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For the past ten years, I have been developing and refining an automated system for measuring and recording the actual signal strength and coverage area of radio transmitters. This system consists of a laptop computer running a program I wrote, IFR-1500 spectrum analyzer, Magellan GPS receiver, several transceivers and various other hardware installed in my 1997 VW Passat TDI wagon. The system can make signal strength measurements on as many as 8 different channels or transmit sites once every 10 seconds, as I drive. The measurements are recorded on the PC along with position data from the GPS receiver. After the measuring run, I plot the measurements tagged with GPS positions on a map using MapInfo 4.5 desktop mapping. The test points appear as dots superimposed on a road map, and are color-coded to correspond to the signal level (in dBm) encountered at each point.
This system has been used to verify the predicted coverage of newly-built transmitting sites after they are constructed. It has been used to compare the performance of different antennas and/or frequency bands at the same location. It has also been used to produce 'before" and "after" comparisons for antenna changes and upgrades. In another application, it discovered some coverage "holes" due to defective antennas in an in-house radio radio system in a newly-built prison.
Sample Coverage Maps in Adobe Acrobat PDF (Portable Document File) format:
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These two maps show the RF carrier coverage of four sites used to support mobile data terminals in utility service trucks. The first document shows the coverage of each site alone on four separate maps. The second document shows the combined coverage of all four sites working together.
Clicking on the link will cause the file to open in your web browser if Acrobat is installed on your system; right-click the link and choose "Save File As.." or "Save Target As.." to save the file to your hard drive for viewing off-line
RF Carrier Coverage (dBm) of Four Individual
Sites
Combined Coverage of the Four Sites
Home Two-Tone SINAD Testing System Top Of Page
Traditionally, radio coverage tests have focused on measuring signal strength. Many large-area-coverage radio systems today use simulcast. This is the use of multiple transmitters on the same frequency at different locations transmitting simultaneously in an effort to cover a larger area than one transmitter alone. The performance characteristic that needs testing is not signal strength (the overlapping pattern of signals from serveral transmitters normally guarantees enough carrier). The real problem is audio distortion that results when signals from several different transmitters reach the mobile receiver at about the same strength. The audio characteristics of the transmitters involved have to be precisely matched and time delays added to compensate for the difference in distance to the user from the various transmitters involved. When these adjustments are not done properly, even an extremely strong signal can be severely distorted.
I have developed a test system to evaluate the effect of simulcast on audio quality. The traditional method of evaluating distortion in mobile radio receivers is the SINAD (SIgnal-to-Noise-And-Distortion) measurement. SINAD tests typically involve modulating the transmitter or signal source with a 1KHz tone. At the receiver speaker terminals, the 1KHz tone is notched out (removed). The remaining signal level is measured and expressed in decibels below the (now-missing) test tone. In an ideal noiseless, distortion-free system, nothing would remain after the test tone is removed, yielding an infinite dB SINAD ratio. In reality, the SINAD reading reflects the effect of scratchy, hissy noise mixed with the desired signal at low signal levels, and the residual distortion of the transmitter and receiver when the signal level is strong enough to cover up background noise. A noisy, scratchy signal on the edge of intelligibility is typically 10-14 dB SINAD. A SINAD ratio greater than 20-22 db is perceived in the average two-way radio as a perfect signal.
My first attempt to measure the distortion created by simulcast involved modulating all the base stations in the system under test with a 1KHz tone, and then reading/recording the SINAD at the mobile radio's speaker. This approach was effective in revealing areas of poor RF coverage but didn't accurately reflect the effects of simulcast in strong signal areas. I then realized that the main mechanism of distortion in simulcast was intermodulation (the mixing of several audio frequencies together to yield new ones that are the sums and differences of the ones originally present). When only one tone is present, there can't be any intermodulation, thus the SINAD meter doesn't see anything to measure. I modified the audio generator used to modulate the base stations to generate two tones (1000 KHz and 1633 KHz). Normal SINAD meters have only one notch filter (to remove the standard 1000 KHz test tone). I built a second notch filter tuned to 1633 KHz and inserted it between the mobile speaker and the SINAD meter. The difference is dramatic. The automated measurements agree very closely with subjective judgements of voice quality. The two-tone test ruthlessly reveals other imperfections in the system: overdriven audio line amplifiers, microwave mux card deficiencies, non-linear transmitter modulators, off-frequency receivers, etc.
Sample Two-Tone SINAD maps in Adobe Acrobat .PDF (Portable Document File) format: (Click on link for direct read in Acrobat-enabled browsers. Right-click on link or click on icon for FTP download to your hard drive for viewing offline.)
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Two-Tone SINAD Test of Three-Transmitter
Simulcast System (Proper Simulcast Alignment)
Two-Tone SINAD Test of Three-Transmitter Simulcast
System (Incorrect Simulcast Alignment )
(This test intentionally
introduced a 180-degree phase reversal in the circuit supplying
audio to one of the transmitters)
Home Some Examples of Carrier Level Coverage Plots Top of Page
The two maps linked below are the results of an investigation into a poor performance complaint for a 900 Mhz radio base station. The "before" plot shows that this this antenna which was supposed to have 10-12 dB forward gain to the west (left of picture) and 25-30 dB front-to-back radio simply wasn't performing; the pattern is practically omni-directional.
An on-site inspection revealed that the antenna had been mounted on a crowded tower firing into a ladder about 3 feet in front of the antenna. The antenna was relocated to have a clear, unobstructed view in it's forward direction. The second plot confirms that the radiation pattern of the antenna has improved dramatically, exhibiting a tremendous increase is signal strength in the forward direction and a proper front-to-back ratio.
The two maps themselves are clickable buttons; you can click on either map to see the other. Once both images are cached in your browser, you can rapidly toggle between the two versions.