The Doppler method
for determining airspeed and in-air RPM has now gained wide
acceptance. See previous articles
on Doppler for more information. The method has application
in R/C pylon race and C/L speed events, where mufflers are
not required. F3D and F2A are two events that benefit particularly
from in-flight evaluation of engine and propeller performance.
However, there is one disadvantage to the method,
which applies during single-handed testing. This disadvantage
relates to the position of the microphone. In the case of
F3D, the recording microphone should be placed at least 200m
past #1 pylon, and in F2A and F2C 50m from the edge of the
circle. This is highly inconvenient in both cases, enough
to deter even the fittest modeller, or in my case, one whose
knees now creak like the doors of a haunted house!
There may be a solution to this problem, which
I now propose. I have not tested this method in the field,
so I cannot guarantee that it will work. So the risk is yours,
a situation which suits me just fine!
There are now on the market little walkie-talkies
(transceivers?) with good range, many features and a reasonable
price (since you need two). The pair I have are Uniden UH-040XR
units that operate on the UHF band at 476 MHz. I paid AUD99
each for them from Dick Smith some time ago. I note that recently
similar units have been going for AUD89, which I would call
good value for such quality items.
The idea is to place one unit at the point for
sound detection, with it fixed on transmit (PTT), using perhaps
a cable tie to hold the switch down. With a range of up to
3 kilometres, there should be no problem with positioning,
even for F3D. This idea will not work if the unit has an internal
timer which switched the unit off as a power saving measure.
My units seem to be OK in that regard.
A second unit is placed in the pits, where it
is set to "Monitor" and connected directly to a
tape recorder input. On the Uniden unit, if the Monitor key
is held down for 2 seconds, it stays on permanently, an ideal
circumstance. The Uniden has a 3mm output socket labelled
SPKR. An ordinary mono (or stereo) audio cable will fit that
socket, and may be run directly to the MIC input on the tape
Well that's just about it. Oh, I use a Digitor
tape recorder, it is very cheap, well featured and battery
life seems to be much better than the old Realistic brand.
The only problem that may apply to the system is whether the
transmitter has a threshold level for sound input. That is
one of the bad features of mobile phones when used for Doppler
in F3D: they tend to cut in and out. I have not been able
to find a threshold on my Uniden units, but if there is one
it will soon show up in field tests!
Now a tip for F3D. There is no reason why multiple
transmit units should not be used. The Spectrogram trace works
as well for multiple models as it does for one, just makes
identification a bit of a problem. So I suggest having 2 transmit
units, one 200m past #1, and one 200m out past #2and #3. This
way you get speed/RPM in the straights, and speed/RPM as an
average in turns of going around 2 and 3.
Do let me know how you get on with this, and
good luck trying!
Addendum to Remote
In the article entitled "Remote Doppler Recording", on my
website at www.supercoolprops.eftel.com, I discussed the possible
use of Walkie-Talkies for facilitating the collection of sound
data. I have now done some testing on this method and report
the results here.
The Uniden UH-040XR is a reasonably priced (AUD100)
and high quality transceiver operating in the UHF region at
477 MHz. The unit features 2 sockets, one a 3.5mm mono audio
socket for connection to an external speaker, the other a
2.5mm mono audio socket for use by an external microhone.
The unit has no automatic gain control (AGC)
feature. When using the inbuilt microphone, it is necessary
to hold the unit within 150mm of the mouth to get good voice
In the case of recording model engine sounds,
this means you need a really noisy situation, such as in control-line
Goodyear and F2C racing. That being the case, you don't need
the system at all, as a standard tape recorder handles this
easily and chances are the operator will be near the flight
In F3D, the operator can easily be 500m away from the desired
recording location, and distance from the model can be anything
from 100m to 700m. To ensure good sound pickup, it is desirable
to boost the sound signal going into the unit: this implies
the application of an external AGC unit connected to the microphone
input on the Tx.
The point of having an AGC, is that weak sounds
are amplified more strongly than loud sounds, so that the
AGC output is largely independent of sound level.
Provision of this AGC unit has held up my work on the acoustic
antenna for some time now. The cost of design and production
of a custom AGC board is rather high. However, a solution
is at hand. The electronics component and kit supply company
Oatley Electronics (www.oatleyelectronics.com) offer a Laser
Communicator kit, part number K073, price AUD31, which rather
interestingly does not include a laser!
However, the transmitter board does feature
a rather useful AGC section. Perhaps this could be modified
to suit the Uniden? I spent half a day trying to make these
mods work, with a rather comprehensive degree of failure.
Now my electronics is rather dated (my Uni education was on
valves!) but I am not that hopeless. Also, my Huang Chang
oscilloscope does not tell lies, and it said I had a good
signal going into the Uniden.
Just when I was ready to toss in the towel,
I realised I had the AGC output connected to the SPEAKER socket
on the Uniden! This degree of idiocy has plagued me all my
life, so read on and perhaps you will find some more.
In modifying the Laser AGC board, it was first necessary to
know just what signal level the Uniden required from an external
microphone, so I purchased the Uniden hand-held Speaker/Microphone
unit which plugs into the aforementioned audio sockets on
the transceiver. Pulling this apart revealed an electret microphone,
a 3.3k resistor, a 470pF capacitor, a switch and a speaker.
No fancy electronics. This was promising, so I measured the
microphone bias voltage, which was -125mV (rather less than
I expected), and the output signal level. This was +-10mV,
about right, and less than the expected 30mV.
This meant that the AGC had to deliver a maximum
signal of +-10mV into the transceiver MICROPHONE socket. Now
this could be tricky, as most audio circuitry is designed
to deliver line-level signals at about 1V. And indeed, the
Laser Tx was similar.
