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Old 20-Sep-2014, 3:14 PM   #82
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 123
UHF Signal Strength vs Antenna Height (AGL)

This post is a continuation of:

UHF Signal Strength vs Antenna Height (AGL)

In the previous post linked above; the signal strength of 27 DTV channels was measured as a small antenna (Terk HDTVi) was raised in height.

The signal strength of the individual 27 measured DFW stations varied considerably, from ~-35dBm to about -90 dBm. Due to the wide range of strength, it might not be very instructive to plot all signals on the same graph.

The following graph shows three of the weaker signals strength plotted versus antenna height.




This graph illustrates what can be seen throughout all the measured signals (strong and weak).

That is:

1. Some channels show maxima and minima in signal strength as antenna elevation is changed.
2. The general trend is increasing signal strength with increases in antenna height.

Channel 50 shows the most distinct pattern of maxima and minima. This is evidence of signal layering; the constructive and destructive summation of two or more wavefronts.

Channel 51, although close in frequency, shows increasing signal strength with small perturbations. The two stations (channels 50 and 51), yield different responses at the receiving antenna, this could be related to differences in path geometry.

The transmitting antennas of these two stations are located on different towers and at different tower heights*. Incidentally, both are low power=15kW.

*Such transmitter/antenna info can be found at the FCC site, or more conveniently at Rabbitears.info.

Channel frequency does matter. The signal wavelength is inversely related to frequency and constructive/destructive combinations are a matter of signal phase (which is wavelength related). Signal strength differences at the receiving antenna are a result of the combination of frequency and path geometry.

In a different environment, it is likely that the above trends continue, but a specific frequency (channel) may or may not respond with observable maxima and minima.

The response of channel 27 is somewhat typical of the measured channels (total number=27).

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Attached Images
File Type: gif Weak Signals vs Height.gif (10.0 KB, 3482 views)

Last edited by tripelo; 20-Sep-2014 at 5:18 PM.
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