Keep on truckin’
Tales From The Cube: Changing both the antenna and the transmitter didn't clear up this mysterious transmitter-shutdown problem. It turns out that antennas with the same part number were indeed different.
By John Bate, Volvo Group North America -- EDN, October 22, 2009
At our truck-manufacturing facility, the technicians in production called up engineering and said they couldn’t fix a problem fault code, something that we had to fix before we could ship the trucks to the customer. Our transmitter was shutting down due to high VSWR (voltage-standing-wave ratio). Production replaced the failed transmitters as well as the antennas, but the fault returned. We had seen some of these intermittent failures in the field so were hoping to find a number of production trucks with the same problem. We armed ourselves with as much test equipment as we could think of—a few spare antennas, a couple of PCs to monitor the transmitter software, and a 75-lb network analyzer—and drove for two hours to our production facilities.
The first thing we did was set up the PCs to tell us the VSWR in the transmitter and reproduce the problem. Bingo! The first time it transmitted, we hit the high VSWR; the software fortunately shut down the system to prevent any more damage. When we unbolted the antenna from the roof of the truck—read: “removed the ground”—the VSWR improved dramatically.
Our specialties are electrical and software engineering, not antenna design. We leave this black art to the antenna manufacturers. We specify only the antenna characteristics, keeping the VSWR as close as possible to 1-to-1 at about 145 MHz. We were stumped but knew that, without a ground plane, we would kill the system’s performance. Out came the network analyzer.
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We set up the analyzer to read VSWR over the transmitter’s receiving and transmitting frequencies of 138 to 150 MHz. We saw a well-shaped VSWR, although high on the transmitting side. We measured this VSWR with and without the antenna on the ground plane. The results showed why we were seeing faults. I opened up the display span to 100 to 200 MHz to see a bit more of the antenna bandwidth. We saw some significant differences and then realized that the antenna was not tuned to our 145-MHz requirement.
We took lots of pictures but still wondered why this problem had begun to show up. The antenna design wasn’t supposed to change without our approval. Upon returning to the engineering center, we reviewed this work with another system engineer. He quickly disassembled the antenna because it “didn’t look right.” We found that the antenna manufacturer had changed the circuitry at the base of the antenna. We put a few of these antennas through the network-analyzer test and found that they were indeed different, even though the part numbers were the same.
Our system engineer quickly called the antenna rep, who told us that the designs did change from time to time—in this case “only twice” since the original design, the rep said, stating that one of our sister companies had requested a new design for a shorter antenna. The antenna engineers thought that they could redesign the base a little bit and have a common base between both our companies, saving a bit of money.
A seemingly small change greatly affected our “fault-free” system. Now we had to chase down and repair possibly hundreds of trucks. After a few trials, I found that cutting an inch off the antenna whip got the VSWR back in line; a few days later, we shipped the customer some fault-free trucks.
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As usual, an interesting tale!
I would have thought that 145 MHz is primarily allocated to the "2-m Ham radio band", 144 - 148 MHz in this region. What is VOLVO doing there?
Also interesting about your 75 lb network analyzer, scalar or vector? May I mention that I have invented one that works beautifully in this range, 138 - 150 MHz, or any other one make it for, that cost and weighs a fraction of the one you have! It can be held in one hand.
Carl, KQ6AX & SM6MOM
Carl G. Lodström - 2009-1-12 22:21:00 PST -
OK. The specified frequency for best VSWR is generally in the center of the band you want to use. What we don't know is "who the customer is". Could be a government agency. There are many government/military operations on 138-144 and 148-150 MHz.
Lets ask John Bate of Volvo Group North America to clarify before we (hams) get in a tither about unlicensed commercial operations on the amateur bands.
John Larribeau (KR6MR)
Senior Engineering Assistant Electronics
SRI International
Production Engineering
John Larribeau - 2009-5-11 07:17:00 PST -
Vendors making changes that should not make a difference to the customer but do are very common in the electronics industry. Companies I have worked for all put requirements into their purchase contracts that require all changes to design to be reported. We still occasionally made mistakes in investigating these reports but generally it worked much better than not knowing at all.
JP Miller - 2009-30-10 12:09:00 PDT -
form, fit and function
Changes by the supplier without your knowledge often results in suprise.
D Amos - 2009-28-10 16:39:00 PDT -
First, the Freq of 145.??? is for Amateur use only. It is against the law for a buisness, or employee's of a buisness to use this freq for use to conduct buisness. I strongly suggest contacting the FCC and getting a Buisness license for such use.
2nd, The Anritsu SiteMaster is a great tool in helping solve issues like you discovered. Be prepared to pay $2K on up for such a devise. MFJ technologies manufactures a great inexpensive SWR meter that has proved to be fairly accurate for the folks on a budjet ($270-$380 depending on model chosen).
3rd, I don't have any electronic components at the base of any of my antennas except for my HF antennas.
The base of my VHF and UHF antennas connect directly to the base connector, with no loading coils, or other electronics. A simple antenna, that is about 19-20" long will do just fine (adjust for SWR on vehicle).
Again, however, the freq of 145.??? is illegal for commercial use. It is for amateurs only, holding an FCC license for that purpose.
JD Bennight - 2009-28-10 08:13:00 PDT
