HERALD Lab #4

Home Page

Ground Reflections

HERALD Functions

Exercise 4.1 : Estimate reflection parameters

Exercise 4.2 : Analyze Rx power level

Exercise 4.3 : Design diversity Rx

Exercise 4.4 : Change reflection parameters

Exercise 1.5 : Move radio site position

_______________________________________________________________

HERALD Functions

The following Herald functions are related to Reflection analysis :

· The Evaluate / Reflections / Plot All command displays the Path Profile and plots all the (geometrically possible) reflection points, even if partially obstructed. The operator is enabled to selected a single reflection, to be analyzed in details; the relevant geometrical and radio parameters are presented in the table above the profile diagram.

· After a reflection point has been selected on the profile diagram (reflected rays plotted in red), the Evaluate / Reflections / Analyze command can be used to display the "Reflection Analysis" dialog. In the upper frame, the operator sets the relevant parameters (reflection coefficient, use of diversity and antenna spacing). In the second frame the results are shown, with details on the Reflection Path Losses (Antenna directivity, Obstruction Loss in the Reflection Paths, Reflection Coefficient, and Divergence Factor), reflection delay and grazing angle. In the lower frame some Notes may appear when critical parameters are estimated.

· The "Rx Power vs. k" button, in the "Reflection Analysis" dialog, displays a diagram where the received power is plotted as a function of the k-factor, at the main and at the diversity (if configured) antennas.

· The "OK - Store results" button, in the "Reflection Analysis" dialog, saves the Reflection Analysis results. Note that the Rx threshold degradation (single and diversity reception, if configured) is estimated, while the operator can enter the final degradation result (to be included in the Link Budget), taking account of some conservative margin, if necessary.

Exercise 4.1 : Estimate reflection parameters

Run Herald and open the "Reflection1_Sample.hpf" project. This project is already initialized (4 GHz, 60 km over-the-sea radio hop with reflection).

· Select the ALPHA-BETA hop and execute the Evaluate / Reflections / Plot All command.

Tip : Use the Herald icon

· The reflection point at 30.9 km should be selected. If another reflection point is selected, change selection by pressing the Tab key

· Examine the Reflection parameters; in particular note that:

· the path difference is 78 cm, much greater than the signal wavelength (7.5 cm at 4 GHz);

· the time delay is 2.6 ns, much shorter than the symbol period in any digital modulation;

· the direct-to-reflected ray angles, at both terminals are very small, so the loss in antenna gain is almost negligible (0.3 dB);

· the reflected ray path is clear (no obstruction loss).

· By pressing the Tab key, select the other reflection point (at about 5.4 km); this reflection is probably NOT significant, it derives from approximations made in the path profile, describing the steep terrain close to the radio site; note, in this case, a substantial loss in antenna gain (6 dB) and a partial obstruction of the reflected ray.

Exercise 4.2 : Analyze Rx power level

Go on with the "Reflection1_Sample.hpf" project, as in the previous exercise.

· Execute the Evaluate / Reflections / Plot All command, then select the reflection point at 30.9 km (use the Tab key, if necessary).

· Execute the Evaluate / Reflections / Analyze command. In the "Reflection analysis" dialog, first consider the "SET PARAMETERS" frame; set the Reflection coefficient to 0 dB, and uncheck the two Diversity checkboxes

· Examine the "RESULTS" frame; note the Total Loss in the reflection path, with all the items that contribute to the final result. Take note of the Rx degradation estimate.

· Click the "Rx Power vs. k" button to get the diagram of received power as a function of k-factor. Note that a small change in the k-factor around the standard value (1.33) causes the Rx power to change significantly. Return to the "Reflection analysis" dialog.

Exercise 4.3 : Design diversity Rx

Go on with the "Reflection1_Sample.hpf" project, as in the previous exercise. Display the "Reflection analysis" dialog.

· Enable the Diversity checkboxes (upper frame), set the antenna spacing to optimum values (Optimum Diversity button) and repeat the steps indicated at the previous point.

· The Rx degradation with diversity is 0 dB. Display the Rx power vs. k diagram and verify that the two curves are well interleaved (not overlapping). For any k-factor value, one of the two antennas receives a signal at a high power level: diversity reception is quite effective (however, consider that 0 dB degradation is computed under the assumption of an ideal diversity switching).

· Return to the "Reflection analysis" dialog and set the antenna spacing at Site ALPHA at 12 m; note that the Rx degradation (with diversity) is increased to 1.8 dB. In the Rx power vs. k diagram, the two curves at site ALPHA are not well interleaved and overlap for k>1.5; this is an example of INEFFECTIVE diversity design.

· Complete your design with optimum diversity spacing and a final estimate of Rx degradation. This may be a 1 dB degradation, since the 0 dB estimate is computed under the assumption of an ideal diversity switching and the Single Rx degradation is estimated 3.9 dB. Set the final estimate in the "Assumed in Link Budget" data cell. Press the "OK - Store results" button to exit, then check that the results are included in the Hop Report and in the Profile Report.

Exercise 4.4 : Change reflection parameters

Run Herald and open the "Reflection1_Sample.hpf" project.. Display the Path Profile with reflection rays. Starting from this state, you have several ways of changing the radioelectrical and/or geometrical parameters in the reflection path. Below are some examples; in each condition, take note of new aspects of reflection analysis.

· Execute the Evaluate / Reflections / Analyze command. In the "Reflection analysis" dialog, modify the Reflection coefficient to 3, 6, or 10 dB. Take note of new results with increasing attenuation of the reflected ray.

· In the Path Profile diagram, select the reflection point at 5.4 km. Repeat all the main steps in previous analysis, assuming a 3 dB reflection coefficient (quite conservative for reflection on dry soil or rocks). Note the role of antenna directivity in increasing the reflection path loss.

Exercise 4.5 : Move radio site position

Run Herald and open the "Reflection1_Sample.hpf" project.. Display the Path Profile with reflection rays.

· Execute the Define / Path Profile command and modify the Hop Length from 59.5 to about 62.0 km; this means that BETA has been moved some 2.5 km far from the coastline.

· Close the "Path Profile" dialog; then execute the Reflections / Plot All command. The new BETA position produces a partial obstruction in the reflected ray, with a substantial signal loss (see reflection data in the table above the profile diagram).

· Check Path Clearance with the new BETA site position (see HERALD Lab #3). It appears that clearance criteria are not satisfied. Increase by some 2-4 m the antenna height at the BETA site, so that path clearance is confirmed. Check again the reflection results (the obstruction loss in the reflected signal is slightly lower, but still useful).

· Execute the Reflections / Analyze command and revise the results with the additional attenuation in the reflected signal. This is an example of the significant benefit offered by a "lucky" positioning of the radio site (unfortunately, hop design is not always so easy as in computer examples ...).

End of HERALD Lab #4

_______________________________________________________________