80m antenna: a 3 element full size Yagi at 200'!! There aren't many of those around, and who ever uses them will rule the band! (based on conventional wisdom). The 3 ele Yagi really does a great job over most arriving signals.

But let's finally look at the 1x2 verticals over salt water (orange with circles). We can see that the verticals have a large advantage over the 80m Yagi at the very low angles. In this example, the verticals have a 16 dB improvement (3 S-units!) advantage over the Yagi. On the higher angles, the vertical still performs well. Overall, we believe the 2 ele vertical array over salt water is a better all around antenna than a full size 3 ele 80m Yagi at 200'!

The next graph examines the take off angles on 10m to Europe. The reference antenna is a 4 element Yagi at 60', an excellent contesting antenna. You can see that the 10m horizontal Yagi still does not cover the very low angles, and there is a null where there are still many signals arriving.

The comparison array is the 2x2 vertical array. The 2x2 verticals match the peak gain of the Yagi, but also offer excellent coverage over all arriving angles. Did we say that verticals by the beach rule!?

The team has done extensive modeling to all areas of the world on all bands, the results are typical of the examples above: Vertical arrays over salt water are very competitive antennas.

Testing the verticals:

To be certain that vertical antennas were the right choice for their waterfront location, the team did extensive testing of the arrays to ensure they could reproduce the results in the field. For more detailed results and installation tips, read the article written by N6BT and K2KW that appeared in CQ Contest Magazine in March 1998. Below are photos of the antenna testing.

Reference antenna on the land-water boundary

The reference antenna was a Force 12 ZR vertical, which is essentially a shortened half wave vertical dipole. Calibrated receiving test equipment (+/- 0.5 dB) was located on the other side of the San Francisco Bay, approximately 17 miles away. This allowed us to measure the RF energy at less than 0.5 degrees take off angle (in the pseudo Brewster angle).

"2 x 2" Vertical array

The white buckets are the bases for the four quarter wave verticals. The 2x2 array is actually a pair of 2 element parasitic vertical arrays spaced about 0.65 wavelengths apart. Each 2 element "antenna bay" is arranged in a driver-reflector parasitic arrangement. The 2 antenna bays are then fed in phase. The result is approximately 7.5 to 8.0 dB gain over a single element. This array is then fixed in a single direction.

Testing the Pseudo Brewster Angle

The team wanted to know how far from the water could they go before they would loose the enhancement of the salt water. After researching information on the affect of the land-water boundary on the pseudo Brewster angle, the team realized there were no definitive studies on this subject.

A vertical over land has little radiation below 12 degrees take-off elevation angle. This is a result of the so-called pseudo Brewster angle, and the lack of a highly conductive plane underneath. When a vertical antenna is placed over a highly conductive plane (salt water) the antenna effectively radiates energy all the way down to the horizon.

So at what distance away from the water will the land start detracting from the low angle energy?

The team set the vertical on the land-water boundary (above), and moved the vertical away from the water in 3' steps. The land-water boundary was the reference point (0.0)... but when the antenna reached approximately 1/4 wavelength from the water, the signal level increased + 3 dB over the reference point! The zone of this enhancement was fairly small. As the antenna reached 1/2 wavelength from the water, the antenna was now -2 dB from the reference point! At the 3/4 wavelength point, the signal was now +2 dB. Obstacles prohibited us from going farther than 3/4 wavelength from the land-water boundary. But it was clear that the land-water boundary had a significant impact on the low angle energy (in the pseudo Brewster angle). It is also clear that additional research is needed in this uncharted area (Team Vertical will do further testing in the summer of 99).

Unfortunately it appears that the gain enhancement is only in the immediate direction of the water. In directions other than the closest point of water, there is more land, thus impacting the pseudo Brewster angle in those directions. While having 3dB of gain is desirable, it is my conviction that the vertical should be placed as close to, or over salt water to ensure optimum performance in as many directions as possible.

More testing needs to be done in this area - maybe in the summer of 2000?

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