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Antenna Stacking Test - ATF-X300
shown with new
style Integrated
Stacking Boom
and
Compact Vertical Tilter
Description:
The main goal I
had when designing this new antenna was to build a smaller antenna that can
match the performance of much larger and heavier antennas. I also wanted to
design a new simpler stacking boom arrangement to save weight and reduce wind
load. I must have tested a hundred different element positions during the
initial development to find the optimum reception on all channels.
I first
tested each antenna separately. In all my previous tests I tested the single
antenna mounted directly to the mast with its standard bracket. When tested
in this configuration both antennas tested exactly the same. In this test, I
placed the antennas on the stacking boom and ran a test on each antenna. I
was surprised to see the difference in reception I got from just a four foot
change in lateral position. I also found out that the optimum horizontal
angle was different for each antenna. There was a 2-5 degree difference in
horizontal angle, so I "toed in" the antennas about 5 degrees, this change
gave me a 5% increase in signal.
I used a
Spectrum Analyzer to check the waveform for each station. The waveforms
allow you to measure signal strength in decibels and to see multipath
interference in the signal. See below for results and comments.
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Set up:
I mounted
the antennas to my new
Integrated Stacking Boom. 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 set the spacing to 48".
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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. |
Dish 6000 Test

Spectrum Analyzer Waveforms:
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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
48" spacing
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Channel 31 |

Channel 31 |

Channel 31 |

Channel 36 |

Channel 36 |

Channel 36 |

Channel 42 |

Channel 42 |

Channel 42 |

Channel 53 |

Channel 53 |

Channel 53 |

Channel 59 |

Channel 59 |

Channel 59 |

Channel 60 |

Channel 60 |

Channel 60 |

Channel 61 |

Channel 61 |

Channel 61 |

Channel 65 |

Channel 65 |

Channel 65 |

Channel 66 |

Channel 66 |

Channel 66 |
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Conclusion:
- As noted above, in all my previous tests, I tested the single antennas
mounted directly to the mast in the same lateral location. When I tested the
X300 in identical locations, both antennas tested exactly the same with the
waveforms appearing the same on each antenna. However, during this test, I
placed the antennas on the stacking boom spaced 48" apart and ran a test on
each antenna separately. I was surprised to see the difference in reception I
got from just a four foot change in lateral position. At first I thought it
was because one antenna was shielding the other, so I removed the antenna not
being tested and found it was actually the change in lateral position that was
accounting for the difference in performance. This is proof that the placement
of your antenna on the roof plays a important part in reception, especially in
multipath conditions!
- It is easy to see the level of multipath in the pictures above by the
sharp notches and bumps in the waveform. A nice, clean signal should appear as
a straight horizontal line. I have found that the flatter this line gets, the
higher the signal number gets on the Dish6000 and the better the channel stays
locked in. When the notches are deep enough (about 7dB) the signal will not
lock in. The stacked antenna arrangement greatly reduces the multipath notches
in the signal.
- Horizontal stacking decreases the vertical beamwidth, this makes the
vertical angle of the antennas very important in locking in the weakest
stations. Example- It would be very difficult to get channel 53 (ABC) 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 on 53 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 horizontal tuning angle also decreases. For the weaker stations, just
+/- 2 degrees makes a big difference. For the best signal I normally manually
enter the angle on the Channel Master rotator, since it moves in 2 degree
increments when you push the arrow buttons.
- Not shown in this test is the reception from San Diego 130 miles away.
When the atmospheric conditions are favorable, all San Diego channels come in
very strong. Horizontal and vertical angles are not as critical when picking
up long distance signals since multipath is rarely a problem, increasing gain
yields the best performance with long range reception.
- I have been running these two X300's now for about two months. They are so
far the best performing antennas I have used to date. I can't tell you exactly
why that is, since all my development was done through trial and error and not
by using a computer program or a laboratory. I did use a spreadsheet for
calculating the progressive element spacing. I was able to modify certain
variables to alter the spacing characteristics, then I did tests to determine
the values which seemed to work best. I then fine-tuned these variables to get
the optimum performance.
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