WT8V Site Assessment
The station is situated on a ridge top in central West Virginia. The topography is very irregular. A prominent feature of the terrain is the existence of other ridges with which the signal must interact. A considerable amount of terrain enhancement exists in some directions depending on the steepness of ground slope, direction of ground slope and the distance from the tower to any blocking (interfering) terrain features in the far field. On ridge tops with steep ground slope below the antenna, the optimum height of the antenna over ground level tends to be much lower than would be optimum for the same antenna over flat terrain.
HFTA (High Frequency Terrain Analysis) was written by Dean Straw N6BV and is included with the ARRL Antenna Book. A terrain assessment was performed using HFTA at WT8V to determine the optimum location on the ridge top for the 20/40M tower and the optimum antenna height over ground level for the antennas. Prior to the WT8V project, considerable work was done in West Virginia to test the relative accuracy of the software predictions over the rugged terrain. The testing was conducted using A versus B comparison of two equivalent gain antennas on the same hill top for the same band, with each at different height over ground and each with a slightly different terrain shot in a given direction. In every case when HFTA predicted one antenna would outperform another in a given direction on a band, the prediction was accurate. The testing was done on 17, 15, 12 and 10 meters.
WT8V runs a full size 4 element mono-bander on 40 meters at only 74’ above ground. The ridge runs roughly east and west and is only about 40’ wide where the tower is sited. When beaming NW to NE, the 40M yagi looks out over steeply sloping ground. For example, at 45 degrees azimuth, the ground elevation falls 73’ in 200’ from the tower, 130’ in 400’ from the tower, and 154’ in 600’ from the tower.
The net result is that the WT8V 40M antenna @74’ outperforms an equivalent 4 element antenna at 160’ at the same hilltop location.
The FOM (Figure of Merit) is the average gain of the antenna over all angles which are useful for the target area. The FOM favors the 74’ WT8V antenna by 2.7 db but note the 160’ hilltop antenna has a deep wide null centered at 13 degrees. The 74’ WT8V antenna has about 16 db more gain than a 160’ antenna on the same tower in the center of this null.
The light blue line shows the pattern for a 4 element mono-bander at 74’ over flat land. One can see the tremendous terrain enhancement present at WT8V’s elevated site compared to an identical antenna at the same elevation over flat ground. The FOM is 4.5 db favoring the hill location but the peak gain differential exceeds 10 db at some lower angles.
The 20M situation at WT8V is similar to 40M. The antenna is a 5 element M2 mono-bander on a 51’ boom. It is located on the tower 22’ below the 40M antenna and is about 52’ over ground level at a height predicted by HFTA to be optimum for the NE on 20.
The only comparison antenna at WT8V is a Mosley tri-bander with 2 elements on 40M. This antenna is located on a 60’ tower about 600’ east of the big tower, at a spot that is not optimum for the NE per HFTA. In A versus B comparison testing, the big mono-banders will hear several layers of stations that cannot be copied on the Mosley. Some of this can be attributed to the basic gain performance difference between the antennas. In terms of absolute signal strength, the difference on the mono-bander far exceeds what the theoretical basic gain difference should be between the antennas, assuming most or least favorable gain figures for the Mosley. The terrain enhancement for the big antennas contributes a great deal to the signal favorability of the mono-banders.
The DX must be heard to be worked. One main difference, especially on 40M, is the very clean pattern of the JK mono-bander. The side and rear rejection of atmospheric noise makes the signal to noise ratio far greater on the JK antenna than on the Mosley. Terrain enhancement aside, this is one of the major advantages of the big antenna at WT8V. Well sited local stations (hilltop) with good performing 2 element 40M yagi antennas note that WT8V has far lower band noise on the big antennas, which means several layers of stations can be heard that are not copiable on the smaller antennas.
Hopefully the above descriptions of the WT8V site will explain the high performance being achieved on transmit and receive with an antenna “only” 74’ and 52’ over ground level. The big WT8V antennas have significant terrain enhancement in all directions except when beaming along the ridge, in which case they play like 74’ or 52’ flatland antennas. Fortunately, with a 40’ wide ridge, it does not take much rotation to be looking down a hill slope again. The main DX directions are not significantly affected by this feature.
One other point might be of interest. Stacks were evaluated at the WT8V site and at other similar hilltop sites locally. Generally, if the optimum height for an antenna as predicted by HFTA is fairly low, such as is the case with the present WT8V mono-banders, going higher degrades performance. In most cases, the poor performance of the higher antenna degrades the overall performance of a stack. At the N8RR site, stacks would not work at all. At WT8V, the performance of a 40M 4 element stack at 160 and 80’ on the present tower site would be essentially equal to the 74’ antenna, certainly not worth the cost and work involved. A 200’ high stack was modeled in the only place where it could be guyed, and the performance predicted by HFTA did not equal the present 74’ antenna, because the terrain shot was inferior at the alternate location. Stacks will work in some elevated locations; it depends on the steepness of the terrain slope and the distance to far field obstructions.
73 Charlie N8RR
HERE'S LOOKING AT 43-DEGREES FROM TOWER 2 LOCATION.
WE CALL THIS SIGNAL MAX VALLEY
Last modified: 2013-11-02 00:47:36, 6757 bytes cached
This user has no active logs