I believe there’s some form of written communication’s disconnects within our correspondence between us. My understanding of the maze of tunnel’s combined with the above ground external and internal building path losses adds up to a sub-standard and unreliable system coverage for your Public Safety two-way radio system. There’s a process to unravel what has transpired throughout the years at your complex. In each reply between us, I’ve been able to extract more information related to the overall system coverage problems that you and your Public Safety personnel are experiencing on a daily basis so let’s continue our dialogue.
I’m attempting to provide you with alternative ideas and solutions to help correct the under lying problems that have
not been properly addressed for the system coverage of your entire complex.
Let’s start at what we’ll both understand with your Motorola equipment and the terminology used by RF Technicians and Engineers alike and now you’ll be better able to understand the terms too.
RF: Radio Frequency, the designated (FCC) Wireless Carrier Channel licensed and assigned for the user.
Watts: A measurement of energy or power. In this case, used as referenced for the Transmitted RF Carrier Frequency when measuring Forward Power Output and/or Reflected Power from the antenna system. A typical method of measuring the RF Power is using a
Bird RF Wattmeter or equivalent (test equipment) on a 50-OHM system. The symbol commonly used for Watts is the letter “
W”.
FM Receiver Sensitivity: The electrical measured amount of RF Carrier energy to fully Quiet
Q a given receiver per engineering and manufacture’s specifications. This is typically measured in microvolts and the symbols are
uV, milli-volts
mV and volts
V. A typical specification for Receiver Sensitivity should be measured directly at the receiver as:
.25uV up to
.5uV for normal specs. When reading the manufacture’s sales literature it may specify
.25uV @ 20 dB Quieting or .18uV @ 12 dB SINAD but you can
not mix and match the reference levels for true Analog FM measurements. It’s got to be referenced either all as Quieting or SINAD.
Something like comparing Apples and Oranges as they’re both fruit but
not exactly the same. So let’s not confuse the issue. The other thing to remember is the sensitivity measurements such as, the larger the number, the worse the performance capability of a receiver will be capturing the Subscriber’s handheld TX signal.
For an example, i.e.
Rcvr A =.5uV @ 20 dBQ versus Rcvr B = 5uV @ 20 dbQ. The handheld can always be captured by Rcvr
A but only
if the person is very close to Rcvr
B will the hand held’s signal be detected and the TX audio signal heard at the console and routed to the repeater’s TX. There could be multiple reasons that Rcvr
B is having this problem. A defective component in the receiver, path losses due to building construction materials, a poor choice of feed line and antenna, the improper implementation of the radio equipment or
all of the above.
Path Loss Path losses can be caused by several things such as: Buildings
–10 to –40 dB depending on the construction materials, Trees and Foliage
–6 dB, Hills
–14 dB @ 50 Feet, Coaxial cable / Feedline
–1.5 dB, Duplexers
-1.5~2 dB and Portables
-13 dB.
System Gain Receiver Sensitivity, Pre-Amplifiers and Base Station Antennas
+3 dB, +6 dB or +9 dB.
1 - UHF Micor TX P.O.
75W and RX
.5uV @ 20 dBQ with the C269 Spectra-TAC WL Encoder Option. Manufactured from 1975 till 1986.
1 – UHF MSR2000 TX P.O.
100W and RX
.5uV @ 20 dBQ with the C269 Spectra-TAC WL Encoder Option. Manufactured from 1982 till 1992.
1 – UHF Quantar TX P.O.
100W and RX
.25uV @ 20 dBQ with the C269 Spectra-TAC WL Encoder Option. Manufactured from 1991 till 2006.
1 – T1786B Spectra-Tac Analog (Audio) Signal Comparator with
3 SQMs and Tone Keying Module. Manufactured from 1974 till 2005.
1 – Centracom
Tone
Remote
Control Console
You might have noticed there’s a difference between the Micor / MSR2000 and the Quantar’s UHF Receiver’s Sensitivity specifications. When the receiver’s sensitivity vary from
.25uV to .5uV for Quieting levels then this can create the next system problem purely by accident. The Failure is known as
”Down Voting”.
Now, here’s something
you can experiment with on the T1786B Spectra-Tac comparator. Since there are
3 SQMs which are designated for the recovered audio path for each individual receiver site.
Disable Only the Quantar’s receiver site using the Disable switch on the module’s Front Panel. This will have
no effect on the site’s transmitter but will force the Micor’s receiver to be selected and audio signal routed to the Spectra-Tac comparator and then Voted. I would recommend this to be done for a minimum of 8 hours and a maximum of 48 hours. Have your Dispatchers keep accurate notes on the performance of the system coverage issues … Good and Bad. Please send me the results for analysis via the PM method unless you would rather post the information for everyone on the forum to evaluate. The choice is yours.
