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Old 3-Aug-2013, 1:22 PM   #48
tripelo
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Join Date: Jan 2010
Location: Dallas, TX
Posts: 127
Long-Yagi Gain Assessment

Previously shown were computer simulations of antennas; Winegard YA-1713 and homebrew long Yagi. For the channels of interest (Channel 8 and 11), the analysis data showed gain for a pair of long Yagis compared to the stacked pair of YA-1713 to be as listed in table below:



Having finalized construction of the long-Yagi, several comparison tests at the test range were conducted.

A test involved an antenna-under-test (AUT) mounted on the test mast, receiving signals from the transmit antenna. The transmit antenna was a portion of a YA-1713 mounted on the main tower (aimed towards the test mast) fed with a crystal oscillator as a transmitter. The frequencies available were the 9th and 10th harmonic of a 20 MHz crystal (180 MHz and 200 MHz). The AUT signal was amplified by RC-9267* preamplifier then received on a Blonder Tongue FSM-11 (signal level meter). Signal levels at 180 MHz and 200 MHz were recorded.

*RC-9267 – Particular interest due to specified low noise figure. This model has band pass filtering to attenuate frequencies outside upper VHF (174-216 MHz).

As with previous testing over several weeks, the test started with the homebrew long Yagis stacked at 89 inches. The two antennas with MT-74 baluns fed an Antronix CMC2002U splitter (used as a combiner). The combiner output was coupled to the preamp/FSM-11 receiver system as listed above. FSM-11 readings were recorded.

Then, the long yagis were changed to the YA-1713s stacked at 43 inches, using the exact cables and Winegard CC-7870 combiner that was used on the main tower. The preamplifier was a RC-9267 instead of the original CM-7777. The FSM-11 meter readings were recorded.

Measured Results:

Freq. Delta
(MHz) (dB)
180 . 3.3
200 . 4.8

That these results generally agree with the computer simulations was bit of luck.

In this test, there were some confounding variables, examples (no particular order):

- Antronix combiner on long Yagis had less loss (maybe ~0.2 dB) than Winegard 7870
- MT-74 balun on long Yagis may have had more loss than YA-1713 balun.
- Preamp SWR; briefly explored near the end of these tests.
- There are others

Suppose the gain improvement was ~4.8 dB, this falls short of the predicted needed value of 7 dB gain above that of the pair of YA-1713s, as previously discussed.

A somewhat compensating factor was that the noise figure of the RC-9267 was considerably better than of a CM-7777.

Later in 2012, lab comparisons in thermal noise background, using a Sencore 1456CM, indicated ~3dB better post-detection S/N with the RC-9267 compared to the older version of CM-7777.

There are probably few locations (certainly not this location) in the USA where the background noise at Upper VHF is as low as thermal noise. So, one would not expect an approximately 1 for 1 improvement in S/N with improvements in noise figure. But, there was some reason to think that the combinations of antenna gain and noise figure improvement might suffice for adequate reception of WHAS-11.

Over-The-Air reception at the test site with long Yagis, RC preamp, and a Channel Master CM-7000 DTV converter was observed for a few weeks. WBNA-8 reception was very solid with an occasional pixelated image, and WHAS-11 was watchable. But, signal dropouts were present on WHAS-11, especially during hours between about 10 AM and 5 PM or so. This viewing experience seemed to indicate a lack of sufficient antenna gain, or possibly some other deficiency in reception of WHAS-11.



Previously collected antennas and materials were available, so building two more long Yagis for a quad stack was feasible.

Decided to observe performance of these antennas on the main tower for a period of time.

.
Attached Images
File Type: jpg Stack Yagi Test.jpg (122.0 KB, 4954 views)
File Type: gif Gain Chart.gif (5.9 KB, 3760 views)

Last edited by tripelo; 3-Aug-2013 at 4:50 PM. Reason: Typo in Table Image
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