Has anyone deployed 450v in town on a structure like a water tower and dealt with the issue of subscribers close to the tower?
Looking at the AP antenna spec, it has 2 degrees electrical downtilt and null fill but the elevation plot still looks like there’s a pretty significant first null at 6 or 7 degrees. The plot looks like the null fill might be a little more effective at 6 GHz than 5 GHz but it’s hard to tell.
Water towers in our area tend to be 120 to 140 feet tall. Let’s take best case and say the AP is 125 feet AGL and SM is mounted at 15 feet. Subscribers 1000 feet from the tower would have around 6.3 degrees uptilt so that would be about the closest they could be, and the assumptions are a bit optimistic.
Of course we could use mechanical downtilt on the AP, but looking at the specs, the 2 degree built in downtilt is already at the point of just starting to lose performance for customers a mile or more away. So the only solutions I see are to use two sets of sectors, one downtilted and one not, or else use a connectorized AP and a different type of antenna with a wider elevation
pattern or better null fill.
What are people seeing in actual deployments?
Ken,
This antenna plot is scaled from the antenna gain perspective. Along the -2 degree axis, the antenna gain is represented to be 17 dBi gain. At -6 degrees, the antenna gain is represented as about 6 dBi. This means that an SM that is -2 degrees downtilt from this antenna receives a particular RSSI. If this antenna were tilted up so that the SM was now at the -6 degree elevation from the antenna, the RSSI received by the SM would be about 10 dB lower than when it was -2 degrees from the antenna.
If your antenna is set to 0 degrees elevation, meaning that the electrical downtilt is -2 degrees, at 1 mile range, an SM might fall within the -2 degrees gain of the antenna. Assuming we have EIRP of +36 dBm, 1 mile of propagation loss @ 5.8 GHz is about 112 dB. If we shorten the range to 0.5 mile, propagation loss is 6 dB less, or 106 dB. If we cut the range in half again, to 0.25 mile, our propagation loss is 6 dB less again, or 100 dBm.
Our RSSI expected from an SM with 25 dBi gain antenna at 1 mile is -51 dBm. At 0.25 mile range, we might fall in the -6 degree elevation from the antenna, so the effective antenna gain is only 6 dBi instead of 17 dBi. We have reduced our propagation loss by 12 dB, and reduced our antennna gain by 11 dB so our net gain is 1 dB, so I expect our RSSI at 0.25 mile to be -50 dBm.
The closer we get to the antenna, as long as we don’t have near-field obstruction such as falling under the shadow of the water tank in this case, you can extrapolate this performance at any range. Shortening the range by half decreases propagation loss by 6 dB, doubling the range increases the loss by 6 dB.
The effective antenna downtilt should be set so that you can reach your farthest planned subscribers at the maximum antenna gain, since the propagation loss is highest to those farthest SM’s. When Link Planner suggests an antenna downtilt when you have the SM locations identified, it runs through the antenna gain calculation at tilts between 10 degrees uptilt to 20 degrees downtilt by default, It does an RSSI calaculation for each SM identified, and recommends the downtilt (or uptilt as the case may be) for the best overall performance to all the SM’s connected on that sector.
The bottom line is that a sectoral antenna does not have an elevatin signal cutoff, just a reduction in gain from the designed maximum gain.
Hope this helps,
Dave.
1 Like
