View Single Post
Old 22-Jul-2013, 6:13 PM   #44
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 127
Research Communications Preamp and Balun

Earlier, it had become apparent that sufficient gain (predicted 7dB above pair of YA-1713) to enable reception of WHAS-11 would be difficult to achieve.

To make most use of the available gain, a pair of Research Communications Preamplifiers (Models RC-9267) were ordered. The primary rationale for this approach was noise figure related.

The advertised VHF noise figure of major manufacturers of preamps ranged from a low of 2.6 dB to around 3.0 dB. The RC preamps were advertised to have a noise figure of 0.4 dB. Signal-to-noise ratio is closely related to noise figure. As an example, at UHF frequencies, a real improvement in noise figure of say; one dB, can result in a signal-to-noise improvement of about one dB. It is more complicated than this example, and also less likely at VHF, but this can be a close approximation for TV reception purposes.

This begs the question of whether advertised noise figures are accurate. With measurements, using normally expensive equipment, the question can be directly answered. Indirect measurements at the lab bench have indicated that the major manufacturer noise figures are overly optimistic. In any event, field tests here demonstrate the RC preamp provides improved S/N ratios.

Testing with the RC preamp encountered a hitch. One day a thunderstorm arose quickly. The test station was abandoned with antennas in place and the RC preamp powered on. As the scene was observed from the back porch of the house about 100 feet away, lightning struck the earth or a tree, about 600-1,000 feet distant. After the storm passed, the RC preamplifier was no longer functioning. On the main tower, two Channel Master preamplifiers survived.

There are several complication factors in the above scenario, but it is clear that:

- The RC preamplifier can be susceptible* to atmospheric discharge.
- The coaxial half-wave loop balun contributed to the demise of the RC preamp.

*More susceptible than the Channel Master preamplifiers.

The outcome was that testing with the coaxial loop baluns combined with RC preamps was terminated for a time.



Some different brands of commercial baluns were compared to half-wave coaxial baluns on both the test bench and also installed at the long Yagis. A selected pair of Philmore MT-74 baluns provided the lowest loss at Upper VHF. The signal loss of the MT-74 baluns was ~ 0.4 dB worse on channel 11, than the half wave loop baluns. The Philmore MT-74 baluns were used with an RC-9267 preamplifier throughout the remaining tests.

* Recent purchases and testing of Philmore MT-74 baluns indicate quality of construction, materials, and performance at VHF, have significantly changed (negatively) compared to the units tested in 2010.

.
Attached Images
File Type: jpg 2 Stack Yagi Test.jpg (169.1 KB, 4805 views)
tripelo is offline   Reply With Quote