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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:

  • XX-XX = Measured signal strength level

  • 0-XX-XX = Signal locks on briefly but keeps dropping out

  • 0-50 = Signal was detected but not able to lock

  • 0 = No signal detected

  • - = 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)

Antenna A

Antenna B

Stacked A+B

28" spacing

Stacked A+B

36" spacing

WB-31A.jpg (56419 bytes)

Channel 31

WB-31B.jpg (53016 bytes)

Channel 31

WB-31AB.jpg (47000 bytes)

Channel 31

WB-31AB-36.jpg (63150 bytes)

Channel 31

WB-36A.jpg (55873 bytes)

Channel 36

WB-36B.jpg (54867 bytes)

Channel 36

WB-36AB.jpg (47815 bytes)

Channel 36

WB-36AB-36.jpg (61864 bytes)

Channel 36

WB-43A.jpg (53943 bytes)

Channel 42

WB-43B.jpg (52784 bytes)

Channel 42

WB-43AB.jpg (48936 bytes)

Channel 42

WB-42AB-36.jpg (59323 bytes)

Channel 42

WB-53A.jpg (55285 bytes)

Channel 53

WB-53B.jpg (52374 bytes)

Channel 53

WB-53AB.jpg (49035 bytes)

Channel 53

WB-53AB-36.jpg (61714 bytes)

Channel 53

WB-59A.jpg (56498 bytes)

Channel 59

WB-59B.jpg (54620 bytes)

Channel 59

WB-59AB.jpg (47946 bytes)

Channel 59

WB-59AB-36.jpg (59648 bytes)

Channel 59

WB-60A.jpg (54801 bytes)

Channel 60

WB-60B.jpg (54724 bytes)

Channel 60

WB-60AB.jpg (47580 bytes)

Channel 60

WB-60AB-36.jpg (60696 bytes)

Channel 60

WB-61A.jpg (55001 bytes)

Channel 61

WB-61B.jpg (54130 bytes)

Channel 61

WB-61AB.jpg (50460 bytes)

Channel 61

WB-61AB-36.jpg (62399 bytes)

Channel 61

WB-65A.jpg (53190 bytes)

Channel 65

WB-65B.jpg (53850 bytes)

Channel 65

WB-65AB.jpg (49792 bytes)

Channel 65

WB-65AB-36.jpg (60045 bytes)

Channel 65

WB-66A.jpg (54400 bytes)

Channel 66

WB-66B.jpg (54897 bytes)

Channel 66

WB-66AB.jpg (49432 bytes)

Channel 66

WB-66AB-36.jpg (60486 bytes)

Channel 66

 

San Diego Stations (130 mile distance)

WB-19AB.jpg (49838 bytes)

Channel 19

WB-23AB.jpg (49488 bytes)

Channel 23

WB-25AB.jpg (51689 bytes)

Channel 25

WB-30AB.jpg (49527 bytes)

Channel 30

WB-39AB.jpg (52562 bytes)

Channel 39

WB-55AB.jpg (50132 bytes)

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

 

Test#3, Date: 3/04/02 - 3/07/02

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)

martyn-ch31.jpg (59488 bytes)

Channel 31

martyn-ch36.jpg (58259 bytes)

Channel 36

martyn-ch42.jpg (57503 bytes)

Channel 42

N/A

Channel 43

martyn-ch53.jpg (58072 bytes)

Channel 53

martyn-ch59.jpg (60143 bytes)

Channel 59

martyn-ch60.jpg (59170 bytes)

Channel 60

martyn-ch61.jpg (58718 bytes)

Channel 61

martyn-ch65.jpg (59978 bytes)

Channel 65

martyn-ch66.jpg (59881 bytes)

Channel 66

 

 

  


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