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 inCQ 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?