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Antenna Stacking Test - JBX08WB

Description:
The main
goal of this test is to find a smaller, lighter, and less expensive pair
of antennas for horizontal stacking. Since you are picking up some extra
gain from stacking, you can afford to lose a little gain on each antenna.
I used two Blake JBX08WB antennas in this test. These antennas are very
similar to the popular JBX21WB tested
here, except they are shorter and have only 8 director elements. As
seen above, these can be mounted at the end of the boom making for a very
neat installation, but a boom support tube is also included. To confirm
that the antennas were working properly and matched in performance, I
first tested each antenna separately. I used a Spectrum Analyzer to check
the waveform for each station. See below for results and comments.
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Set up:
I mounted
the antennas to a Dual Antenna
Boom Assembly
for easy spacing adjustments. The boom assembly was then mounted to a
ATF-V100 Compact Vertical Tilter
for precise positioning. Vertical angle positioning is very critical with
stacked antennas, since the vertical beamwidth gets even tighter. The
antennas were then connected to the
Low-Loss RG-11 feed line through a
Antenna Phasing Harness. I started with 24" spacing, then 36" and 44".
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Results
Definition:
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XX-XX =
Measured signal strength level
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0-XX-XX =
Signal locks on briefly but keeps dropping out
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0-50 = Signal
was detected but not able to lock
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0 = No signal
detected
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- = Not tested
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Note:
Signal level refers to the Dish 6000 indicator which measures the signal
quality based on percentage of errors. As long as the indicator stays above
48-50%, the signal will lock. I assume that if you have over 50% errors, the
Dish 6000 does not have enough good signal to decode properly. A reading of
100% would indicate no errors in signal. I have found that at 47% I will see
pixelation in the picture and below 46% the picture will drop out
completely. |
Test#1

Spectrum Analyzer Waveforms:
Mt. Wilson Stations (15 mile
distance)
Antenna A
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Antenna B
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Stacked A+B
24" spacing
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Stacked A+B
36" spacing
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Stacked A+B
44" spacing
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Channel 31 |

Channel 31 |

Channel 31 |

Channel 31 |

Channel 31 |

Channel 36 |

Channel 36 |

Channel 36 |

Channel 36 |

Channel 36 |

Channel 42 |

Channel 42 |

Channel 42 |

Channel 42 |

Channel 42 |

Channel 53 |

Channel 53 |

Channel 53 |

Channel 53 |

Channel 53 |
N/A
Channel 59 |
N/A
Channel 59 |
N/A
Channel 59 |
N/A
Channel 59 |
N/A
Channel 59 |

Channel 60 |

Channel 60 |

Channel 60 |

Channel 60 |

Channel 60 |

Channel 61 |

Channel 61 |

Channel 61 |

Channel 61 |

Channel 61 |

Channel 65 |

Channel 65 |

Channel 65 |

Channel 65 |

Channel 65 |

Channel 66 |

Channel 66 |

Channel 66 |

Channel 66 |

Channel 66 |
Conclusion:
- I found the shape of the JBX08WB waveforms to be very similar to the
JBX14WB and JBX21WB, the main difference being the gain. The level of gain can
be seen in the height of the signal waveform, the large graduation lines
represent 10dB. This decrease in gain is to be expected, since the 8 element
antenna has less gain than the 21 element. On most channels in my location,
the signal was too weak to lock in even though the shape was free from
multipath.
- Multipath reduction improves as the antennas are moved farther apart. You
can see the level of multipath in the waveform pictures. Channel 42 is a good
example to see how the signal line gets flatter as the spacing increases. A
clean signal should appear as a straight line. The bumps in this line
drastically reduces the signal levels on the Dish6000'. I have found that the
flatter this line gets, the higher the signal numbers get and the better the
channel stays locked in.
- The wider spacing resulted in greater multipath reduction, but it also
makes the antenna more directional. Turning the horizontal rotator just 2
degrees in either direction makes a big difference on the signal numbers and
the shape of the waveform, especially on the weakest channels, the stronger
channels are not as sensitive. Going beyond 44" spacing, the tuning gets too
sensitive, just the wind shaking the antenna around can cause drop outs on
weak channels. It is best to experiment with the spacing in your location, but
generally you can get away with wider spacing the farther away you are from
the transmitters.
- Horizontal stacking also decreases the vertical beamwidth, this makes the
vertical angle of the antennas very important in locking in the weakest
stations. I would never be able to get channel 53 without a vertical tilter.
If I change the vertical position of the antenna just one degree either up or
down, channel 53 will drop out completely. Fine tuning for strongest signal is
just a matter of +/- 0.5 degree. With channels 36 and 65, the critical angle
is +/- 2 degrees and channel 31 is +/- 3 degrees. This angle will change
according to weather and time of day.
- The JBX08WB stacked should work well in areas where signal gain is high
(line of site at 15-30 miles), but severe multipath is preventing reliable
reception.
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