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Re: MTT4BT and Garmin Montana APRS
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Compton,
Certainly. When the MTT4BT or even more so, the MTT4BT-40 (40 Watt MTT4BT transceiver) have a high SWR during transmission, the processor may become "Swamped" with RF voltage building up on the board, and the TT4 loses control.? You will typically see the two bi-Color LEDs continuously flash, as if the four individual LED's are showing the same pattern as they do on immediate powering of the unit. This is typically what the lower power (10) Watt unit will do when the antenna has a very poor match, and is generally harmless. It may potentially toast the final amplifier, and/or result in corrupt firmware, but this can be reloaded by the end-user or by us ( Byonics) You may hear the T/R relay clicking while this is happening.?
?When the high power MTT4BT-40 goes into continuous reset, the device may lock up in the transmit-mode. Nothing but a carrier will be transmitted, and if this goes on too long, the final amplifier, and even the PCB and case can melt. That's the expensive smoke.??The units that have been melted into hideous globs of stinky plastic have usually been caused by things like people removing their antennas before entering car washes and forgetting to turn of the MTT4BT-40's power when disconnecting the antenna. Note that the RF output from either the low power unit ( up to 14 Watts) or the High power unit ( Up to 40 Watts) can cause RF burns and tattooing. ( Important safety tip!)?
Other issues are crappy coax and bad antennas, coax pinched or strangled in car door frames, etc. This is also a problem we have seen in our MT-RTG-50 ( 50 Watt Transmit-only device? that can actually have as much as 90 Watts output, depending upon power supply voltage and impedance) The little "Hershey's kiss" antenna that we provide for free with our low power RTG transmitter is a great example of an awful antenna for the MTT4BT. RG-174u coax, questionable SWR, and low magnet mass are not good things for a higher power transmitter.
The Mass of an MTT4BT transceiver is less than 4 ounces. Imagine putting a rubber duck antenna on a 40 Watt handheld transceiver! It would be bad. The MTT4BT transceivers?were designed for mobile APRS, which means a fairly low duty cycle, and relying on the Mass of an automobile to act as a counterpoise "Ground" for the transceiver. Consider this as if you were a person floating in space, trying to throw a heavy weight,?Isaac?Newton conspires against your putting very much force into the throw. ( This is not a perfect analogy, so physics?people don't go postal on me) As a practical matter, this effectively means as we increase the output power of a transmitter, we need to increase it's Mass for two reasons. One, we need a heat sink that can handle the wasted power, On the MTT4BT this is the PCB itself; there is no other heat sink, but since APRS duty cycles are very low, the PCB is enough. The second reason is that we need enough Mass in the transmitter?to act as a counterpoise to the antennae. That Mass must increase directly to handle the poorer antennas. The MTT4BT's are happiest with a mobile antenna, like an NMO or big magnet mount.?
For the last few months,? all the stocked? MTT4BT-40's and RTG-50 Transmitters ( And a few rare RTG-100 Watt transmitters) have had thermostatic switches added to the power amplifiers. If the amp gets hotter than 50 C, it gets turned off until the unit cools down substantially below that. This should prevent expensive smoke from leaking out of the units if they get locked on. (BTW, if anyone wants to have me add the thermal circuit breaker to their older MTT4BT-40 or RTG-50, I will do so for the cost of return shipping- I understand that return shipping to Australia is a horribly expensive thing, so I will be producing a set of instructions on how to add one yourself if you are so inclined)?
It was suggest in one of the recent posts to use an antenna that does not require a substantial ground plane. We have found that our V6 antenna ( A center-fed, half-wave dipole) works quite well with the MTT4BT-40, although it would not work well on a motorcycle. a 5/8ths wave antenna might also be a good choice.
73,
Allen AF6OF
-----Original Message-----
From: Compton <comptonallen@...> To: [email protected] Sent: Sun, Jun 27, 2021 8:15 pm Subject: Re: [TinyTrak] MTT4BT and Garmin Montana APRS Hi Allen,
Are you able to describe the symptoms of continuous reset you note? ? -- Compton VK2HRX Sydney, Australia |
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