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Antenna Stacking Test - JBX21WB

Description:
This is my first
attempt at stacking UHF antennas. I used two Blake JBX21WB antennas. To confirm
that the antennas were working properly and matched in performance I tested each
antenna separately first. I used a Spectrum Analyzer to check the waveform for
each station. The antennas were very closely matched. Horizontal stacking gives
the best reduction in multipath since the vertical beamwidth is already very
tight on a single antenna. I mounted the antennas to my
ATF-V100 Vertical Tilter
using a custom made boom assembly.
This boom assembly uses a fiberglass tube to prevent the boom from becoming a
active element in the antenna system. Each antenna mount clamps to the boom and
allows for easy adjustments to the spacing between antennas. I started with a
28" spacing, then tried 20" and 36". See below for results and my comments
following the tests.
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
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.
**For easier
printing, click on image to bring up a separate page containing just results.
Test#1, Date: 2/15/02

Test#2, Date: 2/18/02

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

Channel 31 |

Channel 31 |

Channel 31 |

Channel 36 |

Channel 36 |

Channel 36 |

Channel 36 |

Channel 42 |

Channel 42 |

Channel 42 |

Channel 42 |

Channel 53 |

Channel 53 |

Channel 53 |

Channel 53 |

Channel 59 |

Channel 59 |

Channel 59 |

Channel 59 |

Channel 60 |

Channel 60 |

Channel 60 |

Channel 60 |

Channel 61 |

Channel 61 |

Channel 61 |

Channel 61 |

Channel 65 |

Channel 65 |

Channel 65 |

Channel 65 |

Channel 66 |

Channel 66 |

Channel 66 |

Channel 66 |
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San Diego Stations (130 mile distance)
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Channel 19 |

Channel 23 |

Channel 25 |

Channel 30 |

Channel 39 |

Channel 55 |
Conclusion:
- This experiment made a huge improvement in the reception of channels
suffering from bad multipath.
- You can see the level of multipath in the waveform pictures. 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 a flatter signal waveform on all channels.
The wider spacing also makes the antenna much 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.
- My theory is that with narrow spacing the radiation pattern of the antenna
overlaps and almost becomes one. The inside elements may not be doing as much
good in this condition. With wider spacing, the antennas are acting more like
separate antennas that are able to cancel out the bad signals coming into the
antenna at a angle and increase the reception of good signals that are
approaching the antennas head on.
- 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 42, 53, 59, 66 without a
vertical tilter. If I change the vertical position of the antenna just one
degree either up or down, channel 42, 53, 59, 66 will drop out completely.
Fine tuning for strongest signal is just a matter of +/- 0.5 degree. With
channels 36, 61, 65 the critical angle is +/- 2 degrees and channel 31, my
strongest channel, is +/- 3 degrees. This angle will change according to
weather and time of day.
- In test #2, I am going to compare some other spacing. I have already
determined that a 44" spacing works even better than 36", but the tuning angle
is starting to get very sensitive, requiring each station to be fine tuned
within a few degrees.
- Last night on KABC-DT, I was able to watch "Raiders of the Lost Ark" in
OAR 2.35 with DD5.1 without any dropouts at 44" spacing, previously I have
never been able to get this channel at all with any single antenna. This is
the way it should be :)
Additional comments added for Test #2
- Channel 43 and 59 were not broadcasting during this test
- The channels that suffer the worst multipath improved with wider spacing.
The wider spacing helps to cancel out stray signals. The downside is the
tuning angle becomes more sensitive making it difficult to get the exact angle
for each station
- The easy to tune channels 31 and 36 improved a little with the narrow
spacing. The gain went up slightly with narrow spacing. Since these channels
are not as affected by multipath, the additional gain improved the signal
numbers
- As you can see, the exact spacing is not real critical. If you want higher
gain, use less spacing. If you want better multipath rejection, use wider
spacing
Description:
This test was done
in Pacific Palisades, Calif. using the same antenna setup shown above. In this
location, line of sight is blocked by a large mountain ridge just a few miles
away. The signal strength numbers can not be compared to the ones above because
a RCA DTC100 receiver was used.
First test was
using a single JBX21C/D, the remaining tests were done with a pair of
horizontally stacked JBX21WB's.

Mt. Wilson Stations (33 mile distance)
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Channel 31 |

Channel 36 |

Channel 42 |
N/A
Channel 43 |

Channel 53 |

Channel 59 |

Channel 60 |

Channel 61 |

Channel 65 |

Channel 66
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