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Old 29-Apr-2016, 5:12 PM   #100
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Join Date: Jan 2010
Location: Dallas, TX
Posts: 171
Received Signal Strength Data Analysis

(continued from previous post)

Part 3. Received Signal Strength Data Analysis, Five UHF Antennas

Attached are four Graphs depicting a summary of data analysis.

1. Average Relative Gain, (Image below):

An average 2 scans of all channels for each antenna, relative to the HDTVi antenna.

2.Average Relative Gain per Channel (image below)

A two-scan average of all antennas except HDTVi relative to HDTVi
(included four scans, two at beginning and two at end).

3. Average SWR 5 Antennas (image below)

SWR was derived from measured S11 (Return Loss). The curves represent the
smoothed scan averages for each antenna.

4. Mismatch Loss vs. SWR* (image below, see Note at end)

Table showing mismatch loss as a function of SWR.

========== Comments ====================

The CM4228 had overall slightly higher signal levels, this was mainly due to better gain at lower channels.

The HDB91x delivered highest signal strength at mid and higher channels, and with exception at channel 20, 2nd highest at lower channels.

The 91xg pretty much tied with the HDB91x at very highest channels, with average levels at lower channels.

The DB8 was about average at lower channels similar to the two Yagis. At midband signal levels dropped and stayed lower thru upper band. The signal level loss at midband might be related to the SWR (approaching SWR=4, with nearly 2 dB mismatch loss).

Notes: SWR* & Mismatch Loss

SWR is a positive number representing how well the antenna impedance matches a transmission line. For Example: SWR=1 represents a perfect match, Antenna impedance transformed by balun or other means to 75 Ohms perfectly matches a 75 Ohm coaxial line. Greatly simplified a SWR of 2 could be that the antenna impedance is either 150 Ohms or 37.5 Ohms, either of which is mismatched by a factor of 2.

When there is an impedance mismatch at intersection of antenna and transmission line, there is a certain amount of signal that is rejected (cannot be transferred from the antenna into the transmission line). This loss is called 'mismatch loss'.

When modeling antennas in software (i.e. 4NEC2), to obtain the true gain (sometimes called 'net gain') the mismatch loss must be subtracted from the calculated gain.

When measuring antennas in field test or at an antenna range, the effect of SWR is incorporated in measured signal levels. So, no subtraction is required. One could add back in the mismatch loss and derive a gain-like number, this number would be the 'directivity' of an antenna.

In the case of the antennas above, if the mismatch losses were added back to the measured responses, then one would see that the 'directivity' of the DB8 might be nearer to the 'directivity' of the other antennas.

Derivation of SWR & Mismatch Loss

Scattering parameter S11 was measured with reference at antenna/balun terminals. S11 represents a complex (real and imaginary) reflection coefficient, which includes ‘return loss’.

From return loss (RL), both SWR and ‘Mismatch Loss’ (ML) can be can be calculated:

(To be continued)

Attached Images
File Type: gif Avg Relative Gain.gif (7.7 KB, 2715 views)
File Type: gif Avg Relative Sig Level per Ch.gif (14.4 KB, 2766 views)
File Type: gif Avg SWR 5 Antennas.gif (10.9 KB, 3079 views)
File Type: jpg Mismatch Loss vs SWR.jpg (23.0 KB, 2859 views)
File Type: jpg SWR & ML Calculations.jpg (14.2 KB, 2661 views)
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