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Q: What loss values should I include in my coverage calculations?
A: When preparing a coverage map you must specify desired field strength thresholds in order to plot a map. The plot is then used to assess locations of particularly low signal level. If the engineer specifies levels assuming that we live in a perfect world with no localized losses when operating inside buildings and also assumes that all radio users carefully maintain their antennas in a perfectly vertical position to maximize reception, we are in for a rude awakening. Our prediction will be overly optimistic leading us to believe that we have reliable operation in areas where that may not be the case. In order to increase our accuracy in signal prediction in the real world, we must include specifications for these very real losses in our calculations.
Because this information is so important and not readily available, one of the many events at the Fourth Annual User’s Engineering Seminar was an extensive measurement process of building and body field strength losses. Larry D. Ellis, P.E., President of SoftWright was assisted by Mr. Joel Humke, J. C. Humke & Associates, Inc., Aurora, Colorado and Mr. John DeHart, K1VBM, Bay Area Transit Consultants, Oakland, CA. John operated a handheld transceiver outside the Denver Hyatt hotel from a parking lot approximately 250 feet away from the building door. A receiving antenna was mounted on a wooden mast at five feet above ground level. The antenna was connected to a Techtronix Model 7L12 Spectrum Analyzer via 100’of RG58 coax. The visual output of the analyzer was monitored via TV camera and projected on to a 15’ television screen for all to view. Extensive measurements were made at three frequencies 146.52 MHz, 446.00 MHz, and 1294.50 MHz. Measurements were made with optimum antenna orientations, 45 degree cross polarization, transmitting handheld next to and far away from the body with John rotating through 360 degrees permitting monitoring of antenna pattern variations. The same measurements were then repeated after moving the receiving antenna inside the hotel ground floor meeting room. The angle to the handheld transmitter permitted the distance to the receiving antenna to remain essentially the same, with just the building attenuation to be the unknown variable. The outer walls of the hotel are steel and concrete. No windows were present in the path. The following table is a brief summary of our empirical findings.
| Frequency for test - MHz | 146.52 | 146.52 | 446.00 | 446.00 | 1294.50 |
| Location of receiving ant | inside | outside | inside | outside | outside |
| noise floor (dBm) | -95 | -95 | -95 | ||
| above head | -65,-78 | -32,-40 | -65,-72 | -38,-55 | -64,-78 |
| at arm’s length | -62,-90 | -32,-50 | -65,-75 | -35,-58 | -64,-80 |
| vertical at mouth | -72,-90 | -40,-50 | -62,-78 | -37,-64 | -65,-85 |
| 45° X polarized at mouth | -72,-90 | -46,-54 | -70,-85 | -45,-62 | -62,-85 |
| at waist | -85,-90 | -48,-60 | -70,-90 | -48,-75 | -68,-95 |
| max polarization | -62,-90 | -34,-55 | -62,-90 | -40,-60 | -68,-84 |
| 45° cross polarization | -65,-75 | -30,-35 | -67,-72 | -38,-50 | -67,-70 |
From these measurements we can draw a few reasonable conclusions. By subtracting the minimum and maximum received signal levels for those conditions that are affected by proximity to the operator’s body we find the following:
| Frequency for test - MHz | 146.52 | 146.52 | 446.00 | 446.00 | 1294.50 |
| Location of receiving ant | inside | outside | inside | outside | outside |
| above head | 13 dB | 8 dB | 7 dB | 17 dB | 14 dB |
| at arm’s length | 28 dB | 18 dB | 10 dB | 23 dB | 16 dB |
| vertical at mouth | 18 dB | 10 dB | 16 dB | 27 dB | 20 dB |
| 45° X polarized at mouth | 18 dB | 8 dB | 15 dB | 17 dB | 23 dB |
| at waist | 5 dB | 12 dB | 20 dB | 27 dB | 27 dB |
Bear in mind, these differences in dB represent the range of maximum attenuation in each particular situation due to location of the user’s body on the path between the transmitter and the receiver, NOT an aggregate suppression from maximum received signal for each of these situations. In other words, these values represent a range of "worse than normal" conditions. Any handheld operation results in loss due to proximity of the operator’s body. These values represent additional losses resulting from the common practices such as holding the transceiver at a less than vertical angle, standing with the body between the receiver and the repeater, etc. Body loss for typical 45 degree cross polarized usage at the mouth can easily represent a loss of 17-18 dB for UHF and VHF and approximately 23 dB at 1300 MHz. To design a system that is reliable under the worst case ("typical user operation") scenario, these extra losses must be considered.
The following situations were not taking into account changing body positions relative to the path and therefore represent only reduction in received signal level do the maximum cross polarization and typical 45 degree cross polarization. Again, these values represent the difference in received signal levels listed in the first table. These tests were made at a distance of approximately 3 feet from the operator.
| Frequency for test - MHz | 146.52 | 146.52 | 446.00 | 446.00 | 1294.50 |
| Location of receiving ant | inside | outside | inside | outside | outside |
| max polarization | 28 dB | 21 dB | 28 dB | 20 dB | 16 dB |
| 45° cross polarization | 10 dB | 5 dB | 5 dB | 12 dB | 3 dB |
These calculations show that when unaffected by body losses, typical cross polarization losses for typical hand held operation for both UHF and VHF to be 10-12 dB.
An analysis of the data with respect to losses due to operation of a receiving system inside this particular building can also be made. Subtracting the level of the received signal inside the building from that outside the building in similar configuration of operation will yield the following results:
| Frequency for test - MHz | 146.52 | 446.00 |
| outside - inside | outside - inside | |
| above head | 33 to 38 dB | 27 to 17 dB |
| at arm’s length | 30 to 40 dB | 30 to 37 dB |
| vertical at mouth | 32 to 40 dB | 25 to 14 dB |
| 45° X polarized at mouth | 26 to 36 dB | 35 to 23 dB |
| at waist | 37 to 30 dB | 22 to 15 dB |
| max polarization | 28 to 35 dB | 22 to 30 dB |
| 45° cross polarization | 35 to 40 dB | 29 to 22 dB |
Conclusions from this analysis is that for coverage inside the meeting room in this office building we need to include a building loss at VHF of approximately 35-40 dB and at UHF a building loss of 25-30 dB. Carefully including this in the evaluation of how much signal is enough questions will be required if it is important to insure reliable communications inside such a building.
In real life it would be reasonable to include a combination of building and body loss when establishing your company standard of how much received signal is adequate for your operation. The application of this knowledge is very important when designing a land mobile radio system. Building, body and cross polarization losses are real, significant and frequency dependent. The level of reliability of your system is greatly affected depending upon how your users mount, hold and operate their equipment.
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Copyright 1999 by SoftWright LLC