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Old 9-Aug-2013, 3:08 PM   #51
Senior Member
Join Date: Jan 2010
Location: Dallas, TX
Posts: 173
Single Long Yagi on Main Tower

In October 2010, a single long-Yagi was installed at the top of the main tower mast along with an RC-9267 preamplifier. A Blonder Tongue FSM-11 signal level meter was used at the tower top to observe signal strength of both WHAS-11 and WBNA-8.

The sensitivity of the Yagi height above ground was investigated. The Yagi, with RC-9267 and FSM-11 connected, was raised from near the tower top to the upper end of the mast. The FSM-11 was observed as the Yagi was raised, and about every foot or so, the Yagi was fastened in a fixed position for longer observation of the signal level. This process was repeated until the Yagi was at the top position. While raising the Yagi (~11 feet variation in height) there were no obvious signal strength differences from one position to the next. But, signal fluctuation could easily have masked differences in average signal strength at some particular location.

Often, at any position, the signal would fluctuate rapidly, changing as much as 15-20 dB in a time period of 20 –60 seconds. In the depth of the fades, the signal would be at or below the required threshold of detection for 8VSB DTV (at around 15 dB above the noise floor). The fades were frequency selective. For example, the signal level in lower portion of the 6 MHz wide channel might be fairly strong while in the upper portion of the channel the signal could be critically weak, or vice versa.

The image below shows the long-Yagi at the main tower mast.

For several days the single Yagi was left alone while reception quality was observed at the house (using a CM-7000 DTV converter). Reception was fine for WBNA-8 (with occasional image pixelation). Reception for WHAS-11 was good for several hours of the day, but signal loss and breakup was common through midday hours until early evening.

Comment: Rapid Fluctuations and Frequency Selective Fading

The signal observations at the top of the tower with the Blonder Tongue FSM-11 were similar to those seen at the test mast location. Those rapid fluctuations and frequency selective fading were indicative of multipath. The terrain in central KY is hilly with trees, no mountains. This is the path that a 2-Edge signal (according to TVfool) from Louisville must traverse. At first thought, such a path with abundant vegetation as RF absorbers might not seem to support severe multipath. It could be that for a significant part of the time the attenuation of the path was large enough that the propagation mode was tropospheric scatter (Tropo) or a combination of Tropo and 2-Edge. The propagation mode of Tropospheric scatter is always present, but ordinarily the path is so attenuated that such signals are not seen. If this is the case, 2-Edge propagation combined with Tropo scatter may provide multiple paths that combine to cause rapid frequency-selective signal fading. Tropospheric scatter alone often supports multiple varying paths, and even without 2-Edge propagation could be sufficient to cause such fading.

The image below is an excerpt from a recent TVfool report for this location with stations of interest WBNA-8 and WHAS-11 marked.

In 2013, an opportunity arose to replicate the above antenna configuration and observe some of the signal spectrum characteristics with a Sencore SLM1456CM. The graph below illustrates the signal fluctuation.

Notes for graph, below:

- Mid-morning, sunny day, May 9, 2013
- Measurements taken at tower top
- Long-Yagi mounted at top of main tower mast
- Homebrew ferrite balun
- Homebrew pHEMT preamp with power inserter.

The frequency selective signal fading is evident at the upper portion of the 6 MHz spectrum, at about 203 MHz. Relative to Sample 1, the signal level of sample 2 shows a fade of approximately 25 dB.

Attached Images
File Type: jpg Excerpt TVfool.jpg (59.5 KB, 6865 views)
File Type: gif Ch 11 Selective Signal Fade.gif (11.7 KB, 7844 views)
File Type: jpg One Yagi on Tower.jpg (83.2 KB, 7073 views)
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