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24-Jun-2017, 1:15 PM   #109
tripelo
Senior Member

Join Date: Jan 2010
Location: Dallas, TX
Posts: 127
Compare: Antennas Direct DB8e & CM-4228HD

(continued from previous post)

Part 3. Data Collection & Analysis

The amount of data collected represents:

Number of UHF channels: 23
Measurements per channel
DB8e4 and 4228HD: 9
HDTVi: 12

Total (individual channel measurements): 23 x 9 x 2 + 23 x 12 x1= 690

Attached are Graphs depicting a summary of data analyses.

1. Average received power levels, (Image below):

The image below represents an average of 23 UHF channels, nine measurements per UHF channel for each antenna (4228HD and DB8e), 12 measurements per channel for the HDTVi.

The image below shows the received signal strength per UHF channel for the three antennas. Note signal strength is shown in dBmV (dB referenced to milliVolts in a 75 Ohms system. To convert to dBm (milliWatts) subtract 48.75. from the dBmV numbers.

The image below shows the received signal strength of the DB8e and 4228HD antennas in dB, relative to the HDTVi antenna. The HDTVi signal strength is referenced to zero on the vertical dB scale.

The image below shows a linear best-fit approximation of the received signal strength of the DB8e and 4228HD antennas in dB, relative to the HDTVi antenna.

The following image shows SWR for the three antennas, it was derived from S11 (Return Loss), measured at 25 feet AGL.

The fuzziness of the lines in the SWR graph results from simultaneously receiving the broadcast stations in DFW with the Vector Network Analyzer. The reception of these strong signals caused some interference with the live impedance measurements. The antennas were not turned away from the stations as was done in previous measurements

All three antennas have respectively good values of SWR. No mismatch loss for SWR equal to 1, a SWR of 2 results in a mismatch loss of about 0.5 dB.

Antennas Direct Technical data sheet indicates the SWR is less than 2 from 470 to 698 MHZ. The SWR measurement shown above for the DB8e is fairly well in agreement with Antenna Direct published specifications. Published SWR specifications for the 4228HD are not available.

For mismatch Loss vs. SWR, see previous post*.

*Received Signal Strength and Data Analysis

The Antennas Direct DB8 provides highest overall signal strength, with an advantage over the 4228HD at high channels approaching 4-5 dB. TheCM4228HD performs well for channels below about channel 23, with an advantage of more than 1 dB at the lowest end of the band.

The 4228HD appears to have reduced gain as the frequency increases. The reason for this phenomenon was not investigated in this effort. However, it may be related to the transmission line feed for individual driven elements in the 8-element array, and particularly the feed between the two 4-bay segments.

Others have explored this 4228HD gain effect:

HDTV primer: Designed an improved antenna transmission line feed.

Antennahacks: Implemented an improved transmission line feed system.

Test Location and Data Variability

The test site is LOS to the DFW stations, however effects of multipath have been observed at UHF with received signal on some channels

Several repeats of each measurement probably helped reduce any effects of brief transitory signal variability. The individual data sets were fairly consistent. Measurements with the HDTVi, at both beginning-of-test and end-of-test, somewhat verified data consistency.

(to be continued)

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Attached Images
 Compare All Channels.gif (4.8 KB, 3223 views) Avg Sig Strength per Channel.gif (12.3 KB, 3347 views) Per Channel Relative to HDTVi.gif (10.6 KB, 3221 views) 4228HD DB83 HDTVi Linear fit.gif (9.2 KB, 3184 views) SWR Analysis.gif (12.7 KB, 3136 views)