Bob E. Nelson said:
I have to wonder just how accurate the service contour map is on the FCC database. The transmitter output power is slightly higher for that new CP (BMPDVL‑20091203AHE) with the new antenna with less gain. Looking at the polar plot, there will be less signal to the south and therefore more to the north.
That's a good point. Their current antenna has maxima at 40 and 320 degrees but has a 41 degree rotation, so its strongest signals are 1 degree bearing (due north) and 81 degrees (almost due east). The new antenna has its maximum at 0 degrees with no rotation, so its strongest signal is due north. Since the ERP and antenna height is the same, they should go the same distance northward, and looking at the contour maps, that indeed seems to be the case. But the new antenna has noticeably less signal in most other directions (except northwest).
At first that sounds like a higher-gain antenna, which should
reduce the transmitter power needed to get 300 watts ERP. But I also suspect the new antenna has less vertical directivity, so more signal gets beamed downward (for better reception near the tower) - but also skyward (wasted), so in fact its gain is lower. That'd explain why they need more transmitter power to maintain the same ERP.
In fact that may answer my original question as to why they're doing this. Back on page 6 of this thread dhett reported:
dhett said:
Just completed a road trip that took me through the Metroplex on I-20. Tried to pick up the DFW-area digitals from around the I-20/US-67 junction. Everything came in great except KHPK. Detoured down into Duncanville to gas up and eat (same thing?) and tried KHPK again. Nothing better than 25% signal, not enough to get a picture or a real transport stream reading.
So at present, reception with rabbit ears, even near the transmitter, isn't feasible. If the new antenna helps overcome that, that may gain KHPK more viewers than the reduction in coverage area loses. No one within 15 miles of Cedar Hill is going to have an outdoor antenna.
I noticed KSFW/2's license provides for something similar. Their ERP is only 200 watts, but the antenna has a "beam tilt" which increases the effective ERP to 500 watts. (Clever way of getting around the FCC's power limits, BTW.)