To commence the mods, I removed the unwanted
components from my previously built Laser Communicator. This
was no hardship as the existing laser diode I had been using
was recently made illegal, and I had already bought a new
Parts removed were VR1, Q2, R13, R14, R10 and
C10. I also cut the conductor 5V supply track to R11.
VR1 was then replaced with a 500 ohm trimpot
(Dick Smith part #R1763), which was a pin-for-pin replacement.
Now the problem was to make the right new connections.
The signal I wanted appeared on the output side of C4, which
was now open circuit after removing R10. So I connected this
point to the supply side of R11 and broke the track to supply.
Bad mistake, but I didn't know that yet!
The combination of R11 at 47k and VR1 at 500
ohms now formed a voltage divider which would provide my 10mV
to the Uniden mic input. Of course, this was adjustable in
case I got it wrong. And I did. But not by much.
Now I didn't know the input impedance of the Uniden mic in,
so I figured it would be nice to have a unity gain buffer
to drive my 10mV sound signal. As it happened, in removing
Q2, I freed up one half of the LM358 dual op amp, so this
was a candidate for the buffer. Big mistake.
Anyway, I hung a 1.5k resistor across the buffer
output and neg in, mainly because I don't know any better,
and put a 1k resistor and 1uF cap in the output line to the
The 1k was an effort to protect the Uniden input
from over-current and the cap was to block any stray DC, including
the -125mV from the Uniden.
So I fired it up and started looking at waveforms. The input
to plus-in on the buffer op-amp from the C4 cap was fine,
and easily adjusted to 10mV at high sound level. So far so
But the ouput from the buffer was chopped off,
only giving the top-half of the signal!! Disaster! What could
be wrong? The problem was at the ouput pins of the op amp,
so it had to be the op amp that was the problem. I had previously
run a 741 op amp like this, with no problems, so I was both
surprised and disappointed.
The Dick Smith catalogue listed the LM358, so
I had a look at its specs. Would you believe its differential
input voltage was plus only? How dopey. I guess it has something
to do with making it a low power chip. But that didn't help
So I looked at how the other half of the chip was linked to
the electret microphone output. There was an odd combination
of resistors on the plus input, which appeared to be was some
sort of level-shifter.
By reconnecting R11 to supply and running a
line from C4 output via a 47k resistor to the VR1 pin at the
R11 junction I was able to level shift and get the full-wave
output from the buffer. Any DC was blocked by the output cap,
so we were back in business.
After connecting the AGC to the Uniden, strapping
the Uniden Tx switch to transmit and turning on the FM radio
for some music, I went outside with my second Uniden and was
rewarded with a strong clear signal.
Success at last. AGC today, acoustic antenna
I have now conducted some field test with the
AGC connected to the Uniden 040 transceiver. A sound level
at the transceiver of 70 dBA works fine, and this corresponds
to a normal voice level. If any problems remain, they are
most likely to concern the recording and playback quality
of the tape recorder.
I have been using a very cheap Digitor unit, which is well
featured, has good battery life, and has served me well. However,
I do now feel that a lot of the noise on the tape is due to
the tape recorder itself. Having little experience of such
devices, I do not know what to do next. Possibly a mini-disc
player is the way to go, but they are expensive (at least
by my standards!) .
This criticism of the Digitor needs to be qualified.
If, instead of playing back the tape on the recorder itself,
it is played back on my Hitachi Stereo, then the sound quality
is much better. It would be nice to find a device with digital
signal processing that eliminated tape hiss.
Concerning the modifications to the Oatley Laser
AGC board, there is one final point to be made.
It is a bit annoying trying to hold down the Press-to-talk
button with a cable tie.
With my usual lack of alacrity, it eventually struck me as
odd that the hand-held press-to-talk on speaker/microphone
unit actually worked at all. All it does is switch the electret
microphone into the circuit. So why did not the AGC board
also cause the unit to transmit when it was in circuit?
The hand-held unit was again stripped down;
this time a more thorough set of tests were done. It was found
that the speaker part of the circuit had no influence on causing
the Tx to transmit. Not really surprising, but as I say, this
was a thorough check!
Measurements on the electret side were a bit
more interesting. The DC voltages and DC current draw were
measured with the following results:
Tx off -2.32 .03
Tx on -0.127 .11
These measurements were repeated for the press-to-talk switch
on the Tx itself.
Tx off -2.32 .03
Tx on -0.35 .11
Note that these numbers are generated from the unit itself,
not the microphone. Perhaps it was possible to fool the Tx
into thinking the hand-held was connected, when in fact the
AGC was in place.
Accordingly, various resistors were placed across
the AGC output (to ground on the AGC board) to produce a current
draw similar to that above. It was found a value of 8k gave
a current draw of .08mA, and this indeed did cause the Tx
to transmit. Success!! No more switch tie down!
This is, I hope, close to the end of the story.
There remains just one little idiosyncrasy, which concerns
the Tx itself at turn-on.
If the AGC is plugged in live, or without power,
before the Tx is turned on, the Tx goes bananas and gives
an audible error signal. It does not want to go straight into
transmit mode. The cure is to install a switch in series with
the 8k resistor. The order of events may then occur as follows:
1. With the power off on all units, plug the AGC into the
2. Turn on the Tx
3. Turn on the AGC power
4. Switch in the 8k resistor.
Note that there must be 2 switches on the AGC, one for power
and one for the 8k resistor.
Well I hope that's about it. If any more surprises
come along I will make and Third Addendum!