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Old 16-Aug-2013, 3:48 PM   #53
Senior Member
Join Date: Jan 2010
Location: Dallas, TX
Posts: 123
Two Long Yagis on Main Tower

The signals from a single long-Yagi were observed for a few days. Then, in October 2010, a second long-Yagi was installed on the main tower mast.

The second Yagi was installed 89 inches below the top Yagi. Before the antennas were combined, signal levels were observed on a Blonder Tongue FSM-11 signal level meter. Snap-on connectors were temporarily installed to allow quick switching of the preamplifier (RC-9267) between antennas. There was some indication that the signal level from the top Yagi was stronger than the Yagi at the lowest position. Again, as with the single Yagi, signal variation could have masked any real differences in the average signal levels.

Satisfied the antennas were performing normally, both antennas were connected to an Antronix CMC-2002U splitter (reversed, as a combiner). Signal indications on the FSM-11 showed an increase above that obtained separately from either antenna.

The antennas and connections were secured in their final positions. The configuration:

- Two long-Yagis stacked 89 inches apart.
- Two selected Philmore MT-74 baluns
- Antronix CMC-2002U combiner &
- Research Communications Preamp (RC-9267) mounted between antennas
- Suitable lengths RG-6 cable with custom connectors.

Below is an image of the antennas at the main tower.

Reception was monitored inside the house using a Channel Master CM-7000 converter. Reception of WBNA-8 was good with occasional pixellation. WHAS-11 reception seemed to be improved with the antenna stack compared to the single antenna. WHAS-11 reception was good except there were some dropouts in the troublesome daytime hours. The dropouts were less frequent than seen either at the test location with both antennas stacked, or with the single Yagi at the tower top.

Interesting: The depths of frequency selective fading,seen with the single Yagi, imply that gains in signal strength alone would not be sufficient to overcome the effects of the deep fades. Clearly, a 25 dB selective fade could not be mitigated by a 3 dB increase in signal strength (in reality, stack gain is likely less than 3 dB). Those fades shown in the spectrum (earlier) were not difficult to capture, meaning they were fairly frequent during daylight hours. With the stack of two antennas, the fades were less noticeable. The fades continued to exist but were more difficult to capture via the FSM-11. The equalizer in the tuner demodulator can mitigate the effects of some fades, but equalizers have a limited range. One limit being that the signal level at the depth of the fade must remain above the minimum required for DTV detection (~15 dB S/N) --there are other limits. It seems likely the stack of antennas provided something more than merely an increase in signal strength (theoretical ~ 3 dB).

In 2013, an opportunity arose to nearly 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 10, 2013
- Two Long-Yagi, stacked @ 89 inches, mounted at top of main tower mast
- Homebrew ferrite balun
- Homebrew combiner (transmission line type)
- Homebrew pHEMT preamp with power inserter.
- Measurements taken in house, at converter input
- Signal conditioned: attenuator, filters, UVSJ, industrial drop amplifier, attenuator

Frequency selective signal fading is evident at the mid- portion of the spectrum, at about 200 MHz. Relative to Sample 1, the signal level of Sample 2 shows a fade of approximately 10 dB. The fading depth of ~25 dB (as shown earlier) has not yet been observed with the two antenna stack.

Reception Summary (At end of year 2010):

WBNA-8: Acceptable at all hours of the day.

WHAS-11: Near flawless during evening hours through early morning hours. During daylight hours; could be considered acceptable, but occasional loss-of-lock remains.

Attached Images
File Type: jpg 2 Yagi Tower.jpg (82.2 KB, 4685 views)
File Type: gif Channel 11 Fade.gif (10.1 KB, 4130 views)

Last edited by tripelo; 16-Aug-2013 at 4:22 PM. Reason: typo & clarify
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