Volante24
Posts: 31
Joined: 10/12/2006
From: Notre Dame, IN, USA
Status: offline
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A TX or any TX's RF output is tuned to the antenna length. Check your manual you'll find this information and similar warnings about over-heating the module in this case. TX's without modules suffer too, again it’s about the tuned antenna length not where the RF deck is located.
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Dear Michael,
There is no a single word about such precautions in the manuals, at
least for Optic and for Lasers. I can assure you if they were there I
would have spotted them at once -- I remember these things since late
1960s when the transmitters RC enthusiasts built on their own, were
not recommended to operate without antenna for this very reason of
overloading the RF output transistor. Not seeing such notes I reckoned
that all these problems were in the past off the cell phones era,
together with the "ultra-fast" transistors with the unit-gain
frequency of 120 or even 60MHz.
Ok, I have now specifically checked the manuals for both Laser 4/6 and for
Optic, there is no such a warning. It looks like you consider such
precautions to be known. If so, I think Hitec people should really
find a possibility to put this into the manuals.
My question regarding Lasers was due to the fact that the RF output
part there is inside the body of the transmitter on the main PCB with
plenty of room available. Therefore, easier cooling etc. Not to
mention that it would be trivial to incorporate a limiter which take
care of possible overload in case of untuned output. In fact, when I
checked the Rf output, the voltage did not seem to depend on whether
antenna was extended or collapsed on Laser 4. That was not a proof,
yet suggested that the circuit there duly handles this situation.
In fact, I would like to know if Hitec specifies somewhere the current
draw of this stuff. There is an impression that it's too high, the
supplied batteries did not last enough for safe flying, so we had to
put our own 2500mAh pack. We just checked a week ago one Optic 6 and
it turned out to draw 420mA (antenna fully extended); without the
RF unit the drain was only 80mA. Sounds noticeably larger than 305mA
quoted here in post #108. We are then at a higher risk, right?
This can be compared to the 150mAh total drain specified for Laser
4/6, keeping in mind the radiated power claimed the same. What's the
reason for such an inefficiency of the module? Frequency synthesizer
chip itself should not draw more than 20mA anyway.
By the way, since I am a physicist, a discussion I see here suggests
me to recall on the conservation of energy principle. That is true in
general that a circuit at smaller current drain may dissipate more
heat without antenna. However, hardly much in this setting. FCC does
not allow the antenna of your TX, is it Optic, Eclipse or Laser, to
radiate more than the stated power; in our case it is 500mW -- right?
This means that the power dissipated inside the module is always
within these 500mW of the consumed power. This is not an amount to
discuss where the total consumed power is 2W (in the lucky case) or 3W
(for the transmitter we own). In the case quoted by MikeRuth in post
#108, the change in the consumed power (assuming 10V input voltage)
was 0.3W at the radiated power 0.5W, therefore the dissipated power
indeed increased, but inly by 200mW. Too small.
Of course, this is not a bullet proof calculation. It assumed that the
RF unit is fed by the battery voltage. Hitec people probably can tell
us if that is so. Otherwise, a part of the power here is dissipated in
the voltage regulator, although I doubt it would be related to the RF
power output circuit, maybe only to the synthesizer
itself. Conservation of energy does not tell us where precisely the
power is dissipated inside the module. Heating case-wise this does not
matter much, but for the sake of safety of the circuit it does, since
if all the power, or only a part of it goes into the output
transistor, makes a big difference for the latter.
The upshot of this digression is that our measurements of the current
draw cannot tell us responsibly if it is safe or not to run TX in the
transmitting mode with the antenna collapsed. If it is not, the manual
and other warnings supplied with the system must clearly state
this. And it would be helpful -- already for the sake of safety -- if
Hitec disclosed what should be the current drawn by Optic
with or without the RF unit.
Nick
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All Forums >> Radios, Batteries, Clubhouse and more >> RC Radios, Transmitters, Receivers, Servos, gyros >> Radio Manufacturer Direct Support >> Hitec/MultiPlex Radios- Ask Hitec Customer Service >> RE: Optic/Spectra Glitching 
RE: Optic/Spectra Glitching - 
