Antenna Stacking Test

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

Channel 31

Channel 36

Channel 42

Channel 53

Channel 59

Channel 60

Channel 61

Channel 65

Channel 66

San Diego Stations (130 mile distance)

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

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.