Let’s discuss better utilizing the UHF MSR2000 Base Station Repeater versus purchasing a brand new Bi – Directional UHF RF Amplifier. First of all, these are two different RF pieces of equipment that I believe you’re confusing as one for the use in your below ground level tunnel system.
The Motorola MSR2000 UHF Base Station Repeater is currently set up on the same pair of frequencies as your Micor and Quantar stations. Although the MSR2999 is placed in one of the underground tunnels with some type of an antenna of unknown origin or manufacture, the station does exist and occasionally routes the receiver audio back to the comparator and on to the console. I believe when the idea of selling this station into the system, either the /\/\ account representative or your higher echelon could have forgotten or refused the idea of using Andrew’s Radiax* coaxial cable and a 50-Ohm antenna splitter because of the up front cost to purchase and install.
This is something you should highly consider for all of the tunnels interconnected within your complex. I would also consider relocating the MSR2000 to a more central location within the underground tunnel system. Since the MSR2000 is a true repeater with the C269 Spectra-Tac Receiver / Encoder Option, it provides a better way to use both the Transmit and Receive on channel functions to provide the needed coverage.
The UHF Bi – Directional RF Amplifier (BDA) is a good method to boost RF signals used in conjunction with Radiax and/or Heliax coaxial cable systems. The BDA is a pair of Class “A” RF amplifiers operating in a Full Duplex system mode all the time tuned on or near the customer’s frequencies. This amplifies the relatively small signals from Subscriber hand held units in a tunnel system using several thousands of feet of Radiax RF coaxial cable for better signal penetration both on transmit and receive.
The BDA can be used with an existing UHF Base Station Repeater connected on the same feed line or not. The use of a BDA is added into a RF system when all other equipment fail to provide the required system coverage. It’s an expensive process to under take calculating the RF measurement levels and man-hours of time in the poor coverage areas. Then recommending the correct amount of equipment hardware required to purchase and accomplish this task. The installation time to install the BDAs and cable could be sub-contracted out to a local electrical contractor.
Since you’ve consider inverting the antenna at the Micor receiver site, I would recommend adding a tower arm to extend the antenna away from the tower therefore reducing any tower metal shadowing effects from happening. An ounce of prevention is worth ten pounds of cure. This may drive some signal downward but I doubt it will allow better signal penetration into and out of those tunnels. The theory behind the inverted antennas is based upon what is called the
“Radiation Umbrella Effect” or in other words, you can be standing within eye shot of the antenna but with your hand held Subscriber units, you can
not access the repeater system or you’re very noisey. I don’t have any suggestions how to deal with the owner of the RF site 2 miles away from your campus. It sounds like he’s a self maintained owner of the site and the customer owned equipment.
The use of passive antenna systems have their advantages
but you must keep your RF coaxial cables as short as possible and use preferably ½” Heliax with high gain
UHF +6 ~ +12 dB Yagi antennas on the outside pointed directly at the main repeater site and on the inside, use
+3 to +6 dB Gain Verticals mounted near or along the tunnel walls out of the way for pedestrian traffic.
The Spectra-Tac comparator works on a 20 dBQ signal so all of the Base Station receivers will require to be set-up for the worst receiver sensitivity being measured by your System Technician. As an example, i.e.
.4uV @ 20 dBQ and using that as the reference, setting other two receivers at the same levels with the
”And Squelch” feature incorporated on all Motorola Base Stations will better allow the audio comparator to function correctly. The Spectra-Tac audio comparator switches on a 2 dB change in AGC signals strength. The Spectra-Tac comparator only requires a Telco RT2002 Grade line but the Systems Technician should equalize each Telco pair between the remote receiver sites and the comparator.
The BDA would be my last recommended solution to your system coverage situation as there are many other alternatives to solve the RF coverage.
*OEM Suppliers and Reference Material Sources:
http://www.andrew.com/ ½” Heliax, Radiax, Antennas & Connectors
http://www.antenex.com/ UHF Yagi & Vertical Antennas
http://www.comtelcoantennas.com/index.htm UHF Yagi & Vertical Antennas
http://www.kathrein-scala.com/ UHF 380~512 MHz 50-Ohm Power Dividers
http://www.tessco.com/products/productHierarchy.do
This should provide you with some knowledge and alternative ideas for up grading your Two-Way Radio Communications System.
Dan
(guess who)