Automated Radio Signal Coverage Survey System

(Originally Posted in 1997)

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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 PDF  (Portable Document File)  format:  

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 or another PDF reader is installed on your system and integrated with your browser. 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

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Traditionally, radio coverage tests have focused on measuring signal strength.  Many large-area-coverage radio systems 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. (In southern California, where this work was done, the Los Angeles County Sheriff's Dept simulcasts from seventeen separate transmit sites to achieve countywide coverage.) 

In this kind of radio system, the performance characteristic that needs testing is not signal strength (the overlapping pattern of signals from several 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, so that no one signal positively captures the FM receiver.  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 nearly perfect signal.   

Typically this kind of measurement is used to test the sensitivity of an FM receiver only, on a service bench.  I used this measuring technique, instead,  to characterize the entire radio path (dispatch audio-->transmitters-->RF path-->receiver),  rather than a single piece of equipment such as an FM receiver or a stereo amp.

My first attempt to measure the distortion created by simulcast involved modulating all the base stations in the system under test with the customary 1KHz tone used in standard SINAD tests, 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 distortion a.k.a. IMD (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 as long as the FM receiver remains quieted.

I modified the audio generator used to modulate the base stations to generate two tones (1000 KHz and 1633 KHz) simultaneously. The two-tone generator was two Communications Specialists SS-32B tone encoders combined in a resistive "Y" network. (The "B" version of the SS-32 generates higher tone frequencies than the regular version's "subaudible" CTCSS tones.)  

Normal SINAD meters have only one  notch filter (to remove the standard 1000 KHz test tone) before reading the residual noise and distortion.  I built a second notch filter (twin-T active notch based on a 1458 dual op-amp) tuned to 1633 KHz, and inserted it between the mobile speaker and the SINAD meter input. The home-made notch filter was capable of over 40 dB rejection of the 1633 Hz test tone.   [Pro audio types will recognize this kind of test as very similar to the two-tone IMD tests used on high-end audio amplifiers.]

The difference in resulting over-the-air measurements was dramatic.  The automated measurements now agree very closely with subjective judgments of voice quality.  The two-tone SINAD test ruthlessly reveals other imperfections in the system (even on non-simulcast systems):  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.)

 Two-Tone SINAD Test of Three-Transmitter Simulcast System (Proper Simulcast Alignment)
(Note that even with "correct" simulcast alignment, many areas had high levels of simulcast distortion. However the target service area of this channel was the green areas along the 10 and 60 fwys.)

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, simulating a worst-case phasing error. In real life, the phase of the audio supplied to each simulcast transmitter is adjusted in small (10-electrical degrees) increments until the correct phasing and lowest distortion is achieved in the desired service area.)

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.

Note that these are very early maps created by my survey system, with the color coding "backwards" from what really makes sense.  I.e. the best signal is red while the worst signals are green.  

All later maps created by my system have the color coding reversed. The best signals are green while the weakest detectable are red. No signal detected is represented by a non-filled (white) circle.

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. 